3.9 Foundations in Mining Regions

8/3/2019 3.9 Foundations in Mining Regions

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3 .9 F o u n d a t io n s i n m i n i n g r e g io n sDietmar Placzek

1 G e n e r a l r e m a r k s o n m i n i n g - in d u c e d e f fe c t sM i n in g d a m a g e c o n s t it u te s a s p e c i a l d i s c ip l i n e o f m i n i n g e n g i n e e r i n g w h i c h d e a ls w i t hg r o u n d m o v e m e n t s i nd u c ed b y u n d e rg r o u n d m i n in g . T h e a s s e s s m e n t o f s u ch m o v e m e n t sw i t h r e g a r d t o e n g i n e e r e d s t r u c tu r e s a t th e s u r f a c e , i n c l u d i n g t h e d e s i g n o f t h e s t r u c t u r est o w i th s t an d t h e i m p a c t o f f u t ur e m i n in g , i s c o v e r e d b y t h e f i e l d o f " m i n i n g d a m a g ep r e v e n t i o n " .A n y a c t i v i ty t o p r e v e n t m i n i n g - i n d u c e d d a m a g e o f s u r f a c e s t ru c t u re s r e q u i r e s b a s e - l in ei n f o r m a t i o n f r o m a m i n e s u r v e y o r . H i s t a s k i s t o s p e c i f y , f o r a s p e c i f i c s i tu a t io n , t h e o r d e ro f m a g n i t u d e a n d t h e d i re c t io n o f g ro u n d m o v e m e n t s i n s p a c e a n d t i m e . T o s o l v e th i s t a skh e c a r r i e s o u t m e a s u r e m e n t s a n d c o m p u t a t i o n s w h i l s t a l s o c o n s i d e r i n g m i n e p l a n n i n ga n d p r o d u c t i o n d e t a i l s . B a s e d o n t h i s i n f o r m a t i o n t h e g e o t e c h n i c a l e n g i n e e r i s t h e n r e -q u e s t e d t o d e t e r m i n e t h e r e a c t io n f o r c e s w h i c h a r e tr i g g e r e d w i t h i n e n g i n e e r e d s t r u c tu r e sa t o r n e a r t h e s u rf a c e b y m i n i n g - i n d u c e d g r o u n d m o v e m e n t s . A n i n - d e p th k n o w l e d g e o ft h e m e c h a n i c a l p r o p e r t i e s o f th e f o u n d a t i o n s o il a n d o f t h e o b j e c t u n d e r c o n s i d e r a t i o n( e . g . a b u i l d i n g o r a t u n n e l ) i s e s se n t i a l. W i t h t h e s t r u c t u r a l p a r t i c u l a r s o f t h e o b j e c t i nm i n d , t h e s tr u c t u r a l e n g i n e e r t h e n f i n a l l y c h e c k s t h e c o m p a t i b i l i t y o f th e r e a c t i o n f o r c e sw i t h t h e o b j e c t i t s e l f . I n c a s e s w h e r e s u c h c o m p a t i b i l i t y d o e s n o t e x i s t h e t h e n n e e d s t od e s i g n a d a m a g e - f r e e s t r u c tu r e o r t o c o m e u p w i t h p r o p o s a l s f o r th e a l le v i a ti o n o f a n yd a m a g e w h i c h m a y h a v e a l r e a d y o c c u r r e d .I n th e f ie l d o f m i n i n g d a m a g e th e r e i s a d i s t in c t i o n b e t w e e n d e e p a n d s h a l l o w m i n i n g .G e n e r a ll y , sh a l lo w m i n i n g i s c o n s id e r e d t o b e l i m i t e d to m a x i m u m d e p t h s o f a b o u t 1 00t o 1 5 0 m . T h e i m p a c t o f s h a l l o w m i n i n g o n t h e s u r f a c e a n d i ts s t r u c tu r e s c a n b e s u b -s t a n ti a ll y m o r e i n t e n s e t h a n f o r d e e p m i n i n g . I n o r d e r t o d e s c r i b e m i n i n g a t v e r y s h a l l o wd e p t h s t h e t e r m n e a r - s u r f a c e m i n i n g w a s c o i n e d . I n th i s ty p e o f m i n i n g , t h e g r o u n dm o v e m e n t s f o l l o w o t h e r r u l e s t h a n t h o s e f o r d e e p m i n i n g . M o r e o v e r , t h e o c c u r r e n c eo f c o l la p s e s t o th e s u r f a c e a n d o t h e r d i s c r e te s u r f a c e f e a t u r e s , g e n e r a t e d b y n e a r - s u r f a c em i n i n g , c a n n o t b e r e a s o n a b l y p r e d i c t e d i n t e rm s o f ti m e .

2 G r o u n d m o v e m e n t s2 .1 G r o u n d m o v e m e n t s a b o v e d e e p m i n e w o r k i n g sM i n i n g o f a d e e p s e a m i s a s s o c i a t e d w i th c o l l a p s e a n d s u b s i d e n c e o f th e r o o f s t r a ta( F ig . 1 ). I n a m o s t e l e m e n t a r y l a y o u t o f a h o r i z o n t a l s e a m a n d t h e a b s e n c e o f g e o l o g i c a lf a u lt s , s u b s i d e n c e a t t h e s u r f a c e w i l l b e i n f o r m o f a t r o u g h ( t r o u g h t h e o r y a f t e r Lehmann[ 8 ] ) o r b e t t e r s t i l l a g e o sy n c l i n e .T h e f o r m a t i o n o f a s u b s id e n c e t r o u g h is a c c o m p a n i e d b y a 3 - D m o v e m e n t o f th e s u r f a c ep o i n t s w i t h c o m p o n e n t s n o t o n l y i n v e r t i c a l b u t a l s o i n h o r i z o n t a l d i r e ct io n s . G e n e r a l l y ,

C i ht 2002 J h Wil & S R t i d f k l


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560 Dietmar Placzeke x t e n s i o n ~= c o m p r e s s i o n ~= e x t e n s i o n

; ~ g e o s y n c l i n e ~ ;I E ( s u b s i d e n c e t r o u g h ) E Ip l I ~ 1I I

I s u r f a c e' " ' ' ~ : ; : : : ; ; : : : : ; i ; i i : i : J i : : ; : : - ~ I t o p l a y e r s

x - - - L _ _ . ~ . ,L I" ~ r - - -- / r o o f l a y e r s

/ - / / : . . > " . z ' x x / . '. ." " v + " . . . . y l O . . . " . , " / ~ " . . " x / 1 ; , ~

[HHi'"]iH l } ] i i l i H " t l l i ; I H I ] ~ ! HII,,J,]II"M I'H'i',l',',,,(i',H"

it l im i t a n g l epane l , b o r d e r s t h e z o n e o f a f f e c te d o v e r b u r d e n11 1

Fig . 1 . Co l lapse o f the roo f st ra ta o f a longwa l l pane l showing o f the geosy nc l ine ( se t t lemen t t rough)a t the su r face

the movement vectors are directed towards the excavation with an inclination towardsthe subsidence centre line (Fig. 2). Note that there are gradients for both vertical andhorizontal components. Any such gradient induces a tensile or compressive strain alongthe surface of the subsidence trough, which in turn imposes extra loads (extensionalstress or compressive stress) onto an existing structure which might lead to distor-tion, cracking or even structural failure.First and second order derivatives of the vertical movement component, with respect tothe horizontal distance, give the distributions of inclination and curvature (Figs. 2 and 3).The areas of saddle-like curvatures are indicative of surface regions in extension, thosewith trough-shaped curvatures are regions in compression. The trough subsidence pro-file is characterised by the "fracture angle" 13, which controls the area of maximumtensile strain, and the "limit angle" 7, which refers to the surface point where subsidencediminishes to zero. The complement of 7 is termed the "angle of draw". The magnitudesof both angles, 13 and 7 depend on the rock mass conditions and on the inclinat ion of thestrata.The order o f magnitude of the subsidence and the shape of the trough profile depend on anumber of factors. Besides the thickness and the overburden of the seam, dominantfactors are the size of the workings and the type of backfill. For unrestricted collapseof a full-size panel, surface subsidence amounts to about 90 % of the seam thickness.In back-filled panels, however, subsidence is reduced to values commonly of the orderof about 40 to 60 % of the seam thickness.Also of interest is the development of surface subsidence with time. In the Ruhr CoalMining District the following values are common:1st 2nd 3rd 4th 5th year75 15 5 3 2 % of the total subsidence

Copyright 2002 John Wiley & Sons Retrieved from: www.knovel.com


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3.9 Foun dat ions in min ing reg ions 561T h e a b o v e n u m b e r s a r e o n l y i n d ic a t i v e . T h e r e c a n b e s u b st a n t ia l d e v i a t i o n s , d e p e n d i n go n t h e d e p t h o f m i n i n g , t h e s e q u e n c e o f th e g e o l o g i c a l s tr a ta a n d t h e i n te r a c t i o n s w i t ho l d e r p a n e l s a b o v e t h e a c t u a l m i n i n g l e v e l .I f t h e s u r f a c e s u b s i d e n c e t r o u g h ( g e o s y n c l i n e ) i s c o n s i d e r e d o v e r i ts e n t ir e s h a p e , i . e . i nt h e n o r m a l d i r e c t i o n t o th e p r o f i le s o f F ig s . 1 a n d 2 a s w e l l , i t b e c o m e s e v i d e n t t h a t th e r ea r e a l s o d i s t o r t i o n s i n t h e v e r t i c a l d i r e c t i o n a n d r o t a t i o n a l t w i s t s a n d t o r s i o n s i n t h e h o r -i z o n t a l d i r e c t i o n .

s u b s i d e n c e v =

I ~ ( s u b s id e n c q ig h )fu l l - s i ze p a n e l I

p a n e l( p l a n v i ew )v e r t i c a l

s u b s i d e n c e c o m p o n e n t s

i ii V = ~i.~ c u r v a t u r e v = "

h o r i z o n t a ls u b s i d e n c e c o m p o n e n t s/~ / ~ l i c e m e n t s

. , l o _ _ I r a i ne _ _ . _ s t r a i n t e n s i l e s t l a i n . .~ ~ = ~ c o m p r e s s ~ s

l \ s t r am / I

V x ,V y

Fig. 2. Surface effects due to theextraction of a full-size horizontalpanel [8]



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562 Die tmar P laczekv e r t i c a l s u b s i d e n c e c o m p o n e n t s

s u b s i d e n c e

I V z [m m ]t i l t I

c u r v a t u r e i

t 9 = = c u r v a t u r e r a d i u s [m ]h o r i z o n t a l s u b s i d e n c e c o m p o n e n t s

V x ~ Vyd i s p l a c e m e n t s ~ iI V x ' v ' ' ,~

A V x

s t r a i n( t e n s i l e s t r a i n + ; ~ . ~ I V x2 Jc o m p r e s s i v e s t r a in - ) i v l

Fig. 3. Vertical and horizontal comp onents of ground m ovem ents

W h e n c o n s i d e r in g t h e d e v e l o p m e n t o f s u b s i d e n c e f o r a p a r ti c u la r s u r f a ce p o i n t i n c o n te x tw i t h t h e e x t r ac t io n p r o c e s s o f a d e e p p a n e l, a s m e n t i o n e d a b o v e , i t h a s t o b e r e m e m b e r e dt h a t e a c h s u r f a c e p o i n t i s s u b j e c t t o a g e n e r a l 3 - D m o v e m e n t . W h e n a s u r f a c e p o i n tb e c o m e s i n f l u e n c e d b y a n a p p r o a c h i n g m i n e p a n e l i t i s i n i t i a l l y s u b j e c t t o a h o r i z o n t a lt e n s i le s t r ai n . A s t h e p a n e l f a c e m o v e s c l o s e r t o th e s e c t i o n o f t h e s u r f a c e p o i n t , t h e m o d eo f s t r ai n c h a n g e s t o c o m p r e s s i o n . W i t h th e e x t r a c t i o n o f a d d i t i o n a l p a n e l s t h e p o i n t issu b j e c t t o i d e n t i c a l s t r a i n c y c l e s .



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3.9 Found ations in mining regions 56 3

ex trac tio n ~ a ~ - -a ~ extractic ~ ed ~ / ( d ~'~

g e o l o g i c If au l t

F i g . 4 , S t e p a n d s t a i r l i k es u b s i d e n c e a b o v e a g e o l o g i c a lf a u l t o r a n a r e a o f c o n c e n t r a t e dextraction

C o n c e n t r a te d e x t ra c t io n o f m o r e t h a n o n e s e a m i n t h e s a m e a r ea , f o r e x a m p l e o n a c la i mb o u n d a r y o r in t h e v i c i n i ty o f a g e o l o g i c f a u l t, c a n l e a d t o l o c a l l y c o n c e n t r a t e d s u b s i -d e n c e w i th t h e p o s s i b i l i ty o f t h e d e v e l o p m e n t o f a d i s c o n t i n u o u s s u b s i d e n c e t r o u g h( F ig . 4 ) . I n s u c h a re a s , th e e v a l u a t i o n o f t h e g e o t e c h n i c a l l y r e l e v a n t g r o u n d m o v e m e n ti s e x t r e m e l y d i f f i cu l t, i f n o t i m p o s s i b l e .I n re c e n t y e a rs , d i s co n t i n u o u s s u b s i d e n c e w i t h e x t e n s io n a l s t ra in h a s b e c o m e a c o m m o nf e a t u r e in th e n o r t h e r n a r e a o f th e R u h r C o a l M i n i n g D i s t r ic t . T h i s i s c l e a r l y d o c u m e n t e db y o p e n f r a c t u r e s a n d s t e p s in t h e s u r f a c e i n th e o r d e r o f d e c i m e t r e s . T h e f r a c t u r e s a r eo r i e n t e d p a r a l l e l t o t h e l o n g i t u d i n a l a x i s o f t h e p a n e l s .I n t h e L o w e r - R h i n e r e g i o n , w h i c h g e o l o g i c a l l y i s c h a r a c t e r i s e d b y n e a r - s u r f a c e g r a v e ll a y er s , fr a c tu r e s w i t h o u t a n y e x t e n s i o n a l h o r i zo n t a l d i s p l a c e m e n t c o m p o n e n t s a r e c o m -m o n . T h e n e a r - s u r f a c e g r o u n d o f t h e c e n t r a l a n d e a s te r n R u h r C o a l M i n i n g D i s t ri c t i sc h a r a c t e r i s e d b y l a y e r s o f q u i c k s a n d a n d s a n d y s il t w h i c h a r e i n t e r c a l a te d w i t h l im e -s a n d s t o n e s . G r o u n d f r a c t u r e s a r e c o m m o n l y f i l l e d w i t h s o i l f r o m t h e a d j a c e n t l a y e r s .G r o u n d s t e p s a n d g r o u n d f r a c t u r e s a r e d i s c r e t e f e a t u r e s w h i c h c a n o c c u r e i t h e r i n d i v i -d u a l l y o r i n c o m b i n a t i o n . I n t h e l a t t e r c a s e t h e s p a c i n g b e t w e e n t h e s t e p a n d t h e a s s o -c i a te d f r a c t u r e a t th e s u r f a c e i s t y p i c a l l y o f th e o r d e r o f a b o u t 3 0 t o 5 0 m ( f r a c tu r e z o n e ) .

2 .2 G r o u n d m o v e m e n t s a b o v e s h a l l o w a n d n e a r - su r f a c e m i n e w o r k i n g sO v e r th e la s t f e w d e c a d e s , c o m p r e h e n s i v e k n o w l e d g e h a s b e e n g a t h e r e d a b o u t t h e s u b-s i d e n c e d a m a g e c a u s e d b y s h a l l o w a n d n e a r - s u r f a c e m i n i n g . O n e d i s t i n c t u n c e r t a i n t ya s s o c i a t e d w i t h s u c h m i n i n g i s i n s u f f i c ie n t o r e v e n n o n - e x i s t i n g d o c u m e n t a t i o n o ft h e lo c a t io n , s i z e a n d d e p t h o f th e o l d m i n e w o r k i n g s . S o m e o f t h e s e a r e m o r e t h a n1 0 0 y e a r s o l d , o th e r s a r e th e l e f t o v e r s o f u n a u t h o r i s e d " w i l d " e x t r a c t i o n s d u r i n g t h ew a r a n d p o s t - w a r p e ri o d s o f t e n c a r r ie d o u t f r o m p r i v a t e d w e l li n g s f o r s e l f- s u p p l y p u r-p o s e s . A g a i n s t t h is b a c k g r o u n d o f s u c h u n c e r ta i n t ie s , e x p l o r a t o r y d r i l li n g i s n e c e s s a r y ,w h i c h h a s t o b e a d j u s te d f o r l o c a l g e o l o g i c a l c o n d i t io n s . I f n e c e s s a r y , t h e d r i ll h o l e s m a ya l s o b e u s e d f o r g r o u n d s t a b i l is a t i o n p u r p o s e s . E x p e r i e n c e s u g g e s t s t h e fo l l o w i n g :

T h e s u b s i d e n c e t r o u g h th e o r y , d e v e l o p e d f o r d e e p m i n i n g c o n d i t io n s , i s a ls o a p p l i c -a b l e to s h a l l o w m i n i n g , i f a c o m p a r a t i v e l y s t e e p " l i m i t a n g l e " y i s u s e d .I n n e a r - s u r f a c e m i n i n g t h e r e a r e s o m e s u b s i d e n c e f e a t u r e s w h i c h a r e d i s t i n c t i v e l yd i f f e r e n t f r o m t h o s e o f t h e d e e p e r e x t r a c t i o n s . D i s t in c t f e a t u r e s a r e c o l l a p s e s t ot h e s u r f a c e a n d s i n k h o l e s w h i c h c a n o c c u r a v e r y l o n g t i m e ( i n p r a c t i c a l t e r m s u n -



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5 6 4

I / \ I I ~ I II I I I I I I / Is u r f a c e c o l l a p s e

. . . . ' . - - . . ' . . - - - - . . . ' . , ' . . .

I ( I i ! i [ ~ , ~ 1 1 1 I I i II / I I I ~ t l l / I II I I ~ t I II I I I I " ~ ' x t ~ i I ~ [I I \ I I I I II I I ~ I [ I / Isinkhole

Dietmar Placzek

F i g . 5 . N e a r - s u r f a c e m i n ew o r k i n g s a n d t h e i r p o t e n t i a li m p a c t s o n t o t h e f o u n d a t i o ns t r a t a a n d t h e s u r f a c e

l i m i t e d ) a f t e r e x t r a c t io n . S u r f a c e c o l l a p s e d e n o t e s a s i t u a ti o n i n w h i c h c a v i n g o f t h er o o f s tr a t a r e a c h e s t h e s u r f a c e . S i n k h o l e s a r e a s e c o n d a r y f e a t u r e o f a p i p i n g p r o c e s s i nw h i c h t he s o il o f th e u p p e r l a y e r s is w a s h e d o u t b y f l o w i n g g r o u n d w a t e r a n d t r a n s -p o r t e d i n to t h e c o l la p s e d z o n e s o f t h e d e e p e r g ro u n d . B a s e d o n e x t e n s i v e a n d s y s t e m a -t i c i n v e s t i g a t i o n s , Hollmann a n d Niirenberg [ 4 ] d e v e l o p e d a c h a r a c t e r i s t i c l i n e s m e t h -o d w h i c h c o m p r e h e n s i v e l y d e s c r i b e s t h e e f fe c t s o f n e a r - su r f a c e ex t ra c ti o n . B e l o w ac r it ic a l d ep t h , c o l la p s e a b o v e a n u n d e r g r o u n d o p e n i n g ( r o a d w a y a n d r o a d w a y c r o s s -i n g ) c e a s e s w i t h o u t r e a c h i n g t h e s u r f a c e . T h e n e w b l o c k y s t ru c t u r e o f t h e c o l l a p s e dr o o f m a t e r i a l i s t h e n i n a c o n d i t i o n i n w h i c h a n e w o v e r a l l e q u i l i b r i u m o f th e g r o u n d i se s t a b l i sh e d .

3 I n f lu e n c e o f g r o u n d m o v e m e n t s o n t h e f o u n d a t i o nT h e v e r ti c a l a n d h o r iz o n t a l c o m p o n e n t s o f t h e m i n i n g - i n d u c e d g r o u n d m o v e m e n t s( F ig . 3 ) h a v e s p e c i f i c i m p a c t s o n t h e d e s i g n a n d u s e o f s u r f a c e s t r u c tu r e s . D e p e n d i n go n t h e i r d o m i n a n c e , d i f f e r e n t t y p e s o f p r e v e n t i o n m e a s u r e s a r e a p p r o p r i a t e .

3 . 1 I n f l u e n c e o f e q u a l v e r t i c a l s u b s i d e n c eI f t h e v e r t ic a l c o m p o n e n t o f th e s u b s i d e n c e ( s a g g in g ) i s o f eq u a l m a g n i t u d e a c r os s t hef o u n d a t i o n n o a d d i t i o n a l s t r e s s is g e n e r a t e d w i t h i n t h e s t ru c t ur e . T h i s t y p e o f s a g g i n g i st h e r e f o r e n o t c o n s i d e r e d i n t h e d e si g n . H o w e v e r , s a g g i n g m a y r e q u i re a t t e n ti o n w i t hr e g a r d t o t h e c h a n g e d d r a i n a g e c o n d i t i o n s a n d t h e ri s e o f t h e g r o u n d w a t e r l e v e l r e l a ti v eto the s t ru c tu re (F ig . 6 ) .T h e c l o s e p r o x i m i t y o f t h e f o u n d a t i o n o f a b u i l d i n g t o t h e g r o u n d w a t e r l e v e l o r e v e ni m m e r s i o n i n to t h e g r o u n d w a t e r c a n h a v e n e g a t i v e r e p e rc u s s i o n s u p o n t h e u se o f t h eb u i l d i n g ( s o a k i n g ) a n d c a n e v e n h a v e d e t r i m e n t a l e f f e c t s o n i t s b e a r i n g c a p a c i t y ( f o u n -d a t i o n f a i l u r e ; u p l i f t ) .

3 . 2 I n f l u e n c e o f t i lt - d i f f e r e n t i a l v e r t i c a l s u b s i d e n c eD i f f e r e n t i a l v e r t i c a l s u b s i d e n c e c a u s e s a t il t o f a b u i l d i n g w i t h th e m a x i m u m t il t o c c u r -r i n g a t t h e tr a n s i ti o n p o i n t s b e t w e e n t h e c o n v e x a n d c o n c a v e p o r t io n s o f t h e s u b s i d e n c ep r o f i le . T h e t il t g e n e r a t e s a d d i t i o n a l h o r i z o n t a l f o r c e s w i t h i n t h e s t r u c t u re w h i c h r e q u i r et h e e n g i n e e r ' s a t t e n ti o n , p a r t i c u l a r l y f o r r e la t i v e l y s l e n d e r s t ru c t u r e s s u c h a s c h i m n e y s



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3.9 Found ations in mining regions 56 5:::::::::::::::::::::::::::::::::.. .

subsi dence !n the area ~ the structure

~.gro~n~wa~;r~;;;~,"~7~,..__ ~ ~ ........................:~r is i n g g r o u n a w a t e r l e v e l r e l a ti v e ~ ~t o t h e s t r u c tu r e a n d t h e s u r f a c e . . . . . . . . . . . ~ /"

s u b s i d e n c e i n t h e a r e a o f a d r a i n a g e d i t c h .~""~drainage ditch - - I [

_

g r o u n d w a t e r l e v e l s i n k s b y a n a m o u n t s S < V zFig. 6. Effects of surface subsidence

. . . . T

Fig. 7. Effects of tilt for tall structures and structures founded in groundwater basins

a n d s il os , b u t a ls o f o r b u i l d i n g s w h i c h a r e s et in t o a n i m p e r v i o u s f o u n d a t i o n b a s i n(Fig. 7) .I n a dd i t ion to th is , a n a s se s sm e n t o f the usa b i l i t y o f the s t r uc tu r e i s r e qu i r e d f o r e a c hi n d i v i d u a l c a s e ( e . g . c o n s i d e r a t io n s o f t h e f u n c t i o n o f e l e v a t o r s , m a c h i n e t o o l s , a s s e m -b l y l i n e s, b o i l e r o f p o w e r p l a n ts a n d c o n t a i n e r s ).A m o u n t s o f t il t e x c e e d i n g 1 % c a n s e v e r e l y i m p a i r th e f u n c t i o n o f o r d i n a r y b u i l d in g s .



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566 Dietmar Placzek3 . 3 I n f lu e n c e o f c u r v a t u r eMathematically, the curvature is the second order derivative o f the vertical subsidence Vz.Curvature imposes additional moments in the structure. Their size depends on the mo-ment and distortion stiffness of the structurally interconnected parts of the building. Idealflexible structures conform to the curvature of the subsidence trough (geosyncline) with-out generating any additional stresses. Generally, this is not the case for stiff structures.Saddle and trough style curvatures can cause different types of gaps under stiff founda-tions as indicated in Fig. 8.The foundation conditions of stiff structures depend on a number of factors. The shapeand dimensions of the gaps (centre hole and cantilevered positions) depend not only onthe extraction direction of the panel but also on the compressibility of the foundationlayers. Stiff layers can cause significant stress concentrations.Flexible and stiff (rigid) structures are extreme cases. In reality, most masonry brick andsteel-concrete skeleton buildings have an intermediate stiffness. These structures canaccommodate a certain, but limited amount of curvature without any damage.

f l e x i b l e s t r u c t u r e

s a d d l e c o n f i g u r a t i o n t r o u g h c o n f i g u r a t i o n

s t i f f s t r u c t u r e

llllI I ~ ' ~ I

area of supports a d d l e c o n f i g u r a t i o n t r o u g h c o n f i g u r a t i o n

Fig. 8. Effects of surface curvature on flexibleand stiff structures

3 . 4 I n f l u e n c e o f s t r a i nStrain can be in the form of an extension or a shortening of the foundation plane. In theterminology of the mining damage field the forces generated within a structure aretermed tensile and compressive forces, respectively. A convex shape of the subsidenceprofile (saddle) causes an extension, a concave shape (trough) a shortening. Relativedisplacements between the foundation soil and the foundation of the structure generatefrictional forces in the base plane and side walls of the foundation structure. Soil resis-tance is mobilised at the side walls. Figure 9 shows physical models which delineate theprincipal effects of extension and shortening o f the foundation plane parallel to the mainaxis of a structure.



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3 .9 Founda t ions in min ing reg ions 56 7s t r u c t u r e p r i o r t o t e n s i l e s t r a i n s t r u c t u r e p r io r t o c o r n ) r e s s i v e s t r a i n

s t r u c t u r e a f t e r c o m p r e s s i v e s t ra i n ,c o m p r e s s i v e f o r c e s in t h e r o o f o f t h e o p e n i n gFig. 9. Effects of tensile and com pressive surface strain as demonstrated in a physical m odel

::'i"i: a e n d i n g :i"i-: L|t . ":." .,f r i c t i o n _

: t o e r e s i s t a n c e

I I II4 1 1 " - - i i i i ' - - I ~ "

l o o s e n i n g o f t h e s o i lb y t e n s i l e s t r a in

Fig. 10. Effects of tensile strain in the ground onto a pile foundation

P a r t i c u l a r l y p r o b l e m a t i c i s m i n i n g - i m p o s e d g r o u n d s tr a in o n a p i l e f o u n d a t i o n . I f t h e p il eh e a d s a r e s tr u c tu r a ll y c o n n e c t e d , t h e r e s u lt in g m o m e n t s c a n r a re l y b e a c c o m m o d a t e d b yt h e p il es . T h i s c o m m o n l y l e a d s t o a l o s s o f th e b e a r i n g c a p a c i t y o f th e p il e s. F u r t h e r m o r e ,i t h a s t o b e r e a l i s e d t h a t th e e f f e c t i v e s k i n f r ic t i o n c a n b e s e v e r e l y r e d u c e d b y g r o u n ds t ra in (F ig . 10 ) .

3 .5 I n f lu e n c e o f g r o u n d m o v e m e n t s a b o v e n ea r - su r f a c e m i n e w o r k i n g sA s m e n t i o n e d b e f or e , n e a r - su r f a c e e x t r ac t io n o f s e a m s c a n c a u s e s u r f ac e c o l l a p s e a n ds i n k h o l e s d e p e n d i n g o n t h e d e p t h a n d t y p e o f e x t ra c t i o n ( F ig . 1 1). T h e y o c c u r s p o n t a -n e o u s l y a n d p o s s i b l y a v e r y l o n g t i m e a f t e r th e e x t r a c t io n . F o r e x i s t i n g s t r u c t u r e s a t t h es u r f a c e th e r e a r e c e n t r a l h o le a n d c a n t i l e v e r e d f o u n d a t i o n c o n f i g u r a t i o n s . T h e a s s o c i a t e dl o a d s i m p o s e d o n t o th e s t r u c tu r e s d e p e n d o n t h e l o c a t io n a n d e x t e n t o f t h e s u r f a c e s u b -s i d e n c e t r o u g h a n d t o a l e s s e r d e g r e e o n t h e f o u n d a t i o n d e p t h .



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568 Dietmar Placzek

ig. 11. Surface collapse above~ar-surface mine workings

3 . 6 I n f l u e n c e o f d i s c o n ti n u o u s g r o u n d m o v e m e n t sAccord ing to the most recent findings, discontinuous ground movements in the form ofsinkholes can occur not only in near-surface but also in deep mining, especially in areaswithin the extension zones of the geosyncline. Their occurrence is controlled by factorssuch as the composition and structure of the roof strata, depth of the groundwater leveland the extraction sequence in multi-seam mining. The results are localised and causerelatively large centre hole and cantilevered foundation configurations which are usuallyincompatible with the structure of the building.

4 P r e v e n t i v e m e a s u r e s i n a r e a s w i t h d e e p m i n e w o r k i n g s4 .1 Ty p es o f p rev en t i v e m ea su resThe type and extent of preventive measures for surface structures depend on the type andmagnitude of the ground movements, the bearing capacity of the structure and on eco-nomic factors such as its state of usability and the importance and sensitivity of its func-tion. Firstly, the capability of the structure to accommodate the expected ground move-ments has to be checked with regard to its type and function.A distinction is made here between (see also Chapter 3.1): Stiff structures, which essentially remain unchanged in their shape and dimension. Thedeformations are in response to stresses which remain within acceptable limits. Flexible structures, which conform to the subsidence profile without exceeding the

acceptable stress limits.One must also ver ify to what extent the type and the function of the structure can toleratethe damage prevention measures and, if such measures are required, what type of mea-sures should be implemented. The design of such measures can be in accordance witheither [6]:



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3.9 Found ations in mining regions 56 9 t h e P r i n c i p l e o f R e s i s t a n c e , w h i c h m e a n s t h a t a ll a d d i t i o n a l f o r c e s i m p o s e d b y t h eg r o u nd m o v e m e n t s c a n b e a c c o m m o d a t e d b y t h e s t ru c tu r e o r t h e P r i n c i p l e o f D e f o r m a t i o n , w h i c h m e a n s t h a t t h e s tr u c t u r e c o m p l i e s w i t h th e g r o u n dm o v e m e n t s w i t h o u t d e v e l o p i n g a n y d e t r i m e n t a l s t re s se s .D e p e n d i n g o n t h e f u n c t i o n r e q u i r e m e n t s o f t h e s t ru c t u r e a n d t h e e c o n o m i c c o n s i d e r a -t i o n s , t h e d a m a g e p r e v e n t i o n a l t e r n a t i v e s a r e a s f o l l o w s : T o t a l d a m a g e p r e v e n t i o n , w h i c h i m p l i e s t h e p r e v a l e n c e o f t h e h i g h e s t s a f e t y l e v e l.U s u a l ly , t h e b u i l d in g i s d e s i g n e d a s a v e r y s t i f f s t ru c t u re . A n y t y p e a n d a m o u n t o fg r o u n d m o v e m e n t c a u s e s s t r ai n in g o f t h e s t ru c t u re s w h i c h r e m a i n s w i th i n t h e a c c e p -t a b le e l a s ti c l i m i t s o f t h e m a t e r i a l s . T h e o r i g i n a l p o s i t i o n c a n b e r e - e s t a b l i s h e d b ys u i ta b l e u n d e r p i n n i n g a n d l i f ti n g t e c h n i q u e s s u c h a s h y d r a u l i c j a c k i n g . P a r t ia l d a m a g e p r e v e n t i o n w h i c h i m p l i e s th a t m e a s u r e s a r e t a k e n t o a l l e v i a te t h e i m -

p a c t f r o m t h e m o s t d e t ri m e n t a l f a c t o r o f t h e g r o u n d m o v e m e n t s , w h i c h i s u s u a ll y s t r a ini n th e h o r i z o n t a l d i r ec t io n . T h i s s t r a te g y a i m s t o s e c u r e t h e o v e r a l l s t a b i li t y o f t h es t r u c t u r e w h i l s t t o l e r a t i n g m i n o r d a m a g e w h i c h c a n b e r e p a i r e d a s i t o c c u r s .

4 .2 B a s i c c o n s i d e r a t i o n s o n l a y o u t a n d d e s i g n o f s u r f a c e s t r u c t u r e sI n t h e e a r l y p l a n n i n g s t a g e o f a p r o je c t , it is a l r e a d y a d v i s a b l e t o t a k e t h e r e l e v a n t s u b -s i d e n c e f a c t o r s i n t o a c c o u n t , p a r t i c u l a r l y t h e g e n e r a l g e o l o g i c a l c o n d i t i o n s a n d t h e e x -p e c t e d g r o u n d m o v e m e n t s f r o m t h e e x t ra c t io n o f s e a m s . I n p l a n v ie w , t he l o n g i t u d in a l

; t u re

s t a n d

Fig. 12. Structural dam age prevention by sub division of the superstructure into numerou s small structuralunits separated by joints (Football arena "A uf Schalke" in the C ity of Gelsenkirchen)



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570 Die tmar P laczek

~~ s lip ay e i\

Z-Z-~ adding i t h~ ~ ~ i o a m mr:"i:: ::':(i (~:: ::': i::. fou nda ti '

pile e a d

e x t e n s i o n p l a teron e a m

Fig. 13. Separation of the foun dation structure from the superstructure by m ean s of s lip surfaces(example of a pile foundation)

a x i s o f th e b u i l d i n g s h o u l d p r e f e r a b l y b e o r i e n t e d i n th e d i r e c t i o n o f s t r ik e o f t h e s e a m s ,a s th is w i ll r e s u lt in t h e m i n i m u m o f s u b s i d e n c e - r e la t e d p r o b l e m s . C o m p a c t s tr u c tu r e sa r e l e s s p r o n e t o s u b s i d e n c e d a m a g e t h a n e x t e n d e d o n e s . T h e l a t t e r r e q u i r e s t h e i n c o r -p o r a t i o n o f s e p a r a t i o n j o i n t s , w h i c h a r e ty p i c a l l y s p a c e d a t 2 0 t o 3 0 m ( F ig . 1 2 ). F o u n d a -t io n s m a y a l so n e e d a s e p a r a ti o n f r o m t h e s u p e r s t ru c t u re b y p u r p o s e - d e s i g n e d jo i n t s o rs p a c e r s t o a v o i d u n f a v o u r a b l e i n t e r l o c k i n g p r o b l e m s ( F i g . 1 3 ) .T h e c o n s t r u c t i o n m a t e r i a l m u s t b e p a r t ic u l a r l y s u i t a b l e f o r t h e c h a n g i n g c o n d i t io n s i m -p o s e d b y m i n i n g . I n a d e s i g n w h i c h i s b a s e d o n t h e P r i n c i p le o f D e f o r m a t i o n , s o f t a nde l as ti c e l e m e n t s a r e m o r e a p p r o p r i a t e t h a n b r it tl e e l e m e n t s o r t h o s e w i t h a h ig h m o m e n to f i n e r ti a . I f n o t i n c o n t r a d i c t i o n w i t h t h e f u n c t i o n o f t h e s t r u c tu r e , a s t a t i c a l l y - d e t e r m i -n a t e s y s t e m s h o u l d b e p r e f e r r e d f o r m i n i m a l s t r u c t u r a l r e s i s t a n c e a g a i n s t t h e i m p o s e dd e f o r m a t i o n s .

4 .3 B e a r i n g c a p a c i t y a n d f u n c t i o n a l it y o f a s t r u c t u r eT h e d e s i g n o f s t ru c t u r e s r e s t in g o n s u b s i d i n g g r o u n d r e q u i r e s t h e c o n s i d e r a t i o n o f t h es p e c if ic l o a d c a s e - " m i n i n g - i n d u c e d a c ti o n s " . T h e f u n d a m e n t a l r e f e r e n c e f o r t h is c a s e a r et h e " G u i d e l in e s f o r e n g i n e e r e d s t ru c tu r e s i n r e g io n s a f f e c te d b y u n d e r g r o u n d m i n i n g "[ 1 . G e n e r a l l y , t h e g u i d e l i n e s p e r m i t a d e s i g n t a k i n g i n to a c c o u n t a n i n c r e a s e d l o a d b e a r -i n g c a p a c i t y o f t h e m a t e r i a l s a n d i n e x c e p t i o n a l c a s e s e v e n t h e i r u lt i m a t e s t r e n g th . T h el a tt e r p o s s i b i li t y , h o w e v e r , r e q u i r e s a d e t a i l e d e x a m i n a t i o n o f th e f u n c t i o n a l i t y a n d s t ru c -t u r e f o r e a c h i n d i v i d u a l c a s e .S t r a i n - s e n s i t i v e s t r u c t u r e s a n d h i s t o ri c m o n u m e n t s , f o r i n s t a n c e , a r e to b e d i f f e r e n t lyd e s i g n e d w i t h r e s p e c t t o b e a r i n g c a p a c i t y a n d f u n c t i o n a l i t y t h a n b u i l d i n g s c o n s t r u c t e du s i n g m o d e r n m a t e r i a l s .

4 . 4 P r o v i s i o n s f o r t il tI f n o p r e c i s e i n f o r m a t i o n o n t h e p o s s i b l e o r e x p e c t e d t i lt is a v a i l a b l e , th e s t r u c t u r e s h o u l da l w a y s b e d e s i g n e d f o r e x t r a lo a d s a c t i n g in a n y h o r i z o n t a l d i r e c ti o n w i th a m a g n i t u d e o f1 % o f t h e v e r t ic a l lo a d s a c t i n g o v e r t h e c r o s s s e c t i o n c o n s id e r e d . W h e n e x c e e d i n g t h e



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3.9 Foundations in mining regions 57 1

l o n g i t u d i n a l s e c t i o nI

, , , , ' r F : ; : ; : + ~ - - : : ~ - ' ~ ; : : ; ; : : : ; ; : : : ; ; : : : ~ : ~ : ; ; = = ' ] , , , , ", , " ' , , " ' , , " ' , , " ' , , " ' , , " ' , , " 'I I I I I I I I I I I I I I I t l II I I I I I I I , I I I I 1 1 1 1 I t I I I I I II d [ I I I I l I l I I + ' , i I ~ ~ I i~ i I I I # I I ~ l ~ l l l ~ l l l l h r ~ l +1 7 i i i ~ ; ~ . u u m i r @ - - i ~ l + V - iI I I I I I I I I I I I , b , , I t I I [ I I l i /I I I I t l I I I I I I I t I I I I I I I I II I I I I I I I I I I I I I I I I I I I ~ I I [ ! l i ~ ' iI I I I I I I I I I 1 1 I I I I t l I II I I I I I I I I l I ~ i I |I I I I I I I I I I I I I I I I I I I ~ I II I I I I I I I I I I I I I I I I I I ~ I 1 - ~ . .I I I I I I I I I I I I I I I t I I I I " I II I I I I I I I I t I I t l I I I I I I ~ I II I I I I I I I I I I I I I I I I I I I T - I II I I I I I I I I I I I I I I I I I t li I ~ z i i i i i i i + + i ~I I I I I I I I I I I I I I I I I I I t I I ' 11 1 I I t I I I I I I I I I I I t l I t I I I I F i r e . . .4 . S w l m m l n ~ o o o l w i t h t h r e e -~ l ; i ; i ; l i l 1 1 i ; i l l ' ; t 11 1 1 ~ ~ "a i + i s i i , i i t i i + i ~ t ~ 1 , i ~ i ~ p o i n t S U D D O I ' t s y s t e m T h e S U D D O r t s a r eI I I I I I I I I I I I I I I I I I I L x l # i x, , ~ ,, ,, , , + +, l o c a t e d i m m e d i a t e l y b e n e a t h t h e s l d e -I I I I I I I I I I I I I I I I I I " " "L _ 2 22 .~ 7 .2 2 - 2 2 2 2 2 2 2 - - . 2 - - . - - 2 2 - - - - 2 2 - - - - 2 ~ - 2 2 - - 2 - 2 _ ~ w a l l s a t t h e b a s e o f t h e p o o l

/ / /p l a n v i e w w i t h s u p p o r t s

a l l o w a b l e t i lt , th e f o u n d a t i o n s u p p o r t s o f t h e s t r u c t u r e m u s t b e r e a d j u s t e d . T h i s i s b e s t b ea c h i e v e d b y a th r e e - p o i n t s u p p o r t ( F ig . 1 4). I n o t h e r c a s e s , p r e s s u r e c h a m b e r s s h o u l d b ep r o v i d e d i m m e d i a t e l y b e n e a t h t h e f o u n d a t i o n r a f t f o r th e i n s t a ll a t io n o f li f ti n g d e v i c e ss u c h a s h y d r a u l i c j a c k s .A g e n e r a l p r e c o n d i t i o n f o r t h i s t i l t c o r r e c t i o n i s a s t r u c t u r e w i t h a s i g n i f i c a n t b e n d i n gs t i f f n e s s . I t i s , h o w e v e r , a l s o p o s s i b l e t o a p p l y t h e a b o v e t o t a l d a m a g e p r e v e n t i o n t e c h -n i q u e t o f l e x i b l e s t r u c t u r es p r o v i d e d t h a t t h e i r s u p p o r t s a r e r e - a d j u s t a b l e a n d t h a t i t is as t a t i c a l l y d e t e r m i n a t e s y s t e m , e . g . a s t e e l s k e l e t o n b u i l d i n g .

4 .5 P r o v i s i o n s f o r c u r v a t u r eA c c o r d i n g t o [ 1 ], th e f o l l o w i n g r a d ii o f s u b s i d e n c e c u r v a t u r e P z ( w h i c h i s t h e i n v e r s e o ft h e s u b s i d e n c e c u r v a t u r e p r o p e r ) c a n g e n e r a l l y b e a s s u m e d i n a f o u n d a t i o n d e s i g n : F o r asa d d l e - l i k e su b s i d e n c e p r o f i l e Pz = 2 0 0 0 m a n d f o r a t r o u g h - l i k e p r o f i l e Pz --- 5 0 0 0 m .T h e s e r a d i i a r e a p p l i c a b l e i n a ll c a s e s w i t h t h e e x c e p t i o n o f a n e x p l i c i t s p e c i f i c a t i o n o f al o w e r c u r v a t u r e r a d i u s . I f th e f u n c t i o n a l i t y o f th e s t r u c tu r e i s n o t n e g a t i v e l y a f f e c t e d ,s t a ti c a ll y d e t e r m i n e d s y s t e m s o r s t r u c tu r e s w i t h a lo w d e g r e e o f s t if f n e s s s h o u l d g e n e r -a l ly b e p r e f e r r e d . T h e l o w e r t h e s t i ff n e s s o f a s tr u c tu r e , t h e l o w e r t h e i m p o s e d s t r e s s e s(deform ation principle).I n i t i a l l y , a n a s s e s s m e n t h a s t o b e m a d e a s t o h o w f a r t h e s t r u c t u r e c a n c o m p l y w i t h t h ee x p e c t e d c u r v a t u r e w i t h o u t a n y d a m a g e . O r d e r o f m a g n i t u d e e s t i m a t e s a r e p r o v i d e d i nF i g . 1 5 f o r t r o u g h - l i k e p a r t s o f th e s u b s i d e n c e p r o f i le . I t c a n b e s h o w n t h a t f o r c o m m o ns t ru c t u re s , c o n s i s t i n g o f m a s o n r y b r i c k a n d r e i n f o r c e d c o n c r e t e s l a b s o r a s t e e l - c o n c r e t e



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r e f e r e n c eT e r z a g h i (1 9 4 8 ) 7 2 L ,. . 1 7 0 LL e u s s l n k ( 1 9 5 4 )R u s s i a n n o r m s ( 1 9 5 5 )M e y e r h o f f ( 1 9 5 5 )S k e m p t o n ( 1 9 5 7 )R a u s c h ( 1 9 5 5 )B u r l a n d e t a l , ( 1 0 7 8 ) 2 0 L . . . 1 2 5 . L

7 2 - L . - 1 7 0 . L3 1 . L . . . 6 2 . L

p~ [ m l As [cm]L L

5 5 0 " 1 3 5 0L55 0 " " 1 L s oL L2go " r3o

L62 .L 50 0L7 6 " L 6 0 0L1 2 5 L 1 0 0 0

L L1 50 "" 1 0 0 0

' 2 ! ' . . . . . . 2 ! ' . ? : : : ' : ' 2 i ' ; ? : : ' : . , 2 ! " . ? : : ' : . , iI - L - I

L i n [ m ]

572 Dietmar Placzek

through position

Fig. 15. Settlement differences and curvature radii tolerable for surface structures

skeleton, the acceptable amount of bending is in the order of As = L/8 00 . This is equiva-lent to a minimum radius of curvature of Pz = 100 L, or considering common buildingdimensions, Pz = 1000.. .3000 m with As = L2 /(8 Pz)- For steel skeleton structures(predominately industrial plants) the respective amount of bending is usually even great-er (and accordingly, the minimal curvature radius smaller) than the above-mentionedvalues. Some attention, however, is needed to maintain the integrity of the floor androo f deck support systems throughout the various subsidence phases. This is particularlyrelevant if statically determinate systems and flexible structures are subject to excessivebending. Usually, the structure o f a building reacts more sensitively towards saddle-likecurvature conditions. This has the consequence that there is an admissible curvatureradius for saddle-like conditions which is typically larger than for trough-like ones.For structures with a high bending stiffness it is necessary to investigate all of the po-tential support conditions of the foundation (central hole and cantilever situations). Inorder to determine the stress distribution within the structure, either the sub-grade mod-ulus method or the stiffness modulus method may be used (see Chapter 3.1).The stiffer the foundation soil, the more likely is the development of high stress con-centrations within the structure and the formation of large central hole and cantileversettings. By placing soft pads within the foundation plane and by designing relativelysmall foundation areas with comparatively high foundation pressures it is possible tohomogenise and improve the general foundation conditions and to alleviate the impactof major open gaps in the foundation plane.Figure 16 provides an example of a silo with a comparatively high bending stiffness. Itshows the load distribution o f the four external pillars for some selected curvature radii.The influence of the curvature on the magnitude and the distribution of the foundationpressure is evident.Note the influence of the size of the foundation slab on the load redistribution process. Acomparative ly small foundation area of 2.0 x 2.0 m, which is associated with an averagefoundation pressure of (~0 = 0.68 MPa, gives a load redistribution of about half the mag-nitude of a larger foundation area of 3.0 x 3.0 m, and an average foundation pressure ofor0 = 0.30 MPa. In this context it has to be mentioned that when selecting foundationpressures well above the admissible limit, very careful investigations have to be carried



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3 . 9 F o u n d a t io n s i n m i n i n g r e g i o n s 5 7 3o u t fo r th e b e a r i n g c a p a c i t y o f t he f o u n d a t i o n s o i l to a v o i d a n y u n c o n t r o l l e d s e t t l e m e n t sat the funct ional l imit of the s tructure.Fro m Fig . 16 it furthermore can be dep ic t ed that , w he n the foundat ion so i l i s layered , thesof t er layer sho u ld be preserved , i f poss ib le in it s fu l l th ickness . T h is layer ac t s as a sof tc u s h i o n a n d h o m o g e n i s e s a n y s e t tl e m e n t d i f f e r e n c e s . O v e r a l l, t h e re a re a l m o s t i d e n t i ca lcon d i t ion s for the two op po s ing cases o f a s t i f f s tructure res t ing on so f t so i l or a f l ex ib lestructure rest ing on st iff soil .S lab foundat ions are cr i t ica l in a sadd le - l ike curvature prof i le and the as soc iated gaps int h e c a n t i l e v e r - t y p e f o u n d a t i o n ( F i g . 8 ) . T o a c h i e v e a r e a s o n a b l e a d j u s t m e n t o f t h e s e t -t l e m e n t s i t i s a d v i s a b l e t o a v o i d s l a b f o u n d a t i o n s a l t o g e th e r w h e n e v e r p o s s i b l e . A n a lte r-n a t i v e i s a r i n g f o u n d a t i o n , w h i c h c a n a l s o b e d e s i g n e d f o r c o m p a r a t i v e l y h i g h b a s ep r e s s u r e s . A s s h o w n f o r t h e e x a m p l e o f a c y l i n d r i c a l c o n t a i n e r ( F i g . 1 7 ) , t h i s c a n b ea c h i e v e d b y p l a c i n g p a d s i n t h e c e n t r a l p a r t s o f t h e f o u n d a t i o n . I n s t a t i c c o m p u t a t i o n so f t h e a b o v e c a s e i t w a s s h o w n t h a t , f o r t h e t w o l o a d c a s e s c o n s i d e r e d w i t h a n d w i t h o u tm i n i n g - i n d u c e d s u b s i d e n c e , t h e r e w e r e v i r t u a l l y i d e n t i c a l r e a c t i o n s i n t h e c o n t a i n e rstructure.

3 0 . 8 m

Q = 1 0 . 8 M Nl O m _ 1_ l O m l O m

- I - D

F s s ~,+ : s/ ~ / / >~" I - r / ' ; " ~ P ~ , ~ l

5 . 0 m s i l t E s = 2 0 M P aI ~ ' l . r / I i " r I ~ l I ~ 11 L ~ F ~ l ~ r ~ - - ' l I ~ i " f f - - I ~ l I~ I II I I I I i i I i i i i i i i I I I I 1 I I

I I I L I I I h a r d a n d s t i f f m a r l I I [ I i i i ii i i i i i i i i i i F i it r i t t t t t i i t I T i i I t t t I t t t Ib a s e p r e s s u r e w i t h f o u n d a t i o n d i m e n s i o n s o f a / b - - 3 . 0 x 3 . 0 m : ~ 0 = 0 , 3 M P ab a s e p r e s s u r e w i t h f o u n d a t i o n d i m e n s i o n s o f a / b = 2 . 0 x 2 . 0 m : G 0 = 0 . 6 8 M P a

r a d i u s Pz (m)

o o10.000

5.0003.0002.0001.000

p i l l a r f o r c e F s (MN)s ; i s

o i I 2.7 (2.7) 2.7 (2,7) 2.7 (2.7) 2.7 (2.7)3.2 (3.0) 2 2 (2.4) 2.2 (2.4) 3.2 (3.0)3.7 (3.3) 1.7 (2.1) 1.7 (2.1) 3.7 (3.3)4.4 (3.7) 1.0 (1.7) 1.0 (1.7) 4.4 (3.7)5.4 (4.2) 0 (1.2) 0 (1.2) 5.4 (4.2)

( 5. 4) ( O ) ( O ) ( 5. 4)) a t d i m e n s i o n s o f t h e fo u n d a t i o n o f 2,0 x 2,0 m and ~0 = 0,68 MPa

i n f lu e n c e o f t h e t h i c k n e s s o f s i l t l a y e r a tPz = 5 000 m

p i l la r f o r c e F s (MN)t h i c k n e s s(m ) a x i s@ @

5.0 3.7 1.7 1.7 3.73.0 3.9 1.5 1.5 3.91 .0 5 .2 0 .2 0 .2 5 .2

Fi g. 16. Red istribution of foundation pressures in response to surface curvature (trough configuration)



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5 7 4 D i e t m a r P l a c z e k

f

3 4 . 6 0 m

c y l i n d r i c a l c o n t a i n e r

2 9 . 9 0 m

. 2 4 . 0 0 mZ DDD[]2 :

I k - - Z . : - - - - - ~ Z 2 . . . . . . . 2 n r a v e l s a n d . 2 Z i . . . . . . Z 2 z - - -Fig. 17. Subsidence damage preventionfor an ammonia container

T h e r e a c t i o n f o r c e s d e v e l o p i n g d u e t o s u b s i d e n c e i n st if f s t ru c t u r es c a n a l s o b e r e d u c e db y s e p a r a t i o n jo i n t s. T h e o p e n i n g o f th e j o i n t s h o u l d b e o f s u c h m a g n i t u d e t h a t i n c o n c a v ec u r v a t u r e s ( t r o u g h ) t h e re s h o u l d b e n o t r a n s m i s s i o n o f f o r c e s a c r o s s t h e j o i n t. I n c o n v e xc u r v a t u r e ( s a d d le ) p r o v i s i o n s h o u l d b e m a d e f o r a su f f ic i e n t ly b r o a d c l a d d i n g o f th e j o in t .

h 2S ~ d r l l ~ n n c i t i n n : r ~ n A I : =~ _

t r o u g h p o s i t i o n : r e q . A I = a h 2 !

. . . . . . . . . : i i Fig. 18. Minimum width ofseparation joints to accommodatethe influence of curvature



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3.9 Foundations in mining regions 57 5T h e r e l e v a n t w i d t h o f a j o i n t , d e p e n d e n t o n t h e c u r v a t u r e , c a n b e e a s i l y c a l c u l a t e d f r o mFig. 18. I t i s :

A I : a . h / p zwi t h A1 b e i n g t h e r e q u i r e d w i d t h o f th e j o i n t .A f u r th e r c o m p o n e n t o f th e r eq u i r ed j o i n t w i d t h s t e m s f r o m l e n g th c h a n g e s ( s tr a in ) w h i c ha r e c o n s i d e r e d b e l o w .

4 . 6 P r o v i s i o n s f o r e x t e n s i o n a l s t r a i nE x t e n s i o n a l s t r a in o f th e s u r f a c e l e ad s t o r e la t i v e d i s p l a c e m e n t s b e t w e e n t h e s o il a n d t h ef o u n d a t i o n s l a b w h i c h , i n t u r n , g e n e r a t e s h e a r f o r c e s a c t i n g i n t h e h o r i z o n t a l d i r e c t i o n .W h e n t h e s o i l i s i n d i r e c t c o n t a c t w i t h t h e s l a b , i . e . t h e r e a r e n o i n s e r t e d p a d s o r l o w -f r i ct i o n la y e r s , t h e m a g n i t u d e o f t h e s h e a r f o r c e s is a l m o s t in d e p e n d e n t o f t h e m a g n i t u d eo f t h e r e l a t iv e d i s p l a c e m e n t s . S i g n i f i c a n t f a c to r s , h o w e v e r , a r e t h e d e a d l o a d o f t h e s t r u c -t u r e a n d t h e s h e a r r e s i s t a n c e p r o p e r t i e s o f th e s o i l. I n a s i t u a ti o n w i t h l o w f o u n d a t i o np r e s s u r e s t h e s h e a r f o r c e s t e n d t o b e h i g h e r i n c o h e s i v e s o i l s (d u e t o t h e c o h e s i o n b e i n gt h e r e l e v a n t p o r t i o n o f th e s h e a r r e s i s t a n c e ) t h a n i n n o n - c o h e s i v e s o i l s u c h a s s a n d . I n t h iss i tu a t i o n i t i s a d v i s a b l e t o r e p l a c e a n y t h i n t o p l a y e r s o f c o h e s i v e s o i l b y s a n d s o t h a t in t h ef o u n d a t i o n p l a n e t h e r e a r e o n l y f r ic t i o n a l f o r c e s . F u r t h e r m o r e , i t s h o u l d b e n o t e d t h a t atv e r y l o w v e r t i c a l l o a d ( e. g . r e t a i n i n g wa l l s a n d r a i l r o a d s t r u c t u r e s ) t h e f r i c t io n c o e f f i c i e n t~t c a n b e s i g n i f i c a n t ly h i g h e r t h a n i n n o r m a l c i r c u m s t a n c e s d u e t o th e s t r u c tu r a l i n t e r -l o c k i n g o f t h e p a r t i c l e s .F r i c t io n a l f o r c e s c a n b e s i g n i f ic a n t l y r e d u c e d b y s e p a r a t i o n j o i n t s . A s m e n t i o n e d b e f o r e ,t h e o p e n i n g o f t h e j o i n t s m u s t b e s u f f i c i e n t l y l a r g e to a c c o m m o d a t e a l l t h e i n f l u e n c e so f le n g th c h a n g e s ( b o t h in e x t e n s io n a n d c o m p r e s s i o n ) a n d h a v e a c u r v e d s u r f a ce( b o t h s a d d l e a n d t r o u g h l i k e p r o f i le s ) . T h e s e i n f l u e n c e s h a v e t o b e a s s e s s e d w i t h r e s p e c tt o ti m e , i. e . th e s e q u e n c e o f m i n i n g - i m p o s e d s u b s i d e n c e e f f e c t s h a v e t o b e i n c o r p o r a t e di n to t h e a s s e s s m e n t . A s e l e c ti o n o f c o m m o n j o i n t c o n f i g u r a t i o n s i s s h o w n i n F ig . 1 9.I n s t e e l s k e l e t o n s t r u c tu r e s t h e t r a n s f e r o f r e a c t io n f o r c e s c a n b e r e d u c e d , i f n o t c o m -p l e t e l y n e u t ra l i s ed , b y t h e u s e o f s e l f - a l i g n i n g b e a m s a s i n d i c a t e d i n F i g . 2 0 .F o r s t r ip a n d r a f t f o u n d a t i o n s , c o n s i d e r i n g t h e s h e a r s t r e n g t h p r o p e r t i e s o f th e s o il , in -v e s t i g a t i o n s n e e d t o b e m a d e i n t o h o w f a r t h e f r ic t i o n f o r c e s a c t i n g i n t h e f o u n d a t i o n b a s ec a n b e a c c o m m o d a t e d b y th e s t ru c tu r e. I f t h e y c a n n o t b e a c c o m m o d a t e d , a p p r o p r i a tem e a s u r e s a r e t o b e d e s i g n e d s u c h a s a d d it io n a l r e i n f o r c e m e n t a n d t h e i n c o r p o r a t i o n o f al o w - f r i c t i o n s l i p p l a n e i n t h e f o u n d a t i o n b a s e .D e d i c a t e d m e a s u r e s a r e n o t r e q u i r e d f o r e x t e n s i o n a l s t r a i n s o f le s s t h a n 2 % . W h e na l l o w i n g f o r t h e f u ll b e a r i n g c a p a c i t y o f t h e r e i n f o r c e m e n t b a r s ( s e e a l s o S e c t i o n 4 . 3 )i t i s n o t n e c e s s a r y t o d e s i g n a d d i t i o n a l p r o t e c t i v e m e a s u r e s a g a i n s t t h i s e x t e n s i o n .T h i s a p p r o a c h , h o w e v e r , i s n o t v a l i d i f t h e e x t e n s i o n a l s t ra i n i s i n c o n f l i c t w i t h t h ef u n c t i o n a l r e q u i r e m e n t s o f th e b u i l d i n g , i f t h e r e i s a s e n s i t i v e b u i l d i n g s t r u c tu r e w i t hr e g a r d t o e x t e n s i o n a l s t r a i n g e n e r a l l y o r , o f c o u r s e , i f t h e s t r a in e x c e e d s t h e l i m i t o f2 % . I n a l l th e s e c a s e s t h e p r o v i s i o n f o r s l i p l a y e r s to l o w e r t h e e x t e n s i o n f o r c e s i sd e s i r a b l e a n d i s i n f a c t c o m m o n p r a c t i c e . P r e v i o u s l y , g r a p h i t e o r m o l y c o t e w e r e u s e da s l o w - f r i c t i o n m a t e r i a l s .



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5 7 6 D i e t m a r P l a cz e k

1,2n,IFig . 19 . Co ns t ruct ion deta i l s of separa t ionjo in t s [6]

T o d a y ' s p r e f e r re d p r a c t i c e h o w e v e r , i s t h e u s e o f tw o p o l y e t h y l e n e s h e e t s w i t h a s i l i c o ng r e a s e i n b e t w e e n . T h e f r i c t i o n a l c o e f f i c i e n t g fo r t h is P E s a n d w i c h s h e e t d e p e n d s o n t h ef o u n d a t i o n p r e s s u r e cy a n d h a s a n o r d e r o f m a g n i t u d e a s fo l l o w s :

~ t ~ 0 . 3 5 f o r c y < 5 0 k P a~ t ~ 0 . 1 0 f o r c y > 5 0 0 k P a

, - - - ' r - - , r - - , r - - , r - - , r - - , r - " r - - ' r - - " r - - ", * ' - - ~ . . . . * - ' - - ~ . ' - - ~ ' , - - , - ~ " ~ , w , - ~ + ~ * ~

I I I I I I I I I I

_J_ J 3_ ; _J___ J , _J_, _3 _;_, _L ,A i~+ - - !4 r ~ ~- ' - - '~ !1~- -- -- -+, +4-:---i++- i+-~,--~-~: B . : ~ . - ~ : - . a . - 1 - - ~ I - . - t . i - . i ~ . z . _ i - : - . - 4 ~~ . , - ~ ' , - ~ - ' , - ~ - ' , - ~ - ' ', I I ', r l , r ~ - ' , ~ ' r i p

' _ J _ l ' _ J _ , ' _ J , ' _ J . . . . t . . . . L _ ' ' _ J : ' _ J _ ' , ' _ J _ ~ ' J ,I I I I ~ I I I I I- ; - - - - ~ i - ! + ~ : i - - ; ~ - - - ~ + - 4 ; - i - - ~ - i - - - ~ - . - - ~ , ~ i - - ~ 4 i

i i i i i i i i i: , + - - ' - . - , " ~ - ' - - - ' , - ~ - ~ - I ~ , , " . - 1 _ _ _ ' , ~ , % , i _ _ ~ . ~ :

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Fi g . 20 . Exam pl e o f a fl ex ib l e f oun-dat ion des ign . Note tha t only in i t scent re i s the ground f loor des ignedas a s t i f f f rame (af ter [6])



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3.9 Found ations in mining regions 57 7A l o w c o s t a l t e r n a t iv e t o t h e P E s a n d w i c h i s t w o l a y e r s o f u n t r e a t e d b i t u m i n o u s p a p e rw h i c h r e ac t v i s c o - e l a s t i c a l l y a t a c o m b i n a t i o n o f h i g h p r e s s u r e s ( ~ ~ 0 . 5 M P a ) a n d l o wd i s p l a c e m e n t v e l o c i t i e s in t h e s l ip p l a n e . T h e u p p e r l i m i t o f t h e s h e a r s t r es s e s t h e na m o u n t s t o "c ~ 5 0 k P a .

4 . 7 P ro v i s i o n s f o r co m p res s i v e s t ra inS h o r t e n i n g o f t h e f o u n d a t i o n p l a n e g e n e r a t e s n o t o n l y f r i c ti o n f o r c e s ( c o m p r e s s i v e ) w i t h -i n t h e f o u n d a t i o n b a s e b u t a l s o s o i l r e s i s t i n g f o r c e s a t t h e v e r t i c a l f o u n d a t i o n f a c e s( F i g . 2 1 ) . Usu a l l y , t h e r e i s n o t a fu l l m o b i l i s a t i o n o f t h e e a r t h p r e s su r e a t t h e v e r t i c a lf a c e s b e c a u s e t h e m o s t c o m m o n s o il t y p e s r eq u i re s u b s t an t ia l a m o u n t s o f r e la t iv e d i s -p l a c e m e n t s f o r t h i s m o b i l i s a t i o n w h i c h , i n f a c t , r a r e l y o c c u r s i n p r a c t i c e . M e a s u r e m e n t sr e v e a l e d t h a t f o r fu l l m o b i l i s a t i o n o f t h e e a r th p r e s s u r e t o o c c u r a d i s p l a c e m e n t o f t h es i d e w a l l is n e c e s s a r y w h i c h a m o u n t s t o a b o u t 1 / 1 0 h f o r s o f t a n d l o o s e s o i l a n d t o a b o u t1 / 5 0 . h f o r d e n s e a n d s t i f f s o il . T h e a p p r o x i m a t e f u n c t i o n a l r e l a ti o n s h i p b e t w e e n t h em o b i l i s e d e a r th p r e s s u r e a n d t h e a c tu a l d i s p l a c e m e n t i s s h o w n i n F ig . 2 2 .A m o r e a c c u r a t e e x a m i n a t i o n o f t h is r e l a t io n s h i p w a s c a r r i e d o u t b y [1 1] u s i n g a F Ec o m p u t a t io n . W h e n c o n s i d e ri n g m i n i n g - i n d u c e d s u r f a c e c o m p r e s s i o n s o f s a n d w i th i nt h e r a n g e o f 0 . 2 a n d 2 % a n d i g n o r i n g a n y f r i c t io n a l r e s i s t a n c e a t th e s id e w a l l s( 5 = 0 ) i t w a s s h o w n t h a t a c o m p r e s s i o n o f a b o u t 1 % i s r e q u i r e d f o r f u l l m o b i l i s a t i o no f t h e e a r t h p r e s s u r e .T h e m a g n i t u d e o f th e e a r th p r e s s u r e a c t i n g o n t h e s i d e w a l l s o f a f o u n d a t i o n c a n b e s i g -n i f i c a n tl y r e d u c e d b y t h e in s e r t io n o f p a d s . D e p e n d i n g o n t h e c o m p r e s s i b i l i t y o f t h e p a d s ,t h e h o r i z o n t a l d i s p l a c e m e n t s b e t w e e n t h e s o il a n d t h e l a te r a l f a c e s o f t h e s t r u c tu r e c a n b ep a r t i a l ly o r fu l l y a c c o m m o d a t e d b y t h e p a d s . S o f t c la y , b l a s t f u r n a c e s l a g , s l a g w o o l a n d ,i n p a rt ic u l a r, p e a t a r e t h e m o s t s u i t a b l e p a d m a t e r i a l s . I t s h o u l d b e n o t e d t h a t t h e ir d e g r e eo f c o m p r e s s i b i l i t y l a rg e l y d e p e n d s o n t h e q u a l i t y o f in s t a l la t i o n a n d o n t h e s t o r a g e t i m e .

c o m p r e s s i v e s t r a i n

e a r t h p r e s s u r e d u e to c o m p r e s s i v e s t ra i n/ I /e p : : ~ : e a ~ e p : e p

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e p ~ e a r t h r e s i s t a n c ee a ~ a c t i v e e a r t h p r e s s u r e~ R - - f r i c t i o n

Fig. 21. Earth resistance ep mobilisedon a vertical foundation sidewalls inresponse to com pressive strain in theground



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0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0n o r m a l i s e d w a l l d i s p l a c e m e n t s i n %

F i g . 2 2 . I n t e r r e la t i o n s h i p b e t w e e n e a r thr e s i s t a n c e f o r c e s E p a n d t h e d i s p l a c e -m e n t s

F o a m p a d s o f p o l y s t e r e n e ( e . g . tr a d e n a m e P o r e s ta ) o r o f p o l y e t h y l e n e ( e. g . E t h a f o a m )a r e s u i t a b le f o r th e f o r m a t i o n o f la r g e , h o m o g e n e o u s a n d i n s t a l la t i o n - i n s e n s i ti v e l a y e r sw i t h a h ig h c r u s h i n g p o t e n t i a l. T h e s e m a t e r i a l s a re a l s o u s e d f o r in s u l a t i o n p u r p o s e s .M e c h a n i c a l l y , t h e y a r e c h a r a c t e r i s e d b y a n i n it ia l s t r e n g t h o f v i r tu a l l y z e r o a n d av e r y l o w s t if f n e s s m o d u l u s e v e n a t l o w s t re s s l e v e l s (F i g . 2 3 ). P o l y e t h y l e n e f o a m

c o m p r e s s i v e s t r e s s [ k P a ]0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 00

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1 0 0E t h a f o a m 2 2 0I i F i g . 2 3 . C o m p r e s s i b i l i t y b e h a v i o u r o fd i f f e re n t t y p e s o f p a d m a t e r i a l s



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3.9 Foundations in mining regions 5 7 9p a d s a r e p a r t i c u l a r ly s u i t ab l e f o r r e p e a t e d l o a d c h a n g e s . D u e t o th e w i d e v a r i e t y o f t h et y p e o f m a t e r i a l a v a i l a b l e it i s a l w a y s a d v i s a b l e t o e x p e r i m e n t a l l y d e t e r m i n e t h e s t r e s s -s t ra i n c h a r a c t e r is t i c s o f t h e m a t e r i a l e m p l o y e d .I n s o m e c a s e s , a t t e n ti o n m u s t a l s o b e p a i d t o t h e t i m e - d e p e n d e n t m a t e r i a l c h a r a c t e r is t i c so f th e f o a m m a t e r i a l. A s i g n i f ic a n t d e l a y e d d e f o r m a t i o n , f o r i n s ta n c e , is ty p i c a l f o r t h eP o r e s t a m a t e r i a l .

4 .8 P r o v i si o n s f o r d i s c o n ti n u o u s g r o u n d m o v e m e n t sI f , f o r w h a t e v e r r e a s o n s , i t is n o t p o s s i b l e t o a v o i d a c o n c e n t r a t e d e x t r a c t i o n o f u n d e r -g r o u n d s e a m s ( e. g . b y m o d i f i c a t i o n o f t h e p a n e l l a y o u t , h i g h e r d e g r e e o f b a c k f i l l in g ,s e q u e n c i n g o f th e s e a m e x t r a ct io n ) d i s c o n t in u o u s g r o u n d m o v e m e n t s m u s t b e e x p e c t e di n a l m o s t a ll c i r c u m s t a n c e s . I n th i s c a s e s t a b i l is a t i o n o f th e f o u n d a t i o n s o i l i s o f te n u n -a v o i d a b l e , p a r t i c u l a r l y wh e n b u i l d i n g s , p u b l i c i n s t a l l a t i o n s a n d t r a f f i c a r t e r i e s a r e a f -f e c t e d . A s o u t l i n e d i n S e c t i o n 5 . 3 , o n e g r o u n d s t a b i l i s a t i o n o p t i o n i s b a c k f i l l i n g o ft h e f o u n d a t i o n s o i l w i t h a c e m e n t s u s p e n s i o n , i n p a r t i c u l a r u s i n g t h e l o w - s t r e n g t h v a r i -e t y " D ~ i m m e r " . A n o t h e r o p t i o n i s h i g h - p r e s s u r e i n j e c t io n o f t h e a b o v e m a t e r i a l s ( " F E P "t e c h n i q u e ). T h e m a i n o b j e c t i v e o f t h e s e a c t i v it i e s is t o r e e s t a b l i s h a s u f f i c ie n t c o m p r e s -s i v e s t r en g t h i n t h e f o u n d a t i o n g r o u n d a n d , i f p o s s i b l e , t o c r e a te a s t a te o f s l i g h t h o r i-z o n t a l p r e - s t re s s i n g i n t h e g r o u n d . I f n e c e s s a r y , f o u n d a t i o n s e t t le m e n t s o r s u b s i d e n c e c a nb e s t o p p e d o r e v e n b e r e v e r s e d ( g r o u t c o m p e n s a t i o n t e c h n i q u e ) .B e s i d e s t h e s e m e a s u r e s , t h e s t r u c tu r e m a y a l s o b e s e c u r e d b y i n s t a ll in g h y d r a u l i c j a c k sa n d / o r s p r i n g s i n it s f o u n d a t i o n . N o t e , h o w e v e r , t h a t to u s e t h e s e m e a s u r e s t h e s t r u c t u r eh a s t o b e s u f f i c ie n t l y s t i f f o r a m e n a b l e t o s t if f e n in g .I t i s a g e n e r a l r u l e th a t , i n a d d i ti o n t o th e a b o v e m e a s u r e s , d i s c o n t i n u o u s m o v e m e n t s o ft h e f o u n d a t i o n g r o u n d n e e d c o n t i n u o u s m o n i to r i n g . T h i s in c l u d e s d e f o r m a t i o n m e a s u r e -m e n t s a l o n g s u r v e y i n g l in e s a c r o ss f a u l t z o n e s, d e f o r m a t i o n m e a s u r e m e n t s o f th e s t r u c-t u re a n d t h e f o u n d a t i o n a n d l o a d m e a s u r e m e n t s a t j a c k s a n d s p r in g s .

5 P r e v e n t i v e m e a s u r e s i n a r e a s w i t h n e a r - s u r f a c e m i n e w o r k i n g s5 .1 Ty p es o f p rev en t i v e m ea su resN e a r - s u r f a c e s e a m e x t r a c t i o n c a n t r i g g e r s u r f a c e c o l l a p s e a n d s i n k h o l e s w h i c h , i n t u r n ,c a n g e n e r a t e c e n t r e h o l e a n d c a n t i l e v e r e d f o u n d a t i o n c o n f i g u r a t i o n s . A c c o r d i n g t o o b -s e r v a t i o n s s o f a r a v a i l a b l e , m o s t s u r f a c e c o l l a p s e s a n d s i n k h o l e s o c c u r a s s p o n t a n e o u se v e n t s w h i c h , a t th e s u r f a c e , a r e t y p i c a l l y c o n f i n e d t o a d i a m e t e r o f a b o u t 3 t o 6 m .H o w e v e r , in a n a c tu a l s i tu a t i o n th e e x a c t a r e a a f f e c t e d c a n n o t b e d e t e r m i n e d w i t h as u f f i c ie n t d e g r e e o f a c c u r a c y . I t i s t h e r e f o r e n e c e s s a r y t o t a k e a d e q u a t e p r e v e n t i v e m e a -s u r e s f o r th e s t r u c tu r e s l o c a t e d i n th e i m m e d i a t e v i c i n i t y o f s u c h g r o u n d c o l l a p s e s t ru c -t u r e s . E i t h e r t h e b u i l d i n g s m u s t b e s t r u c t u r a l l y r e i n f o r c e d t o c o p e w i t h c e n t r e h o l e a n dc a n t il e v e r e d f o u n d a t io n c o n f i g u r a ti o n s w i th d i m e n s i o n s w h i c h a r e c o n s i d e r e d t o b e t y -p i c a l f o r t h e m i n i n g r e g i o n , o r t h e s u b s u r f a c e g r o u n d m u s t b e r e h a b i l it a t e d t o a d e g r e et h a t f u r t h e r c o l l a p s e s f r o m n e a r - s u r f a c e m i n e w o r k i n g s a r e n o l o n g e r p o s s i b l e o r a r eh a r m l e s s t o t h e s t r u c t u r e .



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58 0 Die tmar P laczek5 .2 P r e v e n t i v e m e a s u r e s f o r s t r u c t u r e sT h e d e p t h o f t h e s e a m e x t r a c t i o n i s t h e c o n t r o l l in g f a c t o r f o r t h e t y p e o f c o n s t ru c t i o n a n dp r e v e n t i v e m e a s u r e s o f b u i ld i n g s w h i c h a re b o t h s t r u ct u ra l ly f e a s ib l e a n d e c o n o m i c a l l yv i a b l e . I f i n n e a r - s u r f a c e e x t r a c t i o n t h e f o o t w a l l o f th e m i n e d - o u t s e a m i s i n c l o s e p r o x i -m i t y t o t h e f o u n d a t i o n , a n o b v i o u s o p t i o n i s p i ll a r o r p i l e f o u n d a t i o n s w i t h t h e ir f o o t in g sp o s i t i o n e d i n t h e u n d i s t u r b e d f o o t w a l l s t r a t a . S u c h f o u n d a t i o n s a r e e c o n o m i c a l l y v i a b l eu p t o f o u n d a t i o n d e p t h s o f a b o u t 1 0 t o 2 0 m , d e p e n d i n g o n t h e s i ze o f th e b u i ld i n g . Ap o s s i b l e m o d i f i c a t i o n t o th i s f o u n d a t i o n t y p e i s th a t p a r t o f t h e f o u n d a t i o n r a f t i s i n f o r mo f a c a n t i l e v e r e d s t r u c tu r e ( F ig . 2 4 ) .I f t h e r e le v a n t m i n e d - o u t s e a m i s a t a d e p t h w h i c h i s b e y o n d t h e e c o n o m i c l i m i ts o f ap i l la r o r p i le f o u n d a t i o n , t h e s t r u c tu r e h a s t o b e e n d e s i g n e d f o r p o t e n t i a l c a n t i l e v e r e d a n dc e n t r a l h o l e f o u n d a t i o n c o n f i g u r a t i o n s . T h i s t y p e o f st r u c tu r e i s t e r m e d a " f u l l p r o v i s i o ns t r u c t u r e " . I n a d o p t i n g s u c c e s s f u l d e s i g n p r a c t i c e s f r o m l i m e s t o n e k a r s t r e g i o n s , t h ef o u n d a t i o n s i n c o l l a p s e - p r o n e m i n i n g a r e a s a r e u s u a l l y d e s i g n e d f o r a c e n t r e h o l e d i a -m e t e r o f 3 m a n d f o r a c a n t i l e v e r l e n g t h o f 6 m ( F ig . 2 5 ).I n a l m o s t a l l p r a c t i c a l c a s e s a n d w i t h o u t u n d u e e c o n o m i c c o n s t r a i n t s , t h e r e q u i r e d s t i f -f e n i n g o f t h e s t ru c t u re s c a n b e a c h i e v e d b y t h e i n c o rp o r a t io n o f a b a s e m e n t . F o r h e a v y -d u t y b u i l d i n g s t h e e n t i r e b a s e m e n t s h o u l d b e d e s i g n e d a s a s t e el c o n c r e t e s t r u c tu r e w h i c h ,t o g e t h e r w i t h i ts c e i l i n g , c o n s t i t u t e s a c o m p a c t s t r u c t u r a l u n it . Fo r l ig h t s t r u c t u r e s su c h a ss i n g l e t o d o u b l e s t o r e y d we l l i n g s i t i s u su a l l y su f f i c i e n t t o r e s t r i c t t h e s t e e l c o n c r e t es t r u c tu r e t o t h e l o w e r p a r t s o f t h e b a s e m e n t u p t o t h e si ll o f th e b a s e m e n t w i n d o w .A l l d o o r o p e n i n g s o f t he b a s e m e n t m u s t b e d e s i g n e d a s s ti f f f r a m e s a n d a l l th e f l o o rsh a v e t o b e r e i n f o r c e d w i t h a c o n t i n u o u s a n d , i f p o s s i b l e , a g r i d a r r a n g e m e n t o f th e r e -i n f o r c e m e n t b a r s .I n v i r t u a ll y a ll c a s e s i n w h i c h t h e re l e v a n t s e a m e x t r a c t i o n w a s a t a d e p t h o f b e t w e e n 1 5a n d 3 0 m , th e a b o v e - m e n t i o n e d d e s i g n c o n c e p t o f s in g l e a n d d o u b l e s t o r y d w e l l i n g s i ss u f f i c i e n t , p r o v i d e d t h a t t h e f o u n d a t i o n a n d t h e r e i n f o r c e d f l o o r s a r e s t r u c t u r a l l y i n t e r -c o n n e c t e d i n t o a s t if f f r a m e w o r k a n d t h a t a ll f lo o r s a r e a d d i t io n a l l y r e i n f o r c e d b y r i n ga n c h o r s. A n a n a l o g o u s d e s i g n c o n c e p t c a n b e e m p l o y e d f o r st a ti c a ll y d e t e rm i n e d , s tr ai n -

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F i g . 2 4 . H o l l o w b o x f o u n d a t i o n o f t h e T h y s s e n h i g h - r i s e b u i l d i n g i n t h e c i t y o f E s s e n . N o t e t h e l o c a t i o no f t h e l a r g e - d i a m e t e r p i l e s i n t h e f o o t w a l l s t r a t a o f t h e m i n e d - o u t c o a l s e a m



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3.9 Foundations in mining regionsc a n t i l e v e r c o n f i g u r a t i o n c e n t r e h o l e c o n f i g u r a t i o n

5 81

1 ~ 3 . o m I _ b _ 1 3 .O r e I I _ a _ J _ 6 . o m _J_ a II - - 7 - 7 - I I - - I - - i - " IFig. 25. Proposal for the design assum ptions of structures in cantilevered and centre hole foundationconfigurations

i n s e n s i t i v e f r a m e w o r k s . F o r s e n s i t i v e e l e m e n t s , h o w e v e r , s u c h a s c r a n e r a i l s , p r o v i s i o nf o r p o s i t i o n i n g c o r r e c t i o n h a s t o b e m a d e .P r e v e n t i v e m e a s u r e s a r e a ls o r e q u i r e d i f th e r e i s a p o s s i b i l it y o f s i n k h o l e s o c c u r r i n g i n t h ef o u n d a t i o n a r e a . T h i s i s th e c a s e i f t h e fo u n d a t i o n l a y e r s a r e p r o n e t o ' q u i c k ' b e h a v i o u r , i np a r t ic u l a r, i f t h e y c o n s i s t o f w a t e r s a t u r a t e d f i n e t o m e d i u m s a n d a n d c o a r s e s il t. U n d e rt h e s e c o n d i t i o n s p i p i n g c a n o c c u r i n w h i c h t h e m a t e r i a l f r o m t h e u p p e r l a y e r s i s t r a n s -p o r t e d b y th e f l o w i n g g r o u n d w a t e r i n t o th e v o i d s o f th e c o l l a p s e d s t r a ta o f t h e d e e p e rg r o u n d . A p r e d i c t i o n o f th e l o c a l o c c u r r e n c e o f p ip i n g a n d s i n k h o l e s i s q u i t e d i ff i cu l t. I t ist h e r e f o r e a g o o d s t r a t e g y f o r t h e s e g e o t e c h n i c a l c o n d i t i o n s t o c o n s i d e r a d e e p f o u n d a t i o ns o l u t i o n i n w h i c h t h e f o o t i n g s a r e p o s i t i o n e d b e l o w t h e c r i t i c a l l a y e r s a n d o n t o s t a b l eg r o u n d . I n c a s e s u c h a s o l u t i o n i s t e c h n i c a l l y d i f f i c u l t a n d u n e c o n o m i c h o w e v e r , s y s -t e m a t ic i n j ec t io n o f th e d e e p e r g r o u n d is r e c o m m e n d e d w i th t h e o b j e c t i v e o f re d u c i n gt h e p e r m e a b i l i t y o f th e l o w e r s t r a ta to a d e g r e e w h e r e p i p i n g i s n o l o n g e r p o s s i b l e . I n -j e c t i o n i s a l so u n a v o i d a b l e i f t h e r e is s u r f a c e c o l l a p s e a n d i f a s tr u c t u r e i s n o t a m e n a b l e t oi n t e r n a l s t i f f e n i n g p r o c e d u r e s .

5 . 3 S t a b i l i s a t i o n o f t h e g r o u n d b y i n j e c t i o n5 .3 .1 G e n e r a l r e m a r k sO n e o f th e m a i n e f f e c t s o f b a c k f i ll i n g a n d i n j e c ti o n o f h y d r a u l i ca l l y b o u n d c e m e n t e dm a t e r i a l s is t h e s t a b i li s a t io n o f g r o u n d w h i c h i s s t r u c t u r a ll y d i s t u r b e d b y u n d e r g r o u n dm i n i n g . T h e o b j e c t i v e i s n o t t o f i l l a l l t h e m a j o r v o i d s , a s i s u s u a l l y t h e c a s e i n d a mc o n s t r u c t i o n , i n a n a t t e m p t t o m a k e t h e u n d e r g r o u n d s u f f i c i e n t l y i m p e r m e a b l e . I nf a c t , t h e m a i n o b j e c t i v e i s t o e s t a b l i s h f i x e d c o n t a c t s b e t w e e n t h e c o l l a p s e d b l o c k s o ft h e u n s t a b l e g r o u n d . T o d e s i g n t h e i n j e c t i o n w o r k s , t h e g r o u n d h a s t o b e s p e c i f i c a l l yi n v e s t i g a t e d f o r i t s v o i d c h a r a c t e r i s t i c s . I t i s e s s e n t i a l t o g a i n d e t a i l e d k n o w l e d g e o nt h e l o c a l g e o l o g i c a l c o n d i t io n s a n d t h e d e p t h a n d o r i e n t a t io n o f th e c o a l s e a m s . F l u s hd r i ll in g w i t h c a r e f u l o b s e r v a t i o n o f th e d e g r e e o f c i r c u l a t i o n l o s s a n d t h e i n j e c t i o n q u a n -t i t i e s i s r equ i red .



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58 2 Die tmar P laczek5 . 3 .2 I n j e c t i o n m e t h o d ( s e e a l s o C h a p t e r 2 . 2 , V o l u m e 2 )T w o i n j e c t io n m e t h o d s a r e a p p l i c a b l e . T h e f i r s t m e t h o d i s to d r il l t h e i n je c t io n b o r e h o l et o i t s f i n a l d e p t h a n d t h e n c a r r y o u t t h e i n j e c t i o n i n a s t a g e d p r o c e d u r e f r o m t h e b o t t o mu p w a r d s . S e a l i n g o f t h e i n d i v i d u a l i n j e c t i o n s e c t i o n s o f t h e b o r e h o l e i s c a r ri e d o u t b ym e a n s o f a p a c k e r . A s t h e g r o u n d i s u s u a l l y i n a l o o s e s t a t e t h e r e i s a h i g h p o t e n t i a lf o r i n j e c ti o n f l u i d c i r c u l a t in g a r o u n d t h e p a c k e r s e a l s . T h i s c a n b e a m a j o r te c h n i c a lp r o b l e m , w h i c h o f t e n j u s t i f ie s t h e u s e o f t h e a l t e r n a t iv e , i . e . th e s e c o n d i n j e c ti o n m e t h -o d . I n t h is m e t h o d i n j e c ti o n i s a l s o c a r r ie d o u t i n a p h a s e d m a n n e r , h o w e v e r , t h i s i s d o n ef r o m t h e t o p t o t h e b o t t o m a n d i n c o n j u n c t i o n w i t h t h e d r il li n g . D r i ll i n g a n d i n j e c t io n a r ec a r r i e d o u t s e q u e n t i a l ly . F i r st , a s t a n d p i p e w i t h a m i n i m u m l e n g t h o f 2 . 0 m i s s e t a t t h eb o t t o m o f th e b o r e h o l e . T h e s t a n d p i p e m u s t b e in f i rm , c e m e n t - g r o u t e d c o n t a c t w i th t h er o c k a t t h e b o r e h o l e b o t t o m o v e r a l e n g t h o f a b o u t 0 . 5 t o 1 .0 m .T h e i n j e c t i o n w o r k m u s t b e c o n t i n u o u s l y m o n i t o r e d f o r al l d e p t h l e v e l s f o r t h e in j e c t io np r e s s u r e , t h e d u r a t i o n o f th e i n j e c t i o n s t a g e a n d t h e n u m b e r o f i n j e c ti o n c y c l e s . T h el e n g t h o f th e r e s p e c t i v e i n j e c te d s e c t i o n s d e p e n d s o n t h e v o i d c h a r a c t e ri s t ic s o f t h eg r o u n d , h o w e v e r , i t s h o u l d n o t e x c e e d 5 m . I f t h e re i s e v i d e n c e o f a m a j o r v o i d , e . g .b y a to t a l c i r c u l a t i o n l o s s o f t h e d r i l l in g f l u i d o r a su d d e n f a l l o f t h e d r i l li n g r o d s , d r i l l in gh a s t o b e s t o p p e d i m m e d i a t e l y . T h e d r i l l e d s e c t i o n t h e n i s t o b e b a c k f i l l e d a n d i n je c t e d a sa n a d d i t i o n a l i n j e c t i o n s t e p .T h e s p a c i n g o f t h e in j e c t i o n b o r e h o l e s d e p e n d s o n t h e d i m e n s i o n s o f th e s t ru c t u r e a n da l s o o n th e t y p e o f th e a n t i c i p a t e d p r e v e n t i o n m e a s u r e . W h e n t h e g r o u n d i s s ta b i l is e d i n ap i l l a r- l ik e f a s h i o n f o r t h e s u p p o r t o f s t r ip a n d r a f t f o u n d a t i o n s , t h e s p a c i n g i s c o n t ro l l e db y t h e s t i f fn e s s o f t h e s u p e r s t r u c t u re , i n p a r t ic u l a r i ts s t r u c tu r a l c o n f o r m i t y w i t h t h e p o i n tl o a d f o u n d a t i o n s u p p o r t s . I n g e o - e n g i n e e r i n g p r a c t i c e , a s p a c i n g o f a b o u t 5 to 7 m i sc o m m o n . A l a r g e r s p a c i n g m a y b e p o s s i b l e f o r t h e s u p p o r t o f f r a m e w o r k s t ru c tu r es .F o r i n j e c ti o n s o v e r a l a r g e r a r e a , th e s p a c i n g o f th e i n j e c ti o n b o r e h o l e s h o u l d n o t e x c e e d5 m . T h i s a p p l i e s t o b o t h d e e p i n j e c t i o n f o r t h e t r e a t m e n t o f t h e c o l l a p s e d s t r a t a a s w e l l a sf o r n e a r - s u r f a c e i n j e c t io n f o r th e p r e v e n t i o n o f s i n k h o l e s i n p i p i n g - s e n s i t i v e s i tu a t io n s .O n e a n d t h e s a m e d r il l ri g c a n b e u s e d f o r p l a c i n g t h e s t e e l p i p e s , d r il li n g t h e i n j e c t i o nh o l e s a n d f o r re - d r i ll i n g o f h o l e s e c t i o n s w h i c h a r e c e m e n t - f i l l e d f r o m p r e v i o u s i n j e c ti o ns t a g e s . R o t a t i o n a l d r i l l i n g w i t h a f u l l - h o l e b i t a n d f l u i d f l u sh i n g i s a d v a n t a g e o u s i n t h i sr e g a r d .T h e i n j e c t i o n g r o u t m u s t b e c o n t i n u o u s l y m i x e d i n a c o n t a i n e r e q u i p p e d w i t h a s t i r r i n gt o o l to a v o i d a n y s e d i m e n t a t io n p r i o r t o in j ec ti o n . T h e g r o u t is c o n v e y e d b y m e a n s o fp u m p s f r o m t h e m i x e r v i a t u b e s a n d h o s e s t o th e i n j ec t io n p o i n t. T h e p u m p i n g p r e s s u r e sa n d t h e i n j e c t e d q u a n t i t i e s m u s t a l s o b e c o n t i n u o u s l y m o n i t o r e d , e . g . b y a p r e s s u r e -v o l u m e c h a r t r e c o r d e r .5 . 3 .3 I n j e c t i o n m a t e r i a lT h e i n j e c te d m a t e r i a l is u s u a l l y a c e m e n t - w a t e r m i x w h i c h m a y c o n t a i n a d d i t i v e s s u c h a ss a n d , b e n t o n i t e o r a n a g e n t t o a c c e l e r a t e s e t t i n g . E a c h b a t c h h a s t o b e p r e p a r e d f o r t h ep a r t i c u l a r v o i d c h a r a c t e r i s ti c s o f t h e g r o u n d d i s c l o s e d b y t h e d r il li n g . I f d ri ll in g r e v e a l e dt h e a b se n c e o f a n y s i z e a b l e v o i d s i t i s a d v i s a b l e t o s t a r t i n j e c t i o n w i t h a t h i n f l u i d . I n t h ec a s e w h e r e a p r e - s p e c i f i e d q u a n t i t y m a y b e e x c e e d e d , a th i c k e r f lu i d s h o u l d b e i n j e c te d i na s e c o n d p h a s e ( T a b l e 1 ). I f t h e d r i ll i n g r e v e a l e d l a r g e r v o i d s , i t i s a d v i s a b l e t o c a r r y o u tb a c k - f i l l i n g f i r s t b y f l u s h i n g s a n d o r g r a v e l p a r t i c l e s i n t o t h e v o i d s p r i o r t o t h e c o m -m e n c e m e n t o f t h e in j e c ti o n w o r k p r o p e r.



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3.9 Foundat ions in min ing reg ionsTa ble 1. Com position o f a rout mix depending on the injected quantity

Injected quantityO - 1000 1

100 0- 2000 12 0 0 0 - 4 0 0 0 14 0 0 0 - 5 0 0 0 1

M ix

III I IIV

CompositionCementCement (+ 1-2% bentonite)Cemen t + 60 -80% sand+ 3-4% bentoni teCement + 200% sand+ 4 -5 % bentonite

5 8 3

w/c-ratio1.0 . . . 1.20.5 . . . 0 .80 . 8 . . . 1 . 01.0 . . . 1.21.4 . . . 2.0

w/c-ratio ~ water-/cement ratio

5 . 3 .4 I n j e c t io n p r o c e d u r eT h e i n je c t i o n w o r k m u s t b e c a r ri e d o u t c o n t in u o u s l y t o a v o i d a n y d i s t u rb a n c e s c a u s e d b ys e d i m e n t a t io n w i t h in t h e g r o u t m ix . T h e c o m p o s i t i o n o f t he m i x s h o u l d d e p e n d o n t h eq u a n t i ti e s i n j e c t e d i n th e p r e c e d i n g s t a ge s . T o a v o i d u n c o n t r o l l e d i n j e c t i o n a l o n g w a yf r o m t h e p o i n t o f a p p l i c a t io n , i t i s g o o d p r a c t i c e t o s p e c i f y a n u p p e r l i m i t o f t h e i n j e c ti o nq u a n t i t y f o r e a c h i n j e c t io n s ta g e . I f i t i s n o t p o s s i b l e t o a c h i e v e s o m e d e g r e e o f c l o s u r e o ft h e v o i d s w i t h i n t h e fi r st i n j e c t i o n s t a g e , a s i n d i c a t e d b y a l a c k o f a n y i n j e c t i o n p r e s s u r eb u i l t - u p , i t i s n e c e s s a r y t o l e t t h e g r o u t m i x s e t i n t h e b o r e h o l e , r e a m i t o u t a f t e r s e t t i n ga n d r e p e a t t h e e a r l i e r i n j e c t i o n s t a g e .F o r s e c o n d a r y i n j e c t i o n a s y s t e m a t i c g r a d i n g o f t h e g r o u t m i x i s n o l o n g e r n e c e s s a r y . Ar e l a t iv e l y t h i c k g r o u t c a n b e u s e d f o r th is p u r p o s e . A s b e f o r e , a n u p p e r l i m i t o f t h e g r o u tv o l u m e s h o u l d b e s p e c i f i e d a n d a p r o p e r s t a b i li s a ti o n o f t h e v o i d s c a r r ie d o u t s u p p o r t e db y a f u r t h e r a u x i l i a r y i n j e c t i o n . I f a f te r t h e t h ir d i n j e c t i o n p h a s e t h e r e i s s ti ll n o s i g n o f ap r e s s u r e b u i l t - u p , i t i s a d v i s a b l e t o d r il l a n e x t r a i n j e c t i o n b o r e h o l e a t a d i s t a n c e o f a b o u t 1t o 2 m a w a y .T h e i n j e c t i o n p r e s s u r e m u s t b e h i g h e n o u g h t o o v e r c o m e t h e f r i c t i o n l o s s e s i n t h e d e -l i v e r y s y s t e m . O n t h e o t h e r h a n d , i t s h o u l d n o t b e s o h i g h t h

3.9 Foundations in Mining Regions

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