Rapid Permeability Testing Pressurisation Method

8/13/2019 Rapid Permeability Testing Pressurisation Method

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doi:10.1144/GSL.ENG.1986.003.01.311986; v. 3; p. 263-272Geological Society, London, Engineering Geology Special Publications

J.M.A. Pontin and M.A. French

Rapid permeability testing by the pressurisation methodGeological Society, London, Engineering Geology Special Publications

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1986 Geological Society of London
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Ground water in Engineering Geology Lond on 1986, pp. 263-272 Printed in England

apid permeability testing by the pressurisation methodJ.M.A. Pontin M.A. French

ABSTRACT: Design and use of equipment developed to determine permeability and compressibility ofsaturated ground is described. The basis of the method is to produce compressible flow of groundwaterby pressurising special boreholes with sealed screened zones.The development of the principles and theory of the method from existing techniques is described.Analysis of data is illustrated and the advantages of the method discussed.

IntroductionAs part of a major site investigation the hydraulic andgeotechnical characteristics of a series of laminatedclays and silts were to be examined. The site was inan inter-tidal area, and the restrictions on access,combined with a fluctuating response of porepressures to the tide, demanded a rapid means oftesting the permeability of those soils.

Conventional falling head tests were found to beinaccurate for the reasons described below and so anew test and meth od of analysis was required in orderto achieve reliable results. This test procedure wasused at the site in July 1982, November 1984 andMarch 1985.

FIG 1 Test site with equipment se t upFrom CRWPS, J.C., BELL, F.G. CULSHAW, M.G., (eds) , 1986, Groundwater in Engineering Geology Geological SocietyEngineering Geology Special Publication No. 3, pp. 263-272


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64 J .M.A . PO N TI N M. A . FR EN C H

heo ry o f perm eab i l it y tes tsin p ressur ised boreho lesncom pressible soil theory

T h e c o n v e n t i o n a l v a r i a b l e h e a d t e s t u s e s t h e t h e o r y ,d u e t o H v o r s l e v ( 1 9 5 1 ) f o r a n i n c o mp r e s s i b l e r i g i ds o il . By c o n s e r v a t i o n , t h e v o l u m e o f w a t e r e n t e r i n gthe so i l in t ime A t i s e q u a l t o t h e c h a n g e i n v o l u me o ft h e w a t e r c o l u mn i n t h e b o r e h o l e . T h i s r a t e o f f l o w ata n y t i me i s a s s u me d t o b e p r o p o r t i o n a l t o t h e p r o d u c to f t h e h e a d d i f f e r e n t i a l a n d t h e s o i l p e r me a b i l i t y .T h e s e t w o e q u a t i o n s a l l o w q ( s e e A p p e n d i x f o rn o t a t i o n ) t o b e e l i m i n a t e d :

q A t = A V wq = F K ( h w - h ~ ) = ( F K / y ~ , ) ( P w - P ~ ) . (1)

I f t h e c h a n g e i n v o l u m e i s e x p r e s s e d i n te r ms o f h ~t h e n t h e d i f f e r e n t i a l e q u a t i o n c a n b e i n t e g r a t e ddi rec t ly .I n a b o r e h o l e o p e n t o t h e a t mo s p h e r e t h i s v o l u mei s t h a t c h a n g e i n t h e w a t e r c o l u mn p r o d u c e d d u r i n g

A t :z l V w = 7 t r r = ( r r~JT ,)Ap,~ . (2 )

T h e c h a n g e i n h e a d , o r p r e s s u r e , t o w a r d t h ee q u i l i b r i u m s t a t e a t ' i n f i n i t e ' t i me c a n t h e n b ee x p r e s s e d b yH H , , = ( h ~ - h ~ ) ( h , , - h ~ ) = e x p - ( t - t o ) i T (3)

T h e f o r m f a c t o r i s r e l a t e d i n a c o mp l e x ma n n e r t o t h ed i me n s i o n s o f t h e b o r e h o l e a n d b o u n d a r i e s o f t h e s oi li n t o w h i c h i t i s i n s e r t e d . ( s e e Br i ti s h S t a n d a r d Co d e5930: 1981 , fo r example ) .F r o m E q u a t i o n ( 3 ) t h e h e a d r a t i o is r e d u c e d t oabo ut 37 of the in it ia l va lue a f te r a t ime equa l to Tf r o m t h e s t a r t o f t h e f a l l i n h e a d , t,,. The va lue of T i sp r o p o r t i o n a l t o t h e s q u a r e o f t h e r a d i u s o f t h e f a ll i n gc o l u mn a n d t h e r e f o r e t h e d u r a t i o n o f t h e t e s t i sr e d u c e d c o n s i d e r a b l y b y s ma l l e r r a d ii . F o r e x a m p l e ab o r e h o l e w i t h u n i f o r m s o i l f l u s h w i t h t h e b o t t o m o ft h e c a s i n g , d i a me t e r D , h a s a b a s i c t i me l a ga p p r o x i m a t e l y e q u a l t o

T = n D Z l 4 x 2 . 7 5 D K = z t D / l l K . (8)Fo r so il pe rm eab i l i ty 10-6m s ~ and D equa l to 200mm , t y p i c a l o f t h e l a m i n a t e d c l a y s i t u a t i o n , T is e q u a lt o a b o u t 1 6 h o u r , i f t h e n o r m a l c a s i n g t e st is ma d e . I n1 h o u r t h e h e a d r a t i o w o u l d f a ll o n l y t o 9 4 o f t h ein i t ia l va lue . This i s obvious ly imprac t ica l in gene ra la n d i s t h e r e a s o n w h y t h e c o n s t a n t h e a d t e s t i sn o r ma l l y u s e d f o r s o i l s o f l o w e r p e r me a b i l i t y .F r o m E q u a t i o n ( 4 ) t h e d i a me t e r o f a s t a n d p i p ee x t e n s i o n t o t h e b o r e h o l e t h a t w o u l d p r o d u c e as p e c i f ie d h e a d r a t i o o f 0.3 7 a f t e r 1 5 mi n u t e s ( a ' r a p i dte s t ' ) a f te r the in i t ia l (Maximum) va lue i s in f ac tabo ut 25 mm wi th a pe rm eabi l i ty o f 10-6m s - l in aniso t rop ic so i l .F o r t h e l a mi n a t e d c l a y s e r i e s t h e p e r me a b i l i t y w a su n d o u b t e d l y a n i s o t r o p i c w i t h t h e h o r i z o n t a l v a l u ee x p e c t e d t o b e mo r e t h a n 1 0 t i me s t h e v er t i c al . W h e nt h i s fa c t o r i s i n t r o d u c e d t h e t i me l a g e q u a t i o n i smo d i f i e d t o :

or T = z t D / 1 1 K i nP w / P , , = ( P w - p = ) / ( P , , - p = ) = e x p - ( t - t , , ) / T w h e r e

w h e r e T i s t h e ' b a s i c t i me l a g ' d e f i n e d b yT = n r % / F KT = y ~ V / F K (4)

f o r h e a d a n d p r e s s u r e r e s p e c t i v e l y . W e h a v e i n t r o -d u c e d t w o f a c t o r s , d e f i n e d p h y s i c a l l y b yF = q / K H , ~ = y , a t / K P , ~ . (5)

V = q A t / A p ~ = q A t / y w A h ~ (6)w h i c h a r e t h e f o r m o r s h a p e f a c t o r F , t h e r a t e o f f l o wp e r u n i t h e a d d i f f e r e n c e i n t o s o i l o f u n i t p e r me a b i l i t ya n d t h e v o l u m e f a c t o r V , t h e v o l u m e o f fl o w p e r u n itc h a n g e i n p r e s s u r e i n t h e b o r e h o l e . T h e l a t t e r i si n d e p e n d e n t o f p er m e a b i l i t y a n d i s e x p r e s s ed b y

V = J t r ~ e / y ~ (7)

K m = X / K ~ K h (9)a n d t h e s t a n d p i p e d i a m e t e r n e e d e d f o r a ' r a p i d ' t e s tr e d u c e d t o l e s s t h a n 1 5 mm.A n e w a p p r o a c h w a s t h e r e f o r e d e v e l o p e d b yr e c o g n i s i n g t h a t t h e s a me e f f e c t c a n b e p r o d u c e d b ym a k i n g A V w Very sma l l in the bas ic Equa t ion (2) .T h i s c a n b e a c h i e v e d i f t h e v o l u m e e n t e r i n g t h e s o i l isd e r i v e d b y w a t e r e x p a n d i n g f r o m a f i x e d v o l u m e , V , , ,w h i ch i s p r e s s u ri s e d a b o v e t h e a m b i e n t g r o u n d w a t e rp r e s s u r e i n t h e b o r e h o l e .T h e d e f i n i t io n o f v o l u m e c o m p r e s s i b i li t y f o r w a t e r ,

cw = a v~/vOap~ lO)i s s u b s t i t u t e d i n t o t h e b a s i c E q u a t i o n s ( 1 ) w i t h t h er e s u l t t h a t t h e b a s i c t i me l a g i s n o w e x p r e s s e d b y

T = C w V w y w / F K (11)G r o u n d w a t e r i n E n g i n e e r in g G e o l o g y , L o n d o n , 1986


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R P I D P E R M E B I L I T Y T E S T I N G 65

i f t h e e x p a n s i o n a n d c o n t r a c t i o n o f t h e v o l u m ec o n t a i n i n g t h e p r e s s u r i s e d w a t e r a r e a s s u m e d t o b en e g l i g i b l e . W i t h t h e n u m e r i c a l v a l u e s a b o v e a n d ab o r e h o l e d e p t h , d , o f 2 0 m t h e 0 . 3 7 h e a d r a t i o w o u l db e a c h i e v e d a f t e r a t i m eT = z t d D C w y w / l l K r , . (12)

w i t hC~ = 4.65 x 10 1~ m 2 N -~

a n dy. , = 9920 N m -3

( f o r s e a wa te r ) , t h i s is e va lu a te d to b e 16 . 6 se c onds .T h e a d v a n t a g e o f u s i n g a p r e s s u r i s e d s y s t e m i sa p p a r e n t o n t h e b a s i s o f t h i s p r i n c i p l e a p p l i e d t o a ni n c o m p r e s s i b l e s o i l.

( b ) t he s t r a tum i s e x te ns ive ( in f in i t e ) a nd un i f o r m ;( c) t he ine r t i a o f t he wa te r c o lum n i s ne g l ig ib l e .I n the g r oundwa te r a nd o i l i ndus t r i e s t e s t s o f t h i sk ind a r e c a l l e d ' s lug t e s t s ' . I t i s we l l known tha t t hem e t h o d i s r e l i a b l e f o r f in d i n g a n e s t i m a t e o f t h e m e a nh o r i z o n t a l p e r m e a b i l i t y in t h e v i c i n it y o f t h eb o r e h o l e ; a c t u a l ly t h i s is t h e ' b o r e h o l e t r a n s m i s s i v i ty 'e q u a l t o K , I . T h e r e l i a b il i t y o f t h e e s t i m a t e o fs to r a t iv i ty a nd ve r t i c a l c om pr e ss ib i l i t y , m ~, is no t soc e r t a in a c c o r d i n g t o P a p a d o p u l o s e t a l . (1973).An e x te ns ion o f t h i s t e s t f o r t he p r e s su r i se db o r e h o l e c o n d i t i o n w a s s u g g e s t e d v e r y r e c e n t l y b yB r e d e h o e f t P a p a d o p u l o s ( 1 9 80 ) , t o b e u s e d i nt e s t ing s t r a t a o f ve r y low pe r m e a b i l i t y a t g r e a t de p th .T h e m o t i v e w a s t o r e d u c e t h e d u r a t i o n o f t h e t e s tpe r iod .W i t h t h e a s s u m p t i o n s a b o v e t h e f o r m a l s o l u t i o n f o ri n s t a n t a n e o u s p r e s s u r i s at i o n i s n o w e x p r e s s e d b y

H ~ H o = F 2 a , f l) (14)ompressible soil theory

I n s i t e i n v e s t i g a ti o n w o r k t h e a c c e p t e d m e t h o d o f i ns i t u p e r m e a b i l i t y t e s ti n g o f c o m p r e s s i b l e s t ra t a i s b yt h e c o n s t a n t h e a d m e t h o d a p p l i ed t h ro u g h h y d r a u l icp i e z o m e t e r s . T h e t h e o r y i s d u e t o G i b s o n ( 1 9 6 3 ,1966) bu t i s l im i t e d to sho r t f i l t e r s e c t ions inr e a sona b ly i so t r op ic so i l s .F o r c y l i n d r i c a l p i e z o m e t e r s , o r b o r e h o l e s w i t h h i g hle ng th to d i a m e te r r a t io , i n we l l - s t r a t i f i e d so i l s , who seh o r i z o n t a l p e r m e a b i l i t y e x c ee d s t h e v e r t i c a l b y m o r et h a n 5 , a n o t h e r t h e o r y c a n b e u s e d . T h i s i s t h ef i n i te - w e l l t h e o r y d e v e l o p e d q u i t e r e c e n t l y i ng r o u n d w a t e r h y d r a u l i c s , h i t h e r t o n e g l e c t e d i n s o i lm e c h a n i c s p ra c t ic e . T h e t h e o r y w a s d e v e l o p e d i n as e r i e s o f p a p e r s b y P a p a d o p u l o s e t a l . (1967, 1967,1973) in the US Ge o log ic a l S u r ve y .O n t h e b a s i s o f t h i s t h e o r y t h e h e a d r a t i o v a r i e sa f t e r a n i n s t a n t a n e o u s c h a n g e i n t h e w a t e r l e v e l o f af in i t e we l l , r a d ius r , ., a c c o r d ing to

H , ,~ q -l o = F I a , f l ) (13)w h e r e

a = r J y w m d / r c 2 f l = K h l t / r 2

a n d F 1 i s a n i n f i n i t e i n t e g r a l w h i c h i s t a b u l a t e d b yP a p a d o p u l o s C o o p e r ( 1 96 7 ). T h e o t h e r s y m b o l s a r ed e f i n e d i n t h e N o t a t i o n . T h e ' e f fe c t i v e ' o r v i r t u a lr a d ius o f t he we l l i s no t ne c e ssa r i ly e qua l t o thephys ic a l bo r e d r a d ius , r w , o r t he r a d ius o f c a s ingwh e r e in the f a l l o f wa te r l e ve l oc c ur s .T h e s o l u t i o n w a s d e v e l o p e d w i t h t h e f o l l o w i n ga s s u m p t i o n s :( a ) t h e f lo w f r o m t h e b o r e h o l e i s r a d i a l a n ds e n s i b l y u n i f o r m f r o m t h e f i l te r l e n g t h , I ;

w h e r e n o w

a = f ew m d / V + C w , f l = n K d t / V + C + 7 +

a n d t h e f u n c t i o n F 2 i s i de n t i c a l t o F 1 wi th the sep a r a m e t e r s .W e c a n n o t e t h a t t h e p r e s s u r i s e d s o l u t i o n i so b t a i n e d f r o m t h e o p e n b o r e h o l e f o r m b y m a k i n g th es u b s t i t u t i o n

n r s = v w c w ~ w 15)w h i c h w a s d e v e l o p e d a b o v e f o r t h e i n c o m p r e s s i b l eso i l t he o r y .T h e p e r m e a b i l i t y a n d c o m p r e s s i b i l i t y a r e f o u n df r o m a ' t y p e c u r v e ' c o m p a r i s o n w i t h m e a s u r e d d a t aw h i c h i s d e s c r i b e d i n d e t a i l b y B r e d e n h o e f tP a pa dopu los ( 1980) .H o w e v e r , i n t h e t e s t i n g o f t h e l a m i n a t e d c l a y s , t h ea p p l i c a t i o n o f E q u a t i o n ( 1 4 ) t o t e s t d a t a p r o d u c e dunr e a l i s t i c r e su l t s f o r t he se c oe f f i c i e n t s . Th e r e a sonw a s t h a t t h e m a t h e m a t i c a l a s s u m p t i o n s u s e d t o d e r i v eth i s e qua t ion we r e no t s a t i s f i e d phys i c a l ly in the t e s to p e r a t i o n .T h e m o s t i m p o r t a n t d e p a r t u r e a p p e a r e d t o b e t h ef a i l u r e t o a c h i e v e a s a t i s f a c t o r y a p p r o x i m a t i o n t oi n s t a n t a n e o u s p r e s s u r i s a t i o n , i . e . t h e p e r i o d o f r i s i n gp r e s s u r e t o t h e m a x i m u m v a lu e w a s t o o l o n g . D u r i n gt h i s p e r i o d w a t e r w a s b e i n g i n j e c t e d i n t o t h e s o i l i nr e l a t iv e l y l a rg e q u a n t i t y c o m p a r e d t o t h e z e r o v a l u ea s s u m e d b y t h e m a t h e m a t i c a l d e s c r i p t i o n o f t h ep r o c e ss . T o c o m p e n s a t e f o r t h i s e f fe c t w e h a v ed e v e l o p e d a n e w a p p r o a c h f o r a w e l l o f f i n i t ed i a m e t e r . T h i s w i ll n o w b e d e s c r i b e d , a n d c a l l e d t h e' H R m e t h o d ' .

G r o u n d w a t e r i n E n g i n e e r in g G e o l o g y , L o n d o n , 1986


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66 J .M .A . PONTIN M. A. FRENCHT h e o r y o f t h e H R m e t h o d w h e r eD u r i n g t h e i n j e c t i o n p e r i o d , to, t h e r a t e o f d i s c h a r g ei n t o t h e b o r e h o l e a n d a d j a c e n t s o i l i s k e p t c o n s t a n t .T h e h e a d i n s id e t h e b o r e h o l e i s t h e n d e s c r i b e d b y

H ~ = ( O / 4 : t K h l ) F ( u . , , a ) (16)

u w = r e / 4 c J , t = t - t oT h i s e q u a t i o n c a n b e s i mp l i f i e d i n t w o s t a g e s t op r o v i d e a v e r y c o n v e n i e n t f o r m f o r d a t a a n a l y s i s . I th a s b e e n e s t a b l i s h e d f r o m t a b u l a t i o n t h a t i f

w h e r e t > 250 r 2 / K h l = tcu~ = r~ Z / 4cv t, o t = r~ Z m J/ V wC.~ y~ t h e n

a n d c v i s t h e c o e f f i c ie n t o f c o n s o l i d a t i o n f o r h o r i z o n t a lr ad ia l f low f rom the we l l . Equa t ion (16) i s a' p r e s s u r i s e d ' f o r m o f t h e s o lu t i o n p r e s e n t e d i n ( 1 4 ) ,w i t h t h e s u b s t i t u t i o n , E q u a t i o n ( 1 5 ) . I n a d d i t i o n ,a c c o u n t h a s b e e n t a k e n o f t h e n o n - r i g id i t y o f t h eb o r e h o l e s y s t e m, s o t h a t C , , h a s b e e n r e p l a c e d b y C~ ,t h e ' s y s t e m c o m p r e s s i b i l i t y '. C~ c a n b e c a l c u l a t e d b yc o n s i d e r i n g t h e b o r e h o l e d e s i g n a n d ma t e r i a lp r o p e r t i e s .T h e r e s i d u a l h e a d d i f f e r e n c e a f t e r i n j e c ti o n h a se n d e d i s e x p r e s s e d b y s u p e r i m p o s i n g a n i ma g e w e l l i nt h e n o r m a l m a n n e r

r u~, . = W u~)w h e r e W ( u ~ ) i s the we l l (The is ) func t ion for' n o n - l e a k y ' g r o u n d w a t e r f l o w .I n t h e i n e q u a l i t y w e h a v e u s e d

' r / ' = V~C~,./Jtf o r t h e p r e s s u r i s e d v e r s i o n . A l s o , i f

H ~ = H w ( t ) - H w (t ) u , , < 0 .05 then= ( 0 / 4 ~ t K h l ) [F(uw, et) - F ( u w , a ) ] (17) W ( u w ) ~ - ) - In uw.

B o r e h o t e f i ll e d w i thw a t e r , s h u t - in a n d S t a r t p u m p i n g - i n S t o p p u m p i n g . E x c e s s h e a d d e c a y sp r e s s u r e e f t t o s h u t - in b o r e h o l ee q u a U s e - i t h 1 1a m b i e n t - - - , - - - - - ,~

F o r f > i t . p r e s s u r er i s e f o l l o w s T h e i sf u n c t i o n ~ _ . . . .T h e i s f u n c ti o n ~ 1 ~ H v ( t l e x t r a p o l a t e d

H w l t lV a r i a t io n i n h e a d [ / [ ~ ]w i t h o u t t e s t / / I ~ Ii ) a c t u a t e l e x t r a p o l a t e d l ~ m ~ I

\ ~ L l ' ' \ [ F o r t a ~ l t ' t = ' g e .\ , J ' I \ I . , , i s ~ ' , p o r t i o n a t

= _ ~ i m e1 I ~ i m , rt : O t c t oL l 1 * '

F I G . 2 . He ad variation in borehole.G r o u n d w a t e r in E n g i n e e r in g G e o l o g y , L o n d o n , 1986


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R P I D P E R M E B I L I T Y T E S T I N G 2 6 7W h e n t h e s e a p p r o x i m a t i o n s , v a l i d i f t a n d t a r e

s u f f i c i e n t ly l a r g e , a r e s u b s t i t u t e d i n t o E q u a t i o n ( 1 7 ) av e r y s i m p l e e q u a t i o n i s o b t a i n e d , v i z :H w = ( O / 4 z t K h l ) In ( t / t ) (18)

T h u s w h e n t h e r e s i d u a l p r e s s u r i s e d h e a d i s p l o t t e da g a i n s t l o g ( t / t ) t h e d a t a s h o u l d u l t i m a t e l y fa l l o n as t r a i g h t l i n e t h r o u g h t h e o r i g i n . T h e s l o p e o f t h i s l i n ep r o v i d e s t h e e s t i m a t e o f K h .

T h e c o m p r e s s i b i l i t y c a n a l s o b e f o u n d , n o t d i r e c t lyf r o m t h i s li n e , b u t i n t h e f o l l o w i n g w a y f r o m t h ep r e s s u r i s i n g p e r i o d .D u r i n g p r e s s u r i s a t i o n , f o r t > t~ ,

H w = (Q / 4 z t K h l ) W ( u w ) (19)A t t h e e n d o f t h e i n j e c t io n p e r i o d t = to t h e v a l u e o fW c a n t h e r e f o r e b e c a l c u l a t e d a n d t h e v a l u e o f t h ea r g u m e n t , u w , f o u n d f r o m t a b l e s o f t h e f u n c t i o n . T h ec o m p r e s s i b i li t y is t h e n c o m p u t e d f r o m

c v = r J ~ 4 u t ow . o

m v = K h / ) w C v

w h e r e t h e n o m i n a l w e l l r a d i u s i s u s e d . I f th e s o i lp e r m e a b i l i t y i s a l t e r e d b y c o n s t r u c t i n g t h e f i l t e rs e c t i o n t h e e f f e c t i v e r a d i u s w i l l b e e i t h e r m o r e o r l e s st h a n rw . A f i rs t a p p r o x i m a t i o n i s t o a s s u m e t h a t t h ed e g r e e o f a l t e r a t i o n is s m a l l . T h e p e r m e a b i l i t y v a l u ei s n o t d e p e n d e n t o n w h i c h r a d i u s i s u s e d .

orehole design and co nstructionT h e p o s s i b i l it y o f p e r f o r m i n g p r e s s u r i s a t i o n t e s t s i nn o r m a l b o r e h o l e c a s in g , e x a m i n i n g th e s o i l o v e r as h o r t , u n c a s e d l e n g t h a t t h e b o t t o m o f t h e h o l e , w a sc o n s i d e r e d b u t r e j e c t e d f o r t h e f o l lo w i n g re a s o n s :( a ) c l e a n i n g th e b o t t o m o f a b o r e h o l e i s an o t o r i o u s l y d i f f i c u lt o p e r a t i o n i n c l a y s t ra t a .( b ) o r d i n a r y c a s i n g j o i n t s a r e li a b l e t o l e a k

( c ) i t c o u l d n o t b e g u a r a n t e e d t h a t t h e l a m i n a t e dc l a y w o u l d n o t s q u e e z e i n t o t h e u n c a s e ds e c t i o n( d ) t h e p e r m e a b i l i t y d e t e r m i n e d f o r t h e t e s te dv o l u m e w o u l d b e i n f l u e n c e d b y v e r t i c a l f l o wf r o m t h e h o l e a n d n o t h o r i z o n t a l fl o w a l o n e .F o r t h e s e r e a s o n s , s p e c i al b o r e h o l e s w e r e d e s i g n e di n c o r p o r a t i n g w e l l - s c r e e n a n d f i lt e r s a n d , a n d u s i n gc a r e f u l d r i l li n g t e c h n i q u e s . D e t a i l s o f d i m e n s i o n s a r eg i v e n i n T a b l e 1 a n d t h e d e s i g n o f t h e p r e s s u r i s a t i o n

b o r e h o l e u s e d i n t h e N o v e m b e r 1 9 84 t e s ts i si l l u s t r a t e d i n F i g . 3 . T o d r i l l t h e h o l e , c a s i n g w a sa d v a n c e d t o t h e t o p o f t h e s t r a t a to b e t e s t e d . B o r i n gw a s t h e n c o n t i n u e d t o t h e b o t t o m o f th e h o l e b y t h er e v e r s e c i r c u l a t i o n m e t h o d u s i n g a 3 5 0 m m d r i ll b it .

1 0 - -

1 2 - -

I & - -

t S - -

I B - -

~ 2 2 - -

I 2 ~ ; - -t-

~ 2 6 - -O2 8 - -

3 0 - -

32 m

3 4 - -

we l l c as i ng / . 05 ram.c e m e n t b e n t o n i t e

~ c e r n e n l b e n t o n i t e PFAb e n t o r d t e

] H y d r o t e c s c r e e n 2 5 0 F m m e s hp l e z o m e t e r s o n d l i t t e r

3 5 0 m m . n o m i n a lt 5 0 m m .

b e n t o n i t e

H y d r o l e c s c r e e n 2 5 0 F m . m e s hr p i e z o m e t e r s ~ n d f i l t e r

- - - m ~ l - ~ - ~ - e n d o f b o r s h o i e

FIG. 3. Con st ruct ion of test boreho le .

TABLE 1. C o n s t r u c t i o n d e t a il s o f p r e s s u r is a t i o n b o r e h o l e s .Bo rehole 1982 Nov i 984Diam eter of boring (mm) 200 350Dep th of boring (mm ) 25.0 33.2Diam eter of screen (mm ) 150 150Up per Dep th to top (m) 14.0 16.7fil ter length (m) 7.25 7.2Lower Dep th to top (m) - 26.7fil ter length (m) - 5.1

W e l l s c r e e n , 1 5 0 m m H y d r o t e c s c r e e n w i t h a 2 5 0~tm f a b r i c f i l t e r , w a s i n s t a l l e d i n t h e h o l e . A s F i g . 3i l l u s t r a t e s , t h e 1 9 8 4 w e l l w a s d e s i g n e d f o r t e s t i n g 2d i f f e r e n t s tr a t a . T h e t w o s c r e e n e d s e c t i o n s w e r es u r r o u n d e d b y p i e z o m e t e r s a n d f i l t e r s w h i c h w e r ep l a c e d b y t r e m m i e p i p e a n d w i t h s o d i u mt r i p o l y p h o s p h a t e a d d e d t o t h e f l u sh w a t e r t o d i s p e r s ec l a y c a k e d t o t h e f a c e o f t h e e x p o s e d d e p o s i t s . A f t e rc o m p l e t i n g t e s t s o n t h e w h o l e l e n g t h o f t h e w e l l , i tw a s g r o u t e d u p t o c o v e r t h e l o w e r s c r e e n e d s e c t i o na n d m o r e t e s t s w e r e c a r r i e d o u t o n t h e u p p e rs c r e e n e d l e n g t h a l o n e . I n t h i s w a y , i t w a s p o s s i b l e t o

G r o u n d w a t e r i n E n g i n e e r i n g G e o l o g y , L o n d o n , 1986


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26 8 J, M. A. PONTIN M. A. FREN CHde te r m ine t he pe r m e a b i l i t y o f bo th o f t he t e s t e ds t r a t a . D ur in g t he 1982 t e s ts , on ly one s c r e e ne dsec t ion was used .

The top of the we l l s t r ing , wi th a sc rew thread , wasl e f t s t a nd ing a bou t a m e t r e a bove be d l e ve l so a s t ope r m i t w or k t o be c a r r i e d ou t a t l ow t i de .

F]o. 4. Borehole sealed by 'top hat'.

E q u i p m e n tT he e qu ip m e n t u se d i s i l l u s t r a t e d s c he m a t i c a l l y i nFigs 1,4,5,6 7.T he m os t im por t a n t c ond i t i on a s sum e d in t hea na ly s i s , w h ic h t he e qu ipm e n t w a s de s igne d to s a t i s f yis tha t the f low ra te in to the we l l dur ing thep r e s su r i s a t i on s t a ge shou ld be c ons t a n t . I n a dd i t i on i ti s c l e a r ly im por t a n t t o pe r f o r m se ve r a l t e s t s on e a c hse c t i on a nd so a m e a ns i s r e qu i r e d o f a c h i e v ingva r ious f l ow r a t e s e a c h o f w h ic h i s , ne ve r the l e s s ,c ons t a n t du r ing a t e s t .A s a c he c k on t he p r e s su r i s a t i on m e thod , p r ov i s ionw a s m a de f o r c a r r y ing ou t a c onve n t iona l f a l li ng he a dt e s t a nd a sho r t - du r a t i on pum ping t e s t a t t he s a m et ime .E le c t r i c i t y w a s p r ov ide d a t 110 V o l t s by a 2 . 5kV AM ode l 25P K ny lo r ge ne r a to r d r ive n by a P e t t e r A B 14 . 5H P d i e se l e ng ine . T h i s w a s u se d t o pow e r t hep u m p , a M o n o G H P u m p c o u p l e d to a V a r S p eva r i a b l e spe e d un i t , t he f l ow m e te r s a nd l i gh t ing f o rn igh t w or k .

F o r t he p r e s su r i s a t i on s t a ge , w a te r w a s pum pe df rom a supp ly ( two 250 I capac i ty ba r re ls ) thro ugh as t ra ine r and in- l ine f i l te r , then v ia a ' c i r cu i t boa rd ' tothe w e l l , w h ic h w a s c a ppe d by a ' t op - ha t ' a nd w a sthe r e f o r e s e a l e d .

T he c i r c u i t - boa r d w a s f ound to be a c onve n ie n t w a yo f m oun t ing va r ious p i e c e s o f e qu ipm e n t w h ic h a r er e qu i r e d f o r a c c u r a t e c on t r o l a nd m e a su r e m e n t .T he se a r e :

Pressure gauge [ ~ ..... - ?(:.art T F : _____ 3-way valve

~ _. ~ / k . . _ . _ ~ . . QI I ~ ~ ~ On-off valves ~ ~Y'T ' . .__J: i~i._ .._ to waste .. .~ ), ,. 0' Ax r'~i] ' x ' " Z Low regulator

Borehote _- J In-lin e filter [ fT f~ Honopump. .. . . ~- / [ , Flowmeter

ress?re ~ [ressurising circuitf ~ : ~--'it P u m p i n g - o u t d r c u i f . . . . .9 _ ~,l IE~_--:-::_---~! _ . .- ~ I : ~ ~ s r r a m e r$creene~ ~ IE ~ .~ .sechon - ~ ~ Water reservoir

FIG. 5. Schema tic diagram of equip men t.Groundwater in Engineering Geology Lond on 1986


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RAPID PERMEABILITY TESTING 69The borehole pressure was measured accurately bya rapid response strain-gauge pressure transducersupported by a cable at a fixed depth in the borehole.The pressure was recorded by a multi-speed penrecorder and d isplayed also in digital form.

FIG. 6. Flow regulator and f low meter.

FIG. 7. Chart recorder.

a) A Flostat Type V automatic flow regulator,which performed well in maintaining a steadyrate of water injection into the boreholeagainst the varying well pressure.b) Two Litre Metre MM flowmeters, one for thepressurising circuit and one for pumping-out.These incorporate an orifice plate whichcauses a small flow to be diverted from themainstream. This flow is measured by aPelton Wheel and so indicates the totaldischarge, which is shown on a digital display.The flow range chosen was 1-65 l/min,displayed to the nearest 0.1 l/min.c) A pressure gauge, for approximate measure-ment of borehole pressure.

These were connected by Hi-flex high pressure hose.Three-way control valves, quick release couplings andon-off valves were also used. Since the pum ped waterwas brackish, all metal parts were of stainless-steel toavoid corrosion.

est methodAccess to the site was achieved by loading equipmentinto a boat and foating out on the ebbing tide. Theboat settled out at low tide and provided a cleanworking area.The well was first flushed out by pumping waterdown to the bottom and allowing it to overflow forsome 20 minutes. In the most recent tests March1985) some development was carried out by alternatepumping and recharging of the well. This was done aday before testing commenced, in order to allow fullrecovery.

A falling head test was performed and then theseries of pressurisation tests as follows:a) Pump watcr in to borehole and allow overflowuntil air is removed. Adjust flow by varyingpump speed and flow regulator setting untildesired flow rate is achieved.b) Shut-in boreh ole and leave until pressureequalises with ambien t.c) Pump in water at a cons tant rate.d) Stop pumping and shut-in borehol e. Observehead decay.e) Repea t c) and d) 2 or 3 times once head hasdecayed to near ambient, varying the injectionrate.This was followed by a short-dur ation about 1 hour)pumping-out test where the well was pumped to thelist of the pump suction and the quasi-steady stateflow thus produc ed was measured.During the pressurisation tests, it was important toobserve certain conditions, namely the time limitsdescribed in the theory and also the avoidance ofhydraulic fracture.This latter phen omeno n was reported by Bjerrum etal. 1972) to occur in clay soils during constant headpermeability tests using small drive-in piezometers.Exper iment and analysis suggest that this will occur if

the applied excess pressure, Au exceeds the effectivehorizontal or vertical stress of the soil. In that case,cracking of the soil may occur leading to a markedincrease in local permeability.Hydraulic fracture was avoided by calculating anupper limit on the applied excess pressure and notexceeding this value.A test suite such as that described above,comprising a falling head test, 3 or 4 pressurisationtests and an approximately hour long pumping outtest was completed over one low tide period of about5 hours. The work required two operators, partly dueto the heavy equipment and party to the need forhaste because of the tide.

Grou ndwa ter in Engineering Geology Lon don 1986


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27 J . M . A . P O N T I N M . A . F R E N C H

T BLE 2. Su mm ary of pressurisation tests .B oreho le 1982 1984 bothsections 1984 uppe rsectionTes t 1 2 1 2Pressur is ingperio d (mi nutes ) 1.5 1.3 4.0 4.1Dec l in ingperio d (min utes) 21.0 17.0 12.0 10.0Max. excess headabove am bient 9 .7 9 .9 6 .30 7 .75(m)Excess head a tend of test (m) 0.17 0.17 0.35 0.09In jec t ion r a te(1/min ) 20 20 3.75 5.35

2 3

8.5 8.5 9.5

12.0 10.9 11.5

4.29 5.76 7.15

0.20 0.70 0.98

1.9 2.75 3.85

nalysis and resul tsT h e o r y p r e d i c t s t h a t v a l u e s o f H , , p l o t t e d a g a i n s t lo g( t / t ) s h o u l d u l t i m a t e l y f a l l o n a s t r a i g h t l i n e a n d F i g .8 i s a n e x a m p l e o f h o w t h i s w a s i n d e e d s o i n a l l o f t h et e s t s. I t is c l e a r h o w e v e r t h a t t h e v a l u e s o f H w n e e d e d

m

6 -

iS

~ 4

mm

_ _IoI

IIIIm 0

tm O8

m* 0o

i00

I II

9 I0 ~ , , , , , ~ 1 I I1 0

t / tFIG. 8 . Grap h of Hw against log ( t / t ) .Gro und wa ter in Engineering Geology , Lo ndo n, 1986

l

I O 0

f o r p l o t t i n g a r e n o t s i m p l y t h e s a m e a s t h e r e a d i n g , h ,p r o d u c e d b y t h e t r a n s d u c e r . T h i s is b e c a u s e t h er e f e r e n c e h e a d i n t h e w e l l is n o t c o n s t a n t d u e p a r t l yt o t h e f l u c tu a t i n g r e s p o n s e o f th e a m b i e n t p o r e w a t e rp r e s s u r e t o t h e t i d e , a n d p r i m a r i l y t o t h e e f f e c t o fh e a d d e c a y f r o m a n y p r e v i o u s t e s t s.

R e f e r r i n g t o F i g . 2 , it c a n b e s e e n t h a t t h er e f e r e n c e h e a d m e a s u r e d b e f o r e t h e t e st m u s tt h e r e f o r e b e e x t r a p o l a t e d i n o r d e r t o p r o v i d e a v a l u eo f H w .R e s u l t s w e r e c a l c u l a t e d a s d e s c r i b e d i n t h e t h e o r ya n d a r e q u o t e d h e r e f o r t h e 1 9 8 2 t e s t s a n d f o r t h eu p p e r s e c t i o n o f t h e 1 9 84 t es t s . T h e s e a r e o f i n t e r e s tb e c a u s e t h e s a m e s o i l s t r a t u m , t h e l a m i n a t e d c la y s ,w a s t e s t e d i n b o t h c a s e s a n d b e c a u s e a l a t e r m a j o rp u m p i n g - t e s t p r o v i d e s a c o m p a r i s o n f o r t h e v a l u e so b t a i n e d . R e s u l t s f o r t h e m o r e r e c e n t t e s t s h a v e n o tb e e n p u b l i s h e d .

T a b l e 3 s u m m a r i s e s t h e r e s u l t s . T h e v a l u e o b t a i n e df o r t h e c o e f f i c i e n t o f p e r m e a b i l i t y , K h i s r e m a r k a b l yc o n s i s t e n t b o t h b e t w e e n s u c c e s s i v e t e s t s a t d i f f e r e n tf l ow r a t e s o n t h e s a m e b o r e h o l e , a n d b e t w e e n t h e1 9 8 2 a n d 1 9 8 4 b o r e h o l e s .T h e s e v a l u e s , h o w e v e r , a r e a p p r o x i m a t e l y 1 /8 .5 o ft h a t d e r i v e d f r o m t h e l a t e r f u ll - s c a le p u m p i n g t e s t . I ts h o u l d b e n o t e d t h a t t h e w e l l u s e d f o r t h a t t e s t w a s o fm o r e s o p h i s t i c a t e d d e s i g n t h a n t h e p r e s s u r i s a t i o nw e l l s , a n d t h a t i t h a d b e e n e x t e n s i v e l y d e v e l o p e d .T h e v a l u e c a l c u l a t e d f o r c o m p r e s s i b i l i t y , m y , i s , a sm e n t i o n e d , s e n s i t i v e t o t h e e f f e c t i v e w e l l r a d i u s u s e di n c a l c u l a t io n s . T h u s T a b l e 3 gi v e s a r a n g e o f v a l u e sf o r m ,, , b a s e d o n t h e t w o e x t r e m e s o f u s i n g th e b o r e dr a d i u s o f t h e h o l e a n d t h e r a d i u s o f t h e w e l l s c r e e n .F o r t h i s r e a s o n i t is fe l t t h a t t h e t e s t w i l l p r e d i c tc o m p r e s s i b i l i t y t o w i t h i n a f a c t o r o f 1 0 , a n d t h i s i si n d e e d t h e e v i d e n c e o f t h e r e s u l t s in T a b l e 3 .


10/11

R A P I D P E R M E A B I L I T Y T E S T IN G 27TABLE 3. Com parison o f results fr om various test metho ds.

Boreh ole 1982 1984 upper sectionTest 1 2 l 2 3Kh pressu risation 3.0 3.0 2.7 3.1 3.5(107m/s) test

fal l ing head 2 .5test*major pumping 26test

mv pressur isat ion 0 .20-- 0 .17- 0 .02- 0 .022- 0 .025-(m2/M N) test 0.36 0.31 0.11 0.12 0.14falling hea d 0.0012test*majo r pump ing 0 .034test

*by Gibson (compressib le) method.onclusion

T h e H R p r e s s u ri s a t io n t e s t h a s b e e n u s e d t o p r e d i c tp e r m e a b i l i t y t o w i t h in a n o r d e r o f m a g n i t u d e o f t h a td e t e r m i n e d b y a l a r g e sc a l e p u m p i n g t e s t . T h i s h a sb e e n a c h i e v e d u s i n g a w e l l w h i c h w a s f a r c h e a p e rt h a n t h e p u m p i n g w e l l a n d w h i c h , s i g n if i c a n t l y , w a sn o t d e v e l o p e d . H a d t h e p r e s s u r i s a t i o n w e l l b e e nd e v e l o p e d ( a s i n t h e m o r e r e c e n t t e s t s ) i t w o u l dc l e a r l y h a v e y i e l d e d a v a l u e f o r p e r m e a b i l i t y c l o s e r t ot h a t o f t h e m a i n w e l l .C om press ib i l i ty was a l so p red ic ted to w i th in a f ac to r o f10 and i t i s f e l t tha t th i s marg in o f e r ro r may be typ ica lfo r va lues o f compress ib i l i ty measu red by th i s t es t .

T h e H R m e t h o d h a s th e a d v a n t a g e o v e r a f u ll -s c a lep u m p i n g t e s t o f s p e e d a n d l o w c o s t. I t s s u p e r i o r i t yo v e r c o n v e n t i o n a l f a l l i n g h e a d t e s t s l ie s p a r t l y i n th ec o n s t r u c t i o n o f a s p e c ia l b o r e h o l e , a l l o w i n g c l e a rd e f i n i t i o n o f t h e s e c t i o n b e i n g t e s t e d a n d r e d u c i n gp r o b l e m s o f s m e a r i n g e t c , b u t a b o v e a l l i n t h ec o n s i s t e n c y b e tw e e n t e s t m e t h o d a n d a n a l y si s .

R e s u l t s o b t a i n e d f r o m t e s t s t o d a t e h a v e p r o d u c e dr e a s o n a b l e e s t i m a t e s o f p e r m e a b i l i t y a n d c o m p r e s si -b i l i t y ( a n d h e n c e d i f f u si v i t y ). M o r e t r ia l s w o u l d l e a dt o s t il l m o r e u n d e r s t a n d i n g o f t h e r e l i a b i l it y o f t h em e t h o d .

ReferencesB ISHOP, A .W . AL-DHA HIR , Z .A . , 1970 . Somecompar isons between laboratory tes ts , in situ tes ts andfull-scale performance with special reference to so ilpermeabil i ty and coeff icient of consolidation .Proceedings o f the Conference o n In Situ Investigationsin Soils and Rocks London, 251.

BJERRUM, L. NASH, J.K .T .L. , KENNARD, R.M . GIBSON,R.E . , 1972. Hyd raulic f ractur ing in field perme abil i tytesting. Geotechnique 22,319.BRAND, E.W . PREMCHITI', J. 198 0. Shap e factors ofcylindr ical p iezometers . Geotechnique 32, 203.BRAND, E. W . PREMCHITT, J. 1982. Res pons e charac teris-t ics of cylindr ical p iezome ters . Geotechnique 32 ,203 .BREDEHOEFT, J.D . PAPADOPULOS, S.S. 198 0. A met ho dfor determining the hydraulic proper t ies of t ightformations. Water Resources Research 16(1), 223.BRITISH STANDARDS INSTITUTION 19 81 . Code o f practice fo rsite investigations BS5930.COOPER, H .H ., BREDEHOEFT, J.D . PAPADOPULOS, S.S .,1967. Response of a f in i te d iameter well to aninstantaneous charge of water . Water ResourceResearch 3, 263.DOMENICO, P.A . MIFFLIN, M .D ., 196 5. W ate r from lowpermeabil i ty sediments and land subsidence. WaterResource Research l, 563.GIBSON, R.E . , 1963. An analysis of system f lexib il i ty and i tsef fect on t ime- lag in pore water pressuremeasu remen ts . Geotechnique 13, 1.GIBSON, R.E . , 1966. A note on the constan t head tes t tomeasure so il permeabil i ty in s i tu . Geotechnique 16,256.

HVORSLEV, M.J . , 1951. Time lag and soil perm eabil i ty ingroundwater observations. Waterways ExperimentalStation US Corps of Engineers Vicksburg, Miss. BullNo 36.NEUZIL, C.E . 1982. On conducting the modif ied ' s lug ' tes t int ight formations. Water Resource Research 18(2), 439.PAPADOPULOS, S.S ., BREDEHOEFT, J.D . COOPE R, H .H .,1973. On the analysis of ' s lug tes t ' data . WaterResource Research 9, 1987.PAPADOPULOS, S.S. COOPER, H .H . 1 967. Dra wd own in awell of large d iameter , Water Resource Research 3241.J .M .A. PONTIN M.A . FRENCH Grou ndw ater Section , Hydrau lics Research Lim ited , Wall ingford ,Oxfo rdsh i re OX 10 8B A.

Gro undw ater in Engineering Geology Lon don 1986


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7 J .M.A . PO N TIN M. A . FR EN C H

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Appendix Notation= K J y , , . , r n v , c o e f f i c ie n t o f c o n s o l i d a t io n f o r h o r i z o n t a l f l o ws y s t e m c o mp r e s s i b i l it yc o mp r e s s i b i l i ty o f g r o u n d w a t e rd e p t h o f b o r e h o l e c a s i n gd e p t h o f b o r e h o l e f i lt e r , d i a me t e r o f b o r e h o l ef o r m o r s h a p e f a c t o r f o r p i e z o m e t e r sf i n it e w e l l f u n c t io n s d e f i n e d i n r e f e r e n c e s= z + p d 7 , , , , h y d r a u l i c h e a d i n w e l l o r p i e z o me t e r a t s o me o r i n f i n i t e t i me a f t e r s t a r t o f t e s te x c e s s h e a d i n w e l l o r p i e z o me t e r a b o v e a mb i e n t a t i n f i n i t e t i me ,a t t h e s t a r t o r d u r i n g e q u a l i s a t i o nc o e f f i c ie n t o f p e r me a b i l i t y , h o r i z o n t a l o r v e r t ic a ll e n g t h o f b o r e h o l e f i lt e r= A e / ( l + e ) A p , c o e f f ic i e n t o f v o l u m e c o m p r e s s i b il i typ r e s s u r e i n w e l l o r p i e z o me t e r a t s o me o r i n f i n i t e t i mee x c e s s p r e s s u r e i n w e l l o r p i e z o me t e r a b o v e a mb i e n t a t i n f i n i t et i me , a t t h e s t a r t o r d u r i n g e q u a l i s a t i o nr a t e o f fl o w f r o m w e l l o r p i e z o m e t e ri n j e c t i o n o r d i s c h a r g e r a t e f r o m w e l lr a d i u s o f b o r e h o l e c a s i n g o r s t a n d p i p er a d i u s o f b o r e h o l et i me f r o m s t a r t o f in j e c t i o n , t i me f r o m e n d o f in j e c t io nin jec t ionb a s i c t i me l a g ( H v o r s l e v )a p a r a m e t e r d e f i n e d i n t e x tv o l u m e f a c t o r o f p i e z o m e t e rv o l u m e o f w a t e r i n w el l o r p i e z o m e t e rw e l l f u n c t i o n f o r n o n - l e a k y a q u i f e r s ( T h e i s )p a r a me t e r s o f t h e f i n i t e w e l l f u n c t i o n s , d e f i n e d i n t e s tu n i t w e i g h t o f g r o u n d w a t e r

G r o u n d w a t e r i n E n g in e e r in g G e o l o g y , L o n d o n , 1986

Rapid Permeability Testing Pressurisation Method

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