Rock movements induced by the construction of the ...

Rock movements induced by the construstion of the ountain trunk sewer (stage 4) Departrnetrt of Civil Etigineerirrg, Uhiversity of Toronto, Toronto, Ont., Catrada M5S IA4

J. C. THOMPSON Can. Geotech. J. Downloaded from www.nrcresearchpress.com by Peking University on 06/06/13 For personal use only.

Department of Civil Etigitreering, University of Waterloo, Waterloo, Ont., Catrada N2L 3GI AND

Departmerrt of Civil Etlgirreering, University of Toronto, Toronto, Ont., Canada M5S IA4 Received October 11, 1978 Accepted May 9, 1979

During the extension of the Hamilton Mountain trunk sewer, instantaneous and long-term movements were monitored in the rock strata containing the excavated trench. Movements as far as 150 m from the trench axis were observed over a period of more than 1 year. Although the movements seem to be continuous and decreasing with time, their spatial distribution and dissipation are not entirely predictable, suggesting the important role of local geological features. Interpretations of the results and recommendations for other work in the region are submitted. Au cours de l'extension du projet "Hamilton Mountain trunk sewer" des mouvements instantanis et h long terme ont CtC enregistrCs dans la format~onrocheuse contenant l'excavation. Des mouvements ont CtC observCs jusqu'h 150 m de l'excavation aprks plus d'un an. Bien que ces mouvements semblent &trecontinus et dkcroissants avec le temps, leur ripartition et leur dissipation ne sont pas entikrement prkvisibles. Ceci suggkre un r6le important des structures gkologiques. L'interprCtation des rCsultats et des recommendations pour d'autres travaux dans la region sont proposees. Can. Geotech. J., 16, 651-658 (1979)

Introduction are common. Local geological conditions appear to Numerous instabilities in the near-surface rock govern the orientation and relative magnitudes of the strata at widely scattered locations across southern principal stresses, but the overall trend seems to Ontario and northern New York State have been follow the general pattern of N40°E for the orientaextensively reported (Rose 1951; Hogg 1959; Acres tion of the maximum principal horizontal stress Consulting Services Ltd. 1972, 1974; Czurda and (Roegiers 1978). Any excavation in a highly stressed rock formation Quigley 1972; Czurda et al. 1973; Jones 1973; White et al. 1973; Lee and Lo 1976; Bowen et al. 1976; must result in a significant redistribution of the in situ Quigley et al. 1978; Fakundiny et al. 1978). The stress field around and (or) under the excavation. The existence of such instabilities strongly suggests the vertical and lateral extent of the redistribution zone presence of high in situ horizontal stresses in the rock will depend on several factors. These include the strata throughout this region. This hypothesis is sup- dimensions of the excavation, the local itz situ stress ported by overcoring stress measurements (Coates conditions, the elastic and time-dependent properties 1964; Sellers 1969; Sbar and Sykes 1973; American of the rock, and the presence of joints and bedding Falls International Board 1974; Lo 1975; Morton planes (particularly those along which there is little et al. 1975; McLennan and Roegiers 1975, 1976; resistance to slippage). The lack of significant interPalmer and Lo 1976). Horizontal stresses in the locking across such planes permits the zone of stress range of 8-10 MPa in the near-surface rock strata redistribution to propagate much further back from are common with values as high as 20 MPa having an excavation than would otherwise be possible. been recorded. Values for the minor and major prin- Until the extent of the redistribution zone can be cipal stresses of the order of 5 and 8 MPa respectively predicted reliably, it is not possible to assess the 0008-3674/79/040651-08$01.00/0 @ 1979 National Research Council of Canada/Conseil national de recherches du Canada

652

CAN. GEOTECH. J. VOL. 16, 1979

Can. Geotech. J. Downloaded from www.nrcresearchpress.com by Peking University on 06/06/13 For personal use only.

/-----

!' I

.L

LINE OF SHEAR BLASTING

EXTENSION

CENTERLINE OF EXCAVATION

1-7-4

2% ALL DIMENSIONS IN METRES

@

TEST PITS (DOORSTOPPER. INCLINOMETER, IRAD GAUGE)

I I I I I

1

HAm

SITE PLAN

-:;

m LAKE ONTARIO

ESCARPMENT

INCLINOMETER STATIONS ONLY

FIG.1. General site plan and location of instrumentation.

DENSITY CONTOLIRS

20 O/o

S

potential for distress that movements may cause to a planned structure, as well as to adjacent buildings and underground utilities. PREDOMINANT POLE I ASSOCIATED PLANE During the summer of 1977, work commenced on the stage 4 section of the Hamilton Mountain trunk sewer (Roegiers and Thompson 1977). Overburden was removed and a 7.5 m wide trench was excavated by blasting to an average depth of 3.5 m in the Eramosa Member of the Lockport Formation. Due to the local continuity of the rock formations throughout this region, movements of the bedrock similar to those reported by Quigley et al. (1978) for earlier STATISTICAL ANALYSIS OF NEAR HORIZONTAL JOINTS SHOULD K T construction stages were anticipated. On each side of BE CONSIDERED AS BEING REPRESENTATIVE the trench they observed several slippages along FIG.2. Stereographic projection of joint survey. horizontal layering of approximately 15 mm in magnitude, suggesting total inward movement of up to to-hard clayey silt till. Beneath this, the bedrock is 100 mm within 5 weeks of excavation. medium-to-dark grey dolostone of the Eramosa Immediately to the north of the trunk sewer is a Member of the Middle Silurian Lockport Formation. 66 m right-of-way for a proposed freeway. To the With the exception of a somewhat more massive south is an extensive tract of land on which develop- caprock, which is about 0.5 m thick, thicknesses of ment is not anticipated before the summer of 1979. individual strata vary between 50 and 150 mm. The As a result, a field programme was designed to dolostone is interbedded with numerous shaley and monitor both the immediate and the long-term ef- bituminous partings. Gypsum coatings and infillings fects, as well as their dissipation with distance from are also evident. Bedding planes are relatively flat on the excavation. A plan of stage 4 and the location of a large scale with small-scale asperities and roughthe instrumentation are given in Fig. 1. ness of up to 10 mm. An on-site survey revealed two major sets of vertical or nearly vertical joints with smooth surfaces Site Geology and Groundwater Conditions (see Fig. 2). These joints appeared to pass without Over most of the site the overburden consists of discontinuity or offset through all of the strata ex0.1-0.4 m of topsoil developed on 3-6 m (approxi- posed by the excavation. mately 5 m along the line of measurement) of stiffDespite continuous pumping within the main ex-

I


ROEGIERS ET AL.

I

LONGITUDINAL STRAIN

(10-3)

0

I

1.O

0.5

CIRCUMFERENTIAL STRAIN

l

1.5

(10-3)

I

B-

2.0

FIG.3. Typical uniaxial test data curves

cavation trench, the phreatic surface remained at about 0.5 m above the bedrock contact as close as 15 m from the excavation periphery. In addition, after pumping the various test pits to draw the phreatic surface down to the bedrock, sevele artesian conditions were encountered as soon as the caprock was pierced. Field Programme

The immediate objective was to determine whether long-term movements were likely to exceed those allowed for in the design of the trunk sewer, and if so, to advise the owners in sufficient time for appropriate changes to be made. The second objective was to determine the time and spatial characteristics of the rock movements and their dependence on the local geology. To meet these objectives, and at the same time obtain data that could be used to develop analytical models of the stress redistribution, efforts to obtain stress and displacement data were concentrated along a transverse axis (section 4-4 in Fig. 1). At five locations inclinometers were installed from the ground surface to a depth of 3 m below the invert of the tunnel (in an effort to ensure anchorage at a depth unaffected by the trunk sewer excavation). Three other inclinometers were located in test pits excavated to bedrock on the south side of the trunk sewer. In the two pits nearest the trunk sewer, and in the main construction trench, absolute and relative stress measurements were made using South African

Doorstoppersl and Irad vibrating wire transducers.

Rock Properties Diamond drill core was taken from all holes necessary for instrumentation purposes. The core recovery and rock quality designation (RQD) were almost 1007,. However, a thin zone approximately 50mm thick, situated just below the caprock, was consistently missing. Samples from representative depths were tested following American Society for Testing and Materials (ASTM) standards and (or) International Society for Rock Mechanics (ISRM) recommended procedures. Two primary rock types were differentiated, dolostone and shaley dolostone. UniaxiaI Compression Tests Ten specimens were tested in uniaxial compression, using a servo-controlled testing frame. In order to obtain the complete stress-strain curve (i.e., including the postpeak behaviour), the circumferential strain was used as the controlling parameter. Two linear variable differential transformers (LVDT) positioned 180" apart, were used to record the longitudinal deformation. This was done to compensate for any eccentricity due to either loading conditions or specimen preparation. The longitudinal deformation and the circumferential strain were graphically 'A cell consisting of encapsulated strain gages attached to the bottom of a borehole and subsequently overcored for stressrelief purposes.

654


TABLE1. Average rock properties-uniaxial compression tests


0 SHALN WLOSTONE

Dolostone (5 tests) Uniaxial compressive strength CO(Pa) Tangent Young's modulus, Et,50 (Pa) Poisson's ratio Shaley dolostone (5 tests) Uniaxial compressive strength CO(Pa) Tangent Young's modulus, Et,50 (Pa) Poisson's ratio

WLOSTONE

300-

1.34X108 3.88X1O10 0.14 1.41 X 108 3.17X108 0.16

recorded as a function of the applied load. Tests were performed perpendicular to the bedding planes. All specimens exhibited a class I behaviour (see Fig. 3); that is, longitudinal deformation always increased in the postpeak region. All curves indicated a degree of nonlinear behaviour. A major portion of the stress-strain curve is concave upwards, suggesting the presence of a high percentage of microcracks, which close as the specimen is loaded. Whether this is an inherent property or the result of relaxation due to sampling cannot be affirmed. In conjunction with this is a low value of the tangent Young's modulus at low stress levels. In any analytical investigations, nonlinear characteristics of this nature should be given proper consideration. The rock tested could be classified as high to very high in strength (see Table 1). There were no remarkable differences in elastic properties between the dolostone and the shaley dolostone. Specimens tested did not contain obvious discontinuities. No discernible pattern of strength increase with depth could be detected.

Triaxial Tests A limited number of tests were performed using Hoek's triaxial cell to examine the effects of the lateral confining pressure. The maximum applied confining pressure was 20.7 MPa, corresponding to the maximum anticipated in situ stress field. Axial and circumferential displacements were recorded by means of rosette-type strain gages. Figure 4 summarizes the results. In all cases, failure occurred suddenly without any appreciable plastic deformation prior to attainment of peak strength. The behaviour was almost perfectly linear elastic, suggesting that the nonlinearity, arising from the closing of microcracks, will only be exhibited under very low confining pressure. Creep and Relaxation Tests Creep tests were performed, keeping the load constant and measuring the resulting deformations. Supplementing these were relaxation tests, which entailed maintaining a prescribed deformation and recording

2

z

v 8

. 3c

l00

0

DOLOSTONE 0.86-1.45

lb

5

DOWSTONE SHALEY

lb

5.42- 6.22

6.9 6.9

l"

.

209.9 170.8

io

CONFINING PRESSURE ( M P ~ )

FIG. 4. Results of triaxial tests: compressive strength (MPa) versus confining pressure (MPa).

FIG.5. Creep rate: stress versus strain rate.

the decay in load. These tests were performed under uniaxial loading conditions. Figure 5 reflects the overall trend and summarizes the creep data generated from a typical test. The equation and the straight line drawn on the figure were obtained by conventional regression analysis conducted on all data and show good correlation. Both rock types exhibited similar steady-state creep behaviour, the rate of which increased drastically as the load increased. It should be noted that no creep was detected below 50% of the average ultimate uniaxial compressive strength, and that the reported equation is applicable only within the range shown on the figure. Long-term testing would be required in order t o establish the validity of the equation at lower stress levels.

ROEGIERS ET AL.

-Can. Geotech. J. Downloaded from www.nrcresearchpress.com by Peking University on 06/06/13 For personal use only.

TOWARDS AWAY FROM EXCAVATION EXCAVATION

NUMBER OF DAYS AFTER INSTALLATION OF 48

FIG.7. Chainage data.

FIG.6. Typical displacement profiles.

Displacement Measurement As mentioned previously, the majority of eKort was devoted to the installation of displacement and stress monitoring equipment at eight locations on a transverse section 200 m from the end of the existing (stage 3) section. The data for the individual inclinometers are available elsewhere (Roegiers et al. 1978). An example of such data is given in Fig. 6. An analysis of all data yields the following observations and conclusions. (i) The compressive wave generated by each blast appears to have caused a small but measurable movement outward at the closest inclinometer locations (see curve 1 in Fig. 6). This movement is conceivably associated with the closing of vertical joints. (ii) Until the blasting face passed the line of inclinometers virtually no movement of the rock was noted. (iii) Displacements were detected in the inclinometers nearest the trench as soon as the blasting face passed them. With time and further progress of the blasting face, displacements were detected at the inclinometers progressively distant from the trench. The pattern of movement observed is consistent with the idealization of the rock strata as a composite of thin, weakly bonded plates, stressed externally in the

plane of the plates, and then notched such that several of the upper plates are cut through. The resulting stress redistribution may also qualitatively match that occurring in the rock strata. (iv) The magnitude of the inward movement (i.e., component perpendicular to excavation) generally decreases monotonically with distance back from the excavation. Seven weeks after the excavation had passed the line of measurements and all blasting had been completed, no movement had been detected in the inclinometers situated further than 80 m from the excavation. However, displacements of up to 5 mm were subsequently detected at these locations. (v) Significant movements were noted in the overburden. As anticipated, these movements exceeded those in the underlying rock. Within the overburden, the decrease in relative movement with depth detected by the inclinometers may reflect to some extent frictional interaction along the bedrock surface, which would restrict inward movement. Also, the possible variation in the degree of consolidation of the overburden with depth may have resulted in slightly different angles of internal friction, affecting the relative stiffness. However, no testing of the soil was performed in order to substantiate this speculation. (vi) Chainage measurements were made across the excavation (Fig. 7). The data acquired essentially substantiated the results obtained from the inclinometers (see Fig. 8). During the time measurements were possible, only limited deceleration of movements was noted. (vii) The expected trend of decreasing displacement with depth had to be rejected on the basis that a number of inclinometers indicated an anomalously larger movement of a deeper layer (see Fig. 9).

CAN. GEOTECH. 3. VOL. 16, 1979 AXIS OF TRENCH


\

ORIGINAL SHAPE

DEPTH BELOW BEDROCK SURFACE

\

FIG.9. Schematic diagram of displacement trends. !OO 7

FIG.8. Maximum recorded displacement as a function of depth and location.

trench. However, their directions are diametrically opposite, as expected. (ix) Although movement perpendicular to the trench decreases with distance from the excavation face, the component parallel to the trench increases. The consequence is a limited variation of the resultant movement as a function of distance (Fig. 10).

(viii) The absolute displacement pattern recorded by the inclinometers indicates no major difference between movement magnitudes north or south of the

Conclusions and Recommendations Rock Properties (i) Since the rock behaviour was definitely influ-

DISTANCE FROM

I

l

-

GRADE SURFACE OFOVERBURDEN

0.5M BELOW BEDROCK SURFACE

8M BELOW GRADE

I

SCALES

I

20mm I I DISPLACEMENT SCALE

I

20m LOCATION SCALE

FIG.10. Displacement vectors as a function of depth and location 10 weeks after blasting.

I


ROEGIER:S ET AL.

l0 km

APPROXIMATE OUTLINE OF ESCARPMENT

FIG. 11. Escarpment morphology.

enced by the level of confining pressure, upper layers of the formation should have a distinct nonlinear character. This results in a much larger displacement near the surface under lower in situ stresses than would be predicted assuming a constant stiffness (i.e., tangent Young's modulus at 50% of the ultimate strength Ettso). (ii) The class I behaviour (Wawersik 1968) reinforces the belief that the instabilities (i.e., "popups" reported in quarry floors in this region and at this site during stage 4 construction) are not rockbursts but rather buckling of the floor due to the loss of the lateral (vertical) constraint provided by the overburden. (iii) Although appreciable creep was recognized during laboratory testing, the authors believe that a model based on creep of the rock itself cannot explain the movements measured in the field. Indeed, the in situ levels of loading are far below those at which creep could be detected. However, the time constraints placed on the laboratory specimens do not necessarily prove that no creep occurs at very low loads. The authors further contend that creep along joints is the critical mechanism of displacement.

Joint Properties (i) Predominant joints at the site reflect the local geometry of the Niagara Escarpment (see Fig. 11). (ii) Movements immediately adjacent to the excavation are more strongly influenced by removal of support than are displacements at more distant locations, which appear to be more strongly controlled by an east-west joint system. Movements (i) The displacement profiles with depth did not reveal sharp discontinuities. However, an unexpected

657

increase in displacements (see Fig. 9) was evident for a particular deeper layer of shaley dolostone. This was not a local effect, as this tendency was recorded throughout most of the site. An explanation of this phenomenon cannot be offered at the present time. (ii) The lateral extent of recorded displacements is substantially larger than would be predicted by classical approaches such as continuum mechanics. This is extremely important when considering the influence of the excavation on neighbouring buried works. (iii) For the first 3 months of monitoring, the displacement rate did not seem to change appreciably. Later measurements, still under investigation, suggest a definite decrease. Continuous measurement over several years may establish values for closure relevant for future excavations, and indicate the time after which any further movements at the location of nearby buried works are unlikely to be significant. (iv) Movements did not occur simultaneously at all measuring stations. More distant instruments recorded the initiation of movement only after the elapse of a greater period of time. The data available to date are not sufficient to determine the precise mechanism(s) governing such behaviour. The authors believe that the following characteristics or a combination thereof may be influential: (a) time-dependent character of joints; (b) water movement along joints; and (c) time-dependent character of the rock. Acknowledgements The authors would like to thank the Ministry of Housing and the Director of the Ontario Geological Survey, Ministry of Natural Resources for permitting publication of the results and providing some funding for this work. They would also like to express their gratitude to Mr. R. J. Weir and Mr. S. Rao of the Ministry of Housing for allowing access to the premises, to Dr. 0 . L. White of the Ministry of Natural Resources for providing some of the instrumentation, and to Mr. T. Carmichael of Ontario Hydro Research for providing help in drilling the necessary boreholes. ACRESCONSULTING SERVICES LTD. 1972. Thorold Tunnelinvestigations to determine the cause of cracking in the structure. Report to the Ontario Department of Transportation and Communications. (Unpublished.) 1974. Sir Adam Beck Niagara generating station No. 1 11. Report to the Hydro-review of stability-phase Electric Power Commission of Ontario. (Unpublished.) AMERICAN FALLSINTERNATIONAL BOARD.1974. (a) Preservation and enhancement of the American Falls a t Niagara; (b) Appendix C-Geology and rock mechanics. Final report to the International Joint Commission.


658


BOWEN,C. F. P., HEWSON,F. I., MACDONALD, D. H., and TANNER,R. G . 1976. Rock squeeze at Thorold Tunnel. Canadian Geotechnical Journal, Vol. 13, pp. 111-126. COATES,D. F. 1964. Some cases in engineering works of residual stress effects. I11 State of stress in the earth's crust. American Elsevier, New York, NY. pp. 679-688. K., and QUIGLEY, R. M. 1972. Cracking of a concrete CZURDA, tunnel in the Meaford-Dundas Formation, Mississauga. Faculty of Engineering Science, University of Western Ontario, Soil Mechanics Research Report, Sn1-3-73. CZURDA,K., WINDER,C. G., and QUIGLEY,R . M. 1973. Sedirnentology, mineral facies and petrofabric of the Meaford-Dundas Formation (Upper Ordovician) in southern Ontario. Canadian Journal of Earth Sciences, 10, pp. 17901804. FAKUNDINY, R. H., POMEROY, R . W., PFERD,J. W., NOWAK, T. A., JR., and MEYER,J. C. 1978. Structural instability features in the vicinity of the Clarendon-Linden fault system, western New York and Lake Ontario. I11 Advances in analysis of geotechnical instabilities, SM study No. 13. University of Waterloo Press, Waterloo, Ont. HDGG, A. D . 1959. Some engineering studies of rock movement in the Niagara area. 01 Engineering geology case histories, No. 3. Geological Society of America, New York, NY. pp. 1-12. JONES,T. B. 1973. How one driller solved the Thorold Tunnel rock squeeze problem. Engineering and Contract Record, 86(9), pp. 24-26. LEE,C. F., and Lo, K. Y. 1976. Analysis of rock squeeze problems by the recoverable strain energy approach. Proceedings, 3rd Symposium on Engineering Applications of Solid Mechanics, University of Toronto, Toronto, Ont., pp. 305315. Lo, K. Y. 1975. I11 sitlr stress measurement in rock-Ontario power generating station. Report to Acres Consulting Services Ltd. (Unpublished.) MCLENNAN, J. D., and ROEGIERS, J. C. 1975. A synthesis of stress measurements in Ontario. Report to Franklin Trow Weir-Jones Associates. (Unpublished.) 1976. Stress conditions around the Niagara Gorge. Proceedings, 3rd Symposium on Engineering Applications of Solid Mechanics, University of Toronto, Toronto, Ont., pp. 281-303.

D. J. 1975. Rock MORTON,J. D., Lo, K. Y., and BELSHAW, performance considerations for shallow tunnels in bedded shales with high lateral stresses. Proceedings, 10th Canadian Rock Mechanics Symposium, Kingston, Ont., pp. 339-379. PALMER, J. H. L., and Lo, K. Y. 1976. 111situ stress measurements in some near-surface rock formations-Thorold, Ontario. Canadian Geotechnical Journal, 13, pp. 1-7. QUIGLEY, R. M., THOMPSON, C. D., and FEDORKIW, J. P. 1978. A pictorial case history of lateral rock creep in a n open cut into the Niagara Escarpment rocks at Hamilton, Ontario. Canadian Geotechnical Journal, 15, pp. 128-133. ROEGIERS, J.-C. 1978. Review of stress data in northeastern United States. Report to Acres American Incorporated. (Unpublished.) J. D., and THOMPSON, J. C . 1978. ROEGIERS, J.-C., MCLENNAN, Hamilton Mountain trunk sewer-stage 4-rock movements and stress relaxation. Report to the Ontario Ministry of Housing. (Unpublished.) ROEGIERS, J.-C., and THOMPSON, J. C. 1977. Stress relief and rock movement associated with the construction of the Hamilton Mountain trunk sewer. I11 Summary of field work, 1977, by the Geological Branch. Edited by V. G . Milne, 0. L. White, R. B. Barlow, and J. A. Robertson. Ontario Geological Survey, Miscellaneous Paper 75. ROSE,C . W. 1951. Niagara River redevelopment-rock squeeze studies. U S . Army Corps of Engineers, Buffalo District, NY, Preliminary Report. SRAR,M. L., and SYKES,L. R. 1973. Contemporary compressive stress and seismicity in eastern North America: a n example of intra-plate tectonics. Geological Society of America Bulletin, 84(6), pp. 1861-1882. SELLERS, J. B. 1969. Strain relief overcoring to measure in sitlc stresses. Report from Terrametrics to U.S. Army Corps of Engineers, Buffalo District, NY. W. R. 1968. Detailed analysis of rock failure in WAWERSIK, laboratory compression tests. Ph.D. thesis, University of Minnesota, Minneapolis, MN. J. R. 1973. WHITE,0. L., KARROW,P. F., and MACDONALD, Residual stress relief phenomena in southern Ontario. Proceedings, 9th Canadian Rock Mechanics Symposium, Montreal, P.Q., pp. 323-348.