Design and Support of Excavations Subjected to High Horizontal Stress

Chapter 19

DESIGN A N D SUPPORT OF EXCAVATIONS SUBJECTED TO H I G H HORIZONTAL STRESS

by John Vasey

Mine Engineer , O'Okiep Copper Company Limited Nababeep, South Afr ica

ABSTRACT

A s h a f t of 1690 m depth i s c u r r e n t l y being sunk t o exp lo i t a l a r g e copper su lph ide orebody. The primary crusher chamber, main pump chamber and t h e o t h e r excavat ions on t h e crusher l eve l were developed from the s h a f t du r ing t h e s ink ing ope ra t ion . The maximum p r inc ipa l s t r e s s i s h o r i z o n t a l and has a magnitude twice t h a t of t he v e r t i c a l s t r e s s . It inc reased l i n e a r l y w i th depth and on the e l eva t ion of t he crusher l e v e l , 1613 m below s u r f a c e , t he s t r e s s e s induced by mining can exceed t h e s t r e n g t h of t h e rock. The development of a workable c rushe r l e v e l l ayou t , and a c rusher chamber des ign and support system a r e desc r ibed . Excavation and suppor t of t h e workings a r e d e t a i l e d and a comparison of expected and a c t u a l r e s u l t s i s made.

INTRODUCTION

O1Okiep Copper Company ope ra t e s mines, m i l l s and a smelter i n the north-western Cape Province , South Afr ica . Since t h e s t a r t of oper- a t i o n s i n 1940, 68 000 k t ( k i l o t o n ) of o re have been mined. The su l - phide o r e r e s e r v e i s 25 527 k t of o r e wi th a copper grade of 1.88%, and t h e annual p roduc t ion r a t e i s 1 700 k t of o r e . The p l an t o con- c u r r e n t l y develop a c rushe r l e v e l o f f t h e s ink ing main production s h a f t , of t h e new Carolusberg Deep Mine, meant t h a t any de lay would adve r se ly a f f e c t t h e 1 s t J u l y 1983 product ion s t a r t da t e . Predic- t i o n s of t h e magnitude of t h e p r i m i t i v e s t r e s s e s , a t the planned e l e v a t i o n of t h e c r u s h e r l e v e l , r e in fo rced the view t h a t severe ground c o n t r o l problems could be encountered dur ing t h e excavation of t h i s l e v e l . It t h e r e f o r e became impera t ive t o produce an ove ra l l l ayou t , and a l s o des igns f o r t h e i nd iv idua l excavat ions such t h a t t he l e v e l could be mined w i t h minimum d e v i a t i o n from the sctledule.

DESIGN, SUPPORT OF EXCAVATIONS

ORE

B,C & D OREBODIES

UPPER MlNE

- - ----------- f SUB-L VENT SF

DEEP MlNE I DEEP O R E B O D I E S ~

.915 LEVEL

.I665 LEVEL

2384 LEVEL

2790 LEV (759

1 1690111 LEVEL

Fig. 1. Ver t i ca l p r o j e c t i o n showing t h e Carolusberg Upper mine and t h e Carolusberg Deep mine.

HISTORY

The Carolusberg Upper Mine, shown i n F ig . 1, came i n t o p roduc t ion i n 1961 and the A, B , C and D Orebodies were mined dur ing t h e cou r se of the next twenty years . A t t h e beginning of 1970 a diamond d r i l l - ing programme aimed a t exp lo r ing t h e p o s s i b l e con t inua t ion of t h e B Orebody was commenced. D r i l l i n g was done from 2790 l e v e l , which i s the lowest l e v e l i n t h e Upper Mine. The f i r s t i n t e r s e c t i o n o f t h e Deep Ore zone was made i n A p r i l 1971.

970 m LEVEL

1199m LEVEL

1361m LEVEL

1573111 LEVEL 1613 m LEVEL

430 STABILITY IN UNDERGROUND MINING

Access t o t h e orebody was gained from a Subve r t i c a l Shaf t of 650m de p th sunk from t h e 2790 l e v e l . To d a t e (March 1982) 10 000 m of development have been completed from t h e Subve r t i c a l Shaf t i nc lud ing a ramp system i n t e r c o n n e c t i n g t h e l e v e l s and a tramming haulage on t h e 1361 m l e v e l .

The s i n k i n g of t h e No.2 Main S h a f t , t o s e r v i c e t h e Deep Mine, was s t a r t e d i n December 1979 and t h e s h a f t bottom, 1690 m below s u r f a c e i s due t o be reached d u r i n g May 1982.

CRUSHER LEVEL LAYOUT

The b a s i s f o r t h e d e s i g n of t he Crusher Level l ayou t , i l l u s t r a t e d i n F ig . 2, was a s f o l l ows :

The Crusher Leve l was r e q u i r e d t o provide :

1. a chamber t o house a 1 .25 m by 1.50 m jawcrusher , v i b r a t o r y f e e d e r and overhead c r ane ;

2. a c c e s s f o r t h e mining of a s s o c i a t e d coa r s e and f i n e o r e b i n s ;

L 1573m LEVEL - \

COARSE \ ORE \ BIN \

~ ~ ~ 0 " ~ ~ H U I I C b L MKI

FINE ORE B IN

SECTION A-A

LOADING POCKE

Fig . 2. Genera l arrangement of t h e Crusher Leve l (1613 l e v e l ) .


3. Filter chambers for dust extraction equipment;

4. a main pump chamber and water storage dam;

5. a facility where all mine waste rock could be sized for hoisting by means of a hydraulic rock breaker;

6. a substation to house the electrical transformers and switchgear.

Geology And Rockmechanics

The Geology Department commenced the compilation of a data file for use in excavation design at the start of the development operation. The techniques of geomechanical logging of diamond drill core, mapping of drives, joint surveys, and in situ stress measurement were used and therefore a reasonable knowledge of the rockmass properties was available prior to starting design work.

The country rock in which the Crusher Level was to be mined is a gneissose quartz-feldspar-biotite granite. It has a well developed foliation generally striking N45"E and dipping 30' to 45OS.E. The gneissose granite, despite its foliation, is a competent strong rock displaying isotropic, homogeneous, elastic properties.

Measurements of the uniaxial compressive strength of the gneissose granite give an average value of 237 MPa (Megapascals).

Joints in the granite occur in parallel sets striking N20°W and dipping 72"W. The average spacing is 10 m. Individual joints within a specific main joint set can have closer spacing of the order of a few centimeters. The joints are generally tight and are often coated with a veneer of calcite up to several millimeters thick.

The primitive stress field has a horizontal major principal stress component orientated N38"W. The magnitude of the horizontal stress has been found, by in situ measurements, to have a value twice that of the vertical stress. A plot of the measured values of stress against depth below surface is shown in Fig. 3. The minor principal stress is horizontal, striking N52OE and is approximately equal to the overburden stress.

Extrapolation of the stress measurements showed that at the elevation of the Crusher Level the primitive stress field would have principal stress components with the following magnitudes;

6; , maximum principal stress 90 MPa d, , intermediate principal stress 45 MPa and 6, , minimum principal stress 45 MPa.


Disturbance of the stress field caused by the mining of an excavation would be likely to induce stresses high enough to exceed the strength of the rock. The subsequent failure of the rock adjacent to the excavation would in turn lead to the instability of the excavation.

STRESS, MPa

Fig. 3. Plot of measured vertical and horizontal stresses against depth below surface.

Mining Experience

The presence of the primitive stress field was observed as a deterioration of the northeast and southwest walls during the sinking of both the Subvertical Ventilation Shaft and the No.2 Shaft. Spit- ting and bursting of the rock near the sinking shaft bottom was observed at various times. This caused delays for sidewall support and sinking rates were reduced due to the need to line close to the shaft bottom.

In the Deep Mine development it was common to find a deterioration in the condition of a drive as its orientation changed from parallel to normal to the horizontal stress component. Development of a drive off No.2 Shaft, on the 1573 m level, only forty meters above the elevation of the Crusher Level, showed that tackled wrongly an excavation mined under the influence of this stress field could take much longer than scheduled. The drive was mined at right angles to the horizontal stress direction and the roof of the drive very quickly became unstable. Fractures developed parallel to the sidewalls from

DESIGN. SUPPORT OF EXCAVATIONS 433

the upper corners of the drive, causing large wedges to loosen in the hangingwall along the length of the drive. Development had to be stopped while repair work was done. The rest of the drive was mined successfully using a high roof profile to mimic the shape to which the drive had previously failed.

At this time, a diamond drill probe hole was completed from the 1573 m level through the position of the Crusher Chamber. The core showed that the rockmass had a rating of class I1 on the CSIR Geo- mechanics Classification (Bieniawski, 1976) and the description good.

LAYOUT DESIGN

Consideration of all the factors having a bearing upon the design of a layout for the Crusher Level, led to the production of the plan shown in Fig. 2. The principal feature of this layout is that the majority of the excavations are orientated parallel or subparallel to the direction of the maximum principal stress. This preferred direction was based on previous mining experience, and a study of the influence of the shape and orientation of excavation upon the gen- eration of induced stresses.

To avoid a detrimental superposition of stresses between the excavations, the rule of thumb which states that the separation of adjacent excavations should be twice the diameter of the first plus twice the diameter of the second, was utilised as far as possible. All drives and excavations were planned with smooth corners in order to avoid the very high stress concentrations caused by sharp corners. In those drives where there was no option but to advance normal to the direction of the maximum principal stress, it was decided to excavate the drive to the failure profile.

The joint sets in the gneissose granite, striking N20°W, are also subparallel to the longer axis of the excavations. It was realized that slabbing of the sidewalls could be a problem which would be taken care of by bolting the sidewalls where required.

CRUSHER CHAMBER DESIGN

Size

The Crusher Chamber is illustrated in Fig. 2. The first stage of the design was to make it as small as possible by reducing the orig- inal engineering size specifications to a minimgm, and hy hol:sing the dust filtering equipment and the electrical substation in separate excavations.

STABILITY IN UNDERGROUND MINING

Computer Simulation

To gain a better understanding of how the chamber would affect the primitive stress field, a two-dimensional Boundary Element program (Crouch, 1976) was obtained for use on the company computer.

Computer simulations were run of the interaction of various excavation shapes with the primitive stress field in order to optimise the shape of the Crusher Chamber. The plan dimensions and orientation of the chamber had already been fixed by consideration of access and function. leaving only the roof and wall shapes to be decided upon.

Inital work confirmed the common-sense view that a flat-roofed excavation, in this stress field, would be difficult to excavate and maintain in a stable condition. A general rockmechanics principle (Obert and Duvall, 1967) developed using the theory of elasticity, states that the ideal opening for any stress field is an ellipse with the correct axes ratio. Computer simulation utilising this principle, showed that amelioration of the effects of the primitive stress could be effected by excavating the chamber with an arched roof, based on an ellipse with a 2 to 1 axis ratio, the longer axis being aligned parallel to the direction of the maximum principal stress. A computer plot showing major and minor principal stress trajectories and magnitudes, for such an elliptical roof, is shown in Fig. 4. It was decided that the Crusher Chamber would be excavated with an arched roof despite the practical difficulties of mining such a shape.

Resolution of the problem of the sidewalls was not as straight- forward. It was felt that the chamber should also have an elliptical form in plan but due to time constraints it was decided to forgo curved walls and use the conventional shape shown in Fig. 5. This computer plot, showing the major and minor principal stress trajectories around the walls of the Crusher Chamber, indicated that there would be a problem with the walls parallel to the direction of the maximum principal stress. As noted previously, the strike direction of the joint planes is sub-parallel to these walls, and the action of the stress acting on the sidewalls would be to buckle the plates of rock formed by the joint planes into the excavation. It was estimated that the rock on the sidewalls could be effected to a depth of at least two meters.

Sequence of Excavation

Shaft sinking in No.2 Shaft was to be stopped at 1605 m level and the main access drive, shown in Fig. 6, excavated to the northeastern boundary of the Crusher Chamber Top Slice. It was planned to excavate the Top Slice completely in one operation unless ground conditions dictated otherwise. This was to be done by slashing from the access drive to the final wall positions in a southeasterly and northeasterly direction.


Permanent support of the Top S l i c e was t o be done while the shaf t was being sunk t o 1613 m l e v e l , from where a d r ive to undercut the Top S l i c e was t o be developed. To complete the excavation of the remainder of the chamber i t was planned to b l a s t a c e n t r a l s l o t , from t h e Top S l i c e t o t h e undercut d r i v e on the 1613 m l eve l and bench the wa l l s back t o t h e i r f i n a l p o s i t i o n s working from the top downwards.

Support Systems

Crusher Top S l i c e . Temporary suppor t , i n s t a l l e d during the excavation of the Top S l i c e , was t o be done wi th 1.8 rn long rockbol ts i n s t a l l e d on a c l o s e spaced p a t t e r n , a s r equ i red . The permanent support system i s i l l u s t r a t e d i n Fig . 7. and d e t a i l e d i n Table 1.

TABLE 1. D e t a i l s of Top S l i ce Permanent Support

Support Length Spacing Grout Spec i f i ca t ions

Type m m x m Untensioned Cement 1:0.37 cement/water r a t i o 6 2 x 2.5 Grouted Cable 2 4 m d i a . 6 x 711 x 7WMC

ungalvanised, grease f r e e . 406KN breaking force .

S t e e l Re- Resin 2 0 m d i a . 450 MPa breaking 3 1.1 x 1.1 i n f o r c i n g Bar Capsules s t r eng th and P l a t e

Weldmesh 50 x 50 mm, 2.5mm d i a . wire galvanised

Shotcre te 25mm minimum thickness

The design of t h e Top S l i c e permanent support system was based on t h e geomechanical information a v a i l a b l e p r io r t o excavation. It was intended t h a t the support system would be a l t e r e d t o accommodate any change i n circumstances found a f t e r excavation.

Crusher Sidewal ls . For the suppor t of the remainder of the Crusher Chamber, below the Top S l i c e , t h e r e were t o be two d i s t i n c t phases. The f i r s t , o r excavat ion phase had t o be completed as soon as pos- s i b l e t o al low s h a f t s ink ing t o recommence, and thus i n s t a l l a t i o n of temporary s idewa l l support could no t be allowed t o take up too much time. Phase two, the permanent suppor t phase could run on f o r up t o t h r e e months a s t h i s would be done while the s h a f t was being sunk to i t s f i n a l depth. There was r e a l l y no s a t i s f a c t o r y so lu t ion t o t h i s dilemma, whereby time was not a v a i l a b l e t o do a serviceable job of suppor t on the s idewa l l s a s excavat ion proceeded, and before movement had taken p lace . It was decided t h a t as a minimum 3 m long, 20 mm diameter , r e s i n grouted s t e e l r e in fo rc ing bars with p la t e s would be i n s t a l l e d on a 1.5 by 1 .5 m p a t t e r n a s benching proceeded. The permanent support of t h e s idewa l l s was t o cons i s t of double the support d e n s i t y us ing t h e same r e i n f o r c i n g ba r s and f i n a l l y , meshing and s h o t c r e t e .


EXCAVATION AND SUPPORT OF THE CRUSHER LEVEL

Crusher Chamber Top Slice

Upon reaching the 1605 m level shaft sinking was temporarily hal- ted. The access drive shown in Fig. 6 was driven from the shaft to the planned position of the Crusher Chamber Top Slice. As expected in a drive mined at right angles to the direction of the horizontal maximum principal stress, the crown of the drive failed. The failed shape of the access drive is shown in Fig. 8 which is a view from the shaft towards the Top Slice. Little time was lost excavating the access drive as the failure had been anticipated and the drive was mined with an outline as close as possible to the failure profile.

As ground conditions were good it was decided that the Top Slice could be mined out in one operation and temporarily supported with 1.8 m rock bolts. Using short rounds to cut the roof to the required elliptical profile, the excavation was opened up from the entrance to the Top Slice by advanring in a northeasterly and southwesterly direction. The completed Top Slice is illustrated in Figs. 9, 10 and 11.

The dark lines on the roof are paint lines marking the planned positions of the drill holes for the permanent supports. The access drive and its stress-induced mode of failure can be seen in Fig. 11.

Mining of the access drive and Top Slice took 9 working days, one day less than scheduled. Upon completion shaft sinking was resumed and the permanent support of the Top Slice was started.

An on-site investigation of ground conditions and rock discontin- uities in the Top Slice indicated that no alteration of the planned permanent support layouts would be necessary. Subsequently, 35 cement grouted cables of 6 m length and 250 resin grouted steel reinforcing bars of 3 m length were installed as permanent support. The Top Slice roof, to within a meter of the footwall, was meshed and shotcreted in order to protect personnel during the excavation of the lower portion of the Crusher Chamber.

Crusher 1 eve1 (1613 m level)

Shaft sinking was once more discontinued at the footwall elevation of the Crusher Level and level development from the shaft started. (Fig. 2.)

Three headings were advanced simultaneously from the shaft station; the pump chamber, crusher access and the decline. The pump chamber and dam were completed without difficulty. A view of the pump chamber from the dam can be seen in Fig. 12.

DESIGN, SUPPORT OF EXCAVATIONS 44 1

Minor amounts of sidewall scaling was evident at the entrance to the pump chamber where a set of the N20°W trending joints were encountered in the sidewall. No rock failure due to stress was experienced in the other two drives until the direction of advance was at right angles to the direction of the maximum principal stress. In the decline this occurred at its junction with the other two declines involving some loss of time while previously supported ground, made dangerous by the stress, was blasted down and the roof of the drive was resupported. In the vicinity of the Crusher Chamber the access drive became dangerous due to the height of the stress-induced failure. This is illustrated by Fig. 13 which shows the entrance to the Crusher Chamber from the south. This failure was markedly more severe and was probably attributable to concentration of the horizontal stress component due to the presence of the Top Slice, only four meters above. In contrast, only ten,meters further along the filter chamber drive, at a position midway between the two filter chambers, ground conditions were very good. This position is shown in Fig. 14, and is a view looking west from the first filter chamber towards the second filter chamber. The remainder of the filter chamber drive, including a small connection drive from the filter chamber to the waste grizzly was started. Stress-inclined failure of the small con- necting drive, mined parallel to the direction of the maximum principal stress, is shown in Figs. 15 and 16 which are views looking northwest towards the waste grizzly chamber.

Excavation of the waste grizzly chamber subsequent to the mining of the filter chamber appears to have protected it from the worst effects of the horizontal stress. A view of the waste grizzly chamber from its access decline is shown in Fig. 17.

The balance of the excavations, essentially the declines and the Substation, were completed without incident. A view down the Fine Ore Bin access decline is shown in Fig. 18, and the Substation, seen from the shaft side, is Illustrated in Fig. 19.

The 1.8 m diameter raisebored core of the Coarse Ore Bin was started adjacent to the Fine Ore Bin access drive and at a similar elevation to the top of the-Fine Ore Bin. (Fig. 2.) This bored raise illustrates well the failure mode of a vertical circular cross section excavation in the stress field. As can be seen in Fig. 20, stress induced spalling of the sides of the raise occurred at right angles to the.direction of the maximum principal stress.

Crusher Chamber

Upon completion of the permanent support work in the Top Slice, preparations were made to excavate the remainder of the Crusher Chamber. A heading was driven on the centre line of the chamber from the southeast end to the northwest end.

DESIGN, SUPPORT OF EXCAVATIONS 445

This heading undercut a series of blast holes drilled from the footwall of the Top Slice. A raise was blasted at the northwestern end of the chamber, to connect the two levels, and this was followed by the blasting of a slot along the full length of the centre line. The slot provided a free face and expansion box into which the rest of the chamber was excavated.

Commencing at the Top Slice footwall, the chamber was benched out back to the final wall position. The vertical extent of the individual benches was restricted to less than two meters, and the support of the final wall was completed before the lower bench was retreated back to its final position.

During this excavation phase the mesh and shotcrete of the Top Slice were, to a small extent, damaged by blasting. Despite this it prevented scaling of the roof and sides from becoming a safety hazard for the people working below.

Altogether the excavation of the Crusher Level took 71 working days, 1 day more than scheduled.

Fig. Nos. 21 and 22 show the Crusher Chamber shortly after excavation. In Fig. 21, a view looking towards the northwestern end, the shotcreted elliptical roof can be seen. The scaffolding for the permanent support work is in the process of being erected. A view of the southeastern wall of the chamber, photographed at the same time, is shown in Fig. 22.

Permanent sidewall support and reapir of the damaged shotcrete commenced upon completion of excavation. Inspection of the Crusher Cham- ber walls revealed that there was a failed layer of rock, varying in thickness from 0.5 m to 1.0 m. It was decided not to remove this layer but to stabilize it with rockbolts, because it was thought that if the confining effect of the failed rock was lost, by removing this layer, the newly exposed walls would commence scaling under the influence of the stress.

Difficulty was experienced installing the 3 m reinforcing bars through the failed rock layer. When a drill hole was completed and the drill steel withdrawn a slab of rock, at some point along the hole, would move down cutting off the back of the hole. This problem was solved by first drilling a short hole into which a split set was installed, and then drilling a parallel hole close by into which the 3 m reinforcing bar could be installed successfully. The permanent support work was completed by meshing and shotcreting all the walls.


Expected and Actual Results

No major problems were encountered during the excavation of the Top Slice and the Crusher Level. This was due, in part, to the layout which took into account most of the known hazards, but also to the good fortune of having a competent rock with few structural discon- tinuities.

Expected difficulties were encountered; e.g. in drives having their direction of advance at right angles to the direction of the maximum principal stress. As noted previously, more severe failure of the Crusher Chamber access drive was experienced than expected, due to the presence of the Top Slice causing a stress concentration over the drive. Better conditions than expected were found for the Waste Grizzly Chamber, which was protected from the horizontal stress by the Filter Chamber drive.

The condition of the sidewalls of the Crusher Chamber were as expected, their degree of failure by slabbing being controlled by the proximity of sets of joint planes to the wall surface. This slabbing failure can be seen in Figs. 23 and 24; which are views of the south- western and northeastern walls of the Crusher Chamber prior to starting the permanent support work. In Fig. 23, a view taken from the shaft side entrance to the Crusher Chamber looking along the northeast wall, the loose slabs have a thickness of approximately 0.75 to 1.0 m. While on the opposite wall, illustrated in Fig. 24 by a view of the wall above the southwest access. The slabbing had a thickness of up to 0.5 m. Failure of the rockmass indicated by the opening up of joint planes, extended back into the walls to a depth of 1.5 to 2 m.

CONCLUSIONS

In conclusion, the excavation of the Crusher Chamber and the Crusher Level were completed successfully, within the scheduled time, in spite of the severe horizontal stress. This was accomplished by using a layout and individual excavation designs which minimised induced stress levels and consequently reduced rockmass failure.

ACKNOWLEDGEMENT

The author acknowledges the permission of the management of O'Okiep Copper Company Limited for the publication of this paper. The assistance of O'Okiep Copper Company Limited staff in its compilation is appreciated.


REFERENCES

Bieniawski, Z.T., 1976 "Rock Mass Classification in Rock Engineering," Proceedings, Symposium on Exploration for Rock Engineering, Johannesburg, Vol. 1, pp 97 - 106.

Crouch, S.L., 1976, "~nal~sis of Stresses and Displacements Around Underground Excavations : An Application of the Displacement Discontinuity Method, "University of Minnesota Geomechanics Report, pp. 123 - 132.

Obert, L., and Duvall, W.I., 1967, "Rock Mechanics and the Design of Structures in ~ock", Wiley, New York, pp 505 - 507.

Quest ion

What technique was used to determine the virgin stress field.

Answer

The over coring technique in which strain relief in a rock core is monitored using the CSIR Triaxial Strain Cell.

Question

In addition to planning the level in terms of minimum horizontal stress forces did any planning go into control blasting and or stress relief due to blast techniques.

Answer

Control blasting was used to cut the roof of the Crusher Chamber to its arched shape. Stress relief using blasting techniques was not utilised.

Design and Support of Excavations Subjected to High Horizontal Stress

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