determination of foundation shear strength at the Oldman River Dam site in southwestern Alberta, .... have also observed bedding-plane shears at Forty Mile.
Determination of the Oldman River Dam foundation sh&r it;ength
P
M. M. DAVACHI,B. J. SINCLAIR,H. H. HARTMAIER,AND B. L. BAGGOTT Acres International Limited, Suite 500, 10201 Southport Road S W, Calgary, Alta., Canada T2 W 4x9 AND
J. E. PETERS UMA Engineering Ltd., 2540 Kensington Road NW, Calgary, Alta., Canada T2N 3S3 Received March 26, 1991
Accepted May 10, 1991 The paper describes the results of site investigation and laboratory testing and the analysis performed for the determination of foundation shear strength at the Oldman River Dam site in southwestern Alberta, Canada. Horizontally bedded claystones, siltstones, and sandstones at the site contain relatively weak bedding-plane shears that adversely affect foundation stability. Data on the bedding-plane shear characteristics were collected by mapping, borehole coring, shaft exploration, and large-diameter sampling. Shear planes of structure-wide continuity were identified. Numerous laboratory direct shear tests were done to measure in situ and residual shear strengths. The design angle of shearing resistance of selected continuous bedding-plane shears was evaluated by summing the representative residual angle of shearing resistance and components of the angle of shearing resistance due to in situ state, roughness, and thickness of the bedding-plane shears. Relatively flat dam slopes were found to be required for stability. The methods used at the Oldman River Dam should be applicable at other sites located in flat-lying mudrock sequences. Key words: Oldman River Dam, foundation shear strength, sedimentary rocks, bedding-plane shear, residual angle of shearing resistance, in situ state, roughness, thickness. L'article prCsente les rksultats des investigations sur le site et des essais de laboratoire, de m&me;huel'analyse rCalisCe pour dCterminer la resistance au cisaillement des fondations sur le site du barrage Oldman River dans le sud-ouest de l'Alberta, Canada. Des lits horizontaux de schistes argileux, de siltstones et de grks sur le site contiennent des plans de cisaillement lit& relativement faibles qui affectent dkfavorablement la stabilitC de la fondation. Des donnCes sur les caractkristiques des plans lit& de cisaillement ont CtC colligCes par cartographie, par les carottes de forage, par les puits d'exploration, et par l'ichantillonnage a grand diamktre. Des plans de cisaillement continus sur la largeur de la structure ont CtC identifiCs. De nombreux essais de cisaillement direct ont CtC faits en laboratoire pour mesurer les rCsistances au cisaillement in situ et rksiduelles. Pour le calcul, I'angle de risistance au cisaillement le long de plans litb continus de cisaillement sClectionnCs a CtC CvaluC en additionnant l'angle residue1 representatif de la rCsistance au cisaillement avec les composantes de l'angle de rCsistance au cisaillement dues 1'Ctat in situ, a la rugositk, et a l'tpaisseur des plans litCs de cisaillement. L'on a trouvC que des pentes relativement douces Ctaient requises pour assurer la stabiliti du barrage. Les mCthodes utilisCes au barrage de Oldman River devraient Ctre applicables sur d'autres sites localisCs sur des alternances de mudrock reposant a de faibles inclinaisons. Mots elks : Barrage de Oldman River, rCsistance au cisaillement de fondation, roches ~Cdirnentaires,plan lit6 de cisaillement, angle de risistance au cisaillement rbiduel, Ctat in situ, rugositC, Cpaisseur. [Traduit par la ridaction] Can. Geotech. .I.28, 698-707 (1991)
Introduction Project description The Oldman River Dam is a flow-regulation project that provides onstream storage of a dependable supply of water for multipurpose use and irrigation service area expansion in southern Alberta. Figures 1 and 2 show the location and project layout. The 76 m high d a m (Fig. 3) is a zoned earth and rockfill embankment with a central core of glacial till and shells comprising rock fill obtained from the spillway excavation and sand and gravel from the river alluvium. Internal filters and drains of processed sand and gravel control seepage through the dam and its foundation. It contains some 8.25 x lo6 m 3 of fill materials. The 25 m high spillway structure comprises a 110 m wide headworks transitioning t o a 40 m wide chute and flipbucket energy dissipator. The Oldman River D a m was diverted through two, 900 m long concrete-lined tunnels, 6.5 m in diameter, in the left riverbank. The other major project features consist of a 1.5 k m long dyke with a maxiPrinted in Canada / Imprime au Canada
m u m height of 10 m, a 1.3 k m long grout curtain u p t o 100 m deep, and 1.5 km of 3 m diameter pressure-relief tunnels in the river banks.
Purpose of paper The foundation bedrock a t the d a m site comprises nearhorizontal mudrocks and sandstones. Thin bedding-plane shears of relatively low shear strength characterize many of the mudrock strata. For the stability analyses of the main project structures, it was necessary t o identify the continuous bedding-plane shears, assess their characteristics, especially those relating t o shear strength, perform extensive shearstrength testing, and finally determine appropriate shearstrength parameters for each of the critical bedding-plane shears. This paper describes the geological and geotechnical investigations, testing, and methodology used t o identify the bedding-plane shears in the bedrock foundation and assess their shear strength for the design of the project structures.
DAVACHI l3T AL.
Foundation investigations Geological and geotechnical investigations that were used to assess the shear strength of the bedrock foundation included the following: (i) detailed geological mapping of numerous surface and underground excavations in rock, including the diversion tunnels; (ii) core drilling, logging, and sampling using Christensen core barrels with Stratopac bits, resulting in 76 mm diameter cores of excellent quality and core recovery; (iii) six exploratory shafts, 1 m in diameter, and a 100 m long test tunnel, logged in great detail; (iv) horizontal cylindrical core samples (200 mm diameter by 300 mm long) extracted from the walls of the exploratory shafts, diversion tunnels, and portal excavations (Baggott et al. 1985); and ( v ) direct shear, index, and material property testing of the large-diameter core samples, as well as borehole cores. Geological setting and conditions Formations and rock types The Oldman River Dam Project is situated on the western limb of the north-northwest-trending Alberta Syncline, which is a gently folded asymmetric syncline east of the Rocky Mountain Foothills. The eastern limb dips very gently westward. The dip of the western limb varies from near horizontal near the axis to moderately steeply eastward at the western limit. The dam site is located about 5 km west of the Alberta Syncline axis. The abutments and valley floor are composed of the Porcupine Hills Formation of Paleocene age. The Willow Creek Formation of Late Cretaceous age disconformably underlies the Porcupine Hills Formation at a depth of approximately 45 m below the valley floor. The dip of the rock units at the dam site is about 0.5" northeast. Rock types in the project area consist of claystones, siltstones, shallow overbank sandstones, and deep-channel sandstones. All strata are nonmarine clastic deposits and tend to be variable in the horizontal and vertical directions. The siltstones and claystones are gradational; collectively they are classed as mudrocks. Major rock units identified for design purpose include, from top to bottom, the Upper Mudrock Sequence, the Basal Sandstone Sequence, and the Lower Mudrock Sequence, as shown in the following table:
Formation
Major rock unit
Upper Murdock Sequence ....................... Basal Sandstone Sequence ........................................ Willow Creek Lower Mudrock Sequence
Porcupine Hills
*
Approx. contact elevation (m)* 1048 1005
In valley bottom at dam centre line.
The Upper and Lower mudrock sequences comprise predominantly mudrocks with minor sandstone beds. Channel sandstones, which were probably deposited in a fluviodeltaic environment, are characteristic in the Upper Mudrock Sequence. The Basal Sandstone Sequence is mostly composed of uniform sandstones in its upper and lower members; the intermediate member comprises alternating sandstone
OldmanI River Dam Site
FIG. 1. Project location.
and mudrock. Bed thickness is approximately 0.2-2 m in the mudrock sequences and up to 5 m in the Basal Sandstone Sequence. Thin beds of black or dark grey, carbonaceous claystone and siltstone occur throughout the mudrocks. These strata are continuous across the dam site and serve as stratigraphic marker beds. Geological conditions relating to foundation shear strength are discussed below. Bedding-plane shears General Detailed field investigations indicated the presence of planes of relative weakness, i.e., bedding-plane shears, in the mudrocks. These features, which are generally parallel to the bedding, were found at various locations and stratigraphic levels. Although they have undergone relative displacement, the magnitude of the movement is usually undetectable because of the apparent absence of stratigraphic offset. Special emphasis was given to identification and description of these features, since their presence strongly influences foundation shear strength. (1) They usually occur along contacts between relatively strong and weak rocks, such as sandstone and claystone, or in claystones situated between stronger siltstones or sandstones. (2) They generally parallel bedding and, hence, are usually near horizontal in flat-lying mudrocks. (3) They may occur as single shear planes, groups of closely spaced subparallel shear planes, or as brecciated zones, i.e., numerous intersecting fracture planes. (4) Single shear planes are usually less than 2 mm thick and occasionally up to 10 mm thick. Groups of shear planes or brecciated zones can have a total thickness of up to
CAN. GEOTECH. J. VOL. 28, 1991
,
100 50
FIG. 2. Project layout.
FIG.3. Oldman River Dam section. Elevations in metres.
75 mm. Thickness is highly variable. Upper and lower wallrock contacts of the shears may not be parallel, depending on thickness. (5) Thin shear planes, i.e., less than 2 mm, often contain fillings of silt and clay gouge, whereas thicker ones may contain small angular rock fragments and gouge. (6) They are commonly associated with thin, dark, carbonaceous claystones marker beds. (7) They are commonly continuous for distances of hundreds of metres, and some up to 1000 m. (8) Depending on mudrock unit thickness, curved shear planes or splays dipping at angles of up to 50' occasionally
develop off the main bedding-plane shear and terminate in a short distance within the adjacent beds or continue across the bed to merge asymptotically with the bedding-plane contacts. (9) Sometimes the splayed shear planes form a braided network of shears within a claystone bed. (10) Their geometry can be measured using the roughness concept. Roughness generally varies inversely with base length of asperity measured. (1 1) Their frequency is greatest in the upper 10-15 m of bedrock. This may be attributed to weathering. Bedding-plane shears are common features in interbedded
701
DAVACHI ET AL. i
claystone and siltstone sedimentary rock sequences in the plains region of western Canada and elsewhere. Morgenstern (1988) describes these features, their origin, and their impact on foundation shear strength. According to Morgenstern, dam foundations possessing featuers similar to beddingplane shears observed at the Oldman River Dam Project include (i) Gardiner Dam, Saskatchewan (constructed); (ii) Nipawin Dam, Saskatchewan (constructed); (iii) W.A.C. Bennett Dam, British Columbia (constructed); (iv) Peace Canyon Dam, British Columbia (constructed); (v) Site C Dam Project, British Columbia (preliminary design stage); and (vi) Dunvegan Dam, Alberta (feasibility design stage). Davison (personal communication) and UMA-SNC (1979) have also observed bedding-plane shears at Forty Mile Coulee Dams, Alberta (constructed), and Dickson Dam, Alberta (constructed). Matheson and Thomson (1973) describe bedding-plane shears found in two bridge pier foundations in Edmonton, Alberta. Ferguson and Hamel (1981) describe similar and other deformation features in dam and rock foundations in flat-lying sedimentary rock in eastern United States. Morgenstern (1988) and others attribute bedding-plane shears to the following general mechanisms: (i) tectonic activity, (ii) glaciotectonic activity, and (iii) valley rebound. Each of these mechanisms was responsible for the development of bedding-plane shears at various times in the geological history of the Oldman River Dam foundation. Proximity of the Rocky Mountains to the dam site and the fact that the Alberta Syncline is gently folded suggest that the effects of tectonic forces exerted during the Laramide orogeny probably extended as far east as the Oldman River Dam Project area. These forces were compressive, horizontal and northeast-southwest oriented. At the dam site the degree of folding was slight but may have been enough to cause some shear movement between relatively strong and weak strata. The fact that beddingplane shears occur in nontectonic areas, such as eastern Alberta and Saskatchewan, indicates that tectonic activity, however, is not required for their formation. As in many parts of Canada, the project area was glaciated several times and, hence, was subjected to the effects of repeated ice movement. The shearing forces imparted to the upper bedrock may have exceeded the strength of weaker mudrocks and caused shear movement shear between beds. The depth of rock affected by action can be considerable. stress due to river ting is likely to have caused additional shearing, particularly along bedding-~laneshears that may already ha'' been weakened by previous tectonic or glaciotectonic activity. AS downcutting occurs, the existing horizontal stresses in the valley walls are relieved and adjacent beds move differently toward the valley centre. Below the valley floor, the reduction in vertical stress results in uplifting and arching of beds. As a consequence, shearing ~ c c u r salong bed surfaces, as well as occasion all^ obliquely across beds. Valley rebound the phenomena can extend to valley bottom and into the valley slopes. No evidence of landsliding along the bedding-plane shears was found in the dam site area. Continuity of bedding-plane shears Stability analyses required that continuous bedding-plane shears beneath a particular structure be identified; these
shears were considered to be those whose minimum continuity was equal to the upstream-downstream length of the respective structure. Correlation of riverbank surface mapping and borehole core data and consideration of the slight bedding dip enabled the identification of all bedding-plane shears. Assessment of continuity of the bedding-plane shears was then based on relative observed occurrence, as observed in excavations, trenches, shafts, and borehole core. If relative observed occurrence of a given bedding-plane shear exceeded 40% over all potential exposures, then the bedding-plane shear was inferred to be continuous. The 22 continuous bedding-plane shears that were identified are distributed as follows: Area
No. of continuous shears
Riverbanks Valley bottom
11 11
Zones of slickensides In some claystones an unusual number of wavy, multidirectional, slickensided planes were identified. These planes are often discontinuous across the borehole core, as indicated by the fact that cores usually remained intact upon removal from the core barrel. Zones of slickensided planes tend to be characteristic of particular beds and can be correlated from one borehole to another. Several mechanisms are possible for the formation of the zones of slickensiding, including valley rebound and tectonic and (or) glaciotectonic activity. Alternatively, the slickensided planes may have resulted from shearing of the sediments during sedimentation or subsequently, while in the plastic state (compaction shear). Because of their multidirectional, inclined orientation and discontinuous nature, the slickensided planes were not considered as critical to the stability of structures as the beddingplane shears. They did, however, degrade the shear strength of the claystones. Shear strength along bedding-plane shears General principles During the early design stages, it was recognized that the shear strength along bedding-plane shears would control foundation stability and, hence, stability of overlying structures. The bedding-plane shears were often filled with relatively weak, sheared Although relative displacement had occurred along the shears, the amount of movement was unknown. ~t was also recognized that for a given bedding-plane shear, the amount of relative shearing movement that had occurred is not necessarily constant over its entire surface and depends on its depth and the degree to which it may have been affected by the shearing mechanisms described earlier. Therefore the in situ shear strength at any point along a given '.hear plane is between its residual and peak shear strength value, and probably close to the former. For stability analyses, it was necessary to determine an angle of shearing resistance value for each of the beddingplane shears which was considered continuous and critical to the design of project structures. These particular beddingplane shears were designated "design bedding-plane shears," and their angles of shearing resistance called "design angles of shearing resistance."
CAN. GEOTECH. J. VOL. 28, 1991
,
LEGEND: :. intact Claystone Sample Sheared Along Bedding 50 mm x 50 mm Direct Shear Test
0
1
0
m
~
)
4
a
5
0
0
Bedd~ngPlane Shear Sample Obtained From Dr~llhole 50 mm x 50 mm Direct Shear Test
+
Bedding Plane Shear Sample Obtained From Shaft or Trench 100 mm x 100 m m Direct Shear Test
w
RESWN NWX Cf DIE*RINC RESSTWCE (EG)
(C)
FIG. 4. Shear strength plots. (a) Residual shear strength, bedding-plane shear V12. ( 6 )In situ shear strength, bedding-plane shear V12. (c) In situ vs. residual angle of shearing resistance, bedding-plane shear V12.
The following factors were considered to determine the design angle of shearing resistance that should be assigned to each design bedding-plane shear: ( i ) laboratory test results, including residual in situ angles of shearing resistance and (ii) field-determined, site-wide characteristics of the bedding-plane shear, including roughness and thickness. Accordingly, the design angle of shearing resistance was determined by summing the following components: ( i ) residual angle of shearing resistance; (ii) in situ state component of angle of shearing resistance; (iii) angle of shearing resistance attributable to shear-plane roughness; and ( i v ) angle of shearing resistance attributable to shearplane thickness. Component i was the primary factor determining the design angle of shearing resistance. Direct shear testing of
a number of samples from different locations on a single shear plane indicated a range of residual values. From this range of values, a representative residual angle of shearing resistance was conservatively selected. Component ii accounts for the fact that the in situ angle of shearing resistance indicated by direct shear testing often exceeded the residual value. Accordingly, a small additional component of the angle of shearing resistance in excess of the residual value was available. Component iii recognizes the fact that shear-plane roughness contributes to the angle of shearing resistance. Field studies indicated that bedding-plane shears, when examined over wide areas, were usually not perfectly planar. Therefore, a small component of the angle of shearing resistance due to roughness could be utilized.
DAVACHI ET AL.
TABLE1. Representative residual angle of shearing resistance
$A
+A
44
703
, '
'
'
Elevation (m)
representative
range
mean
Difference
(")
("1
("1
(")"
(070)~
Total no. of 4: tests
Rl8 R17 R16 R14 R13 R9 R8 R7 R6 R1
1115 1107.7 1105 1100 1095.5 1088.5 1087.5 1086.5 1084.5 1068.2
11.5 16.0 12.5 15 15 11 12 12 12.5 10.5
Riverbanks 9.9-22.5 15.6-17.4 10.8-24.9 14.3-18.4 13.5-18.3 8.3-21.3 10.3-23.4 11.9-12.8 12.3-15.8 8.7-14.7
14.1 16.2 14.9 15.6 16.3 12.4 16.8 12.2 13.9 11.6
1.6 0.4 1.7 0.7 1.5 2.7 1.7 0.1 0.2 1.8
33 50 33 43 25 35 12 50 33 14
18 4 18 7 4 19 28 4 3 22
V18 V17 V16 V15 V12 V11 V10
1049.4 1047.7 1046.8 1043.9 1030.7 1027.6 1025.6
15.0 21.0 15.5 22.5 12 14.5 13.5
Valley bottom 12.0-23.7 20.5-29.7 14.5-26.0 22.4-29.8 10.9-29.4 14.4-19.3 13.0-25.2
16.6 26.1 18.3 25.7 20.0 16.4 17.5
3 .O 0.5 1.O 0.1 1.1 0.1 0.5
30 17 32 20 13 23 21
7 6 15 5 23 4 14
Bedding-plane shear No.
Measured less than selected
ODifference between selected representative residual angle and minimum measured residual angle. b~ercentageof measured residual angles less than selected representative residual angle.
Component iv recognizes that all other conditions being equal, thin shear planes with rock-to-rock contact have higher angles of shearing resistance than thick shear planes with fillings. Therefore a small component of the angle of shearing resistance could be added in the case of thin shear planes. Zero effective cohesion was used for all the bedding-plane shears.
I I I
Residual angle of shearing resistance Testing methodology Apart from routine classification and property tests, the primary thrust of laboratory testing was to determine the shear-strength parameters of the bedding-plane shears in the 100 mm and 50 mm direct shear box apparatus. A total of 243 direct shear box tests were done on specimens of bedding-plane shears and intact rock cut from cores taken from boreholes, tunnels, exploratory shafts, and rock exposures (Thurber 1986). All specimens were tested to their residual value. The direct shear box tests generally followed the procedures specified in ASTM D3080 (ASTM 1986). The rate of shearing during peak-strength testing was 0.0048 mm/ min, which is slower than the 0.02 mm/min calculated by ASTM D3080. After completing the peak cycle, at least five fast residual cycles were run at a rate of 0.18 mm/min. Then the test was stopped for tlooor 12 h, whichever was longer. To complete the test, two slow residual cycles were run at rates in the order of 0.016 mm/min. If the 0.016 mm/min rate was faster than the calculated ASTM rate, then a comparison was made between the cycles at the 0.016 mm/min rate and the last two cycles at 0.18 mm/min for evidence of shear strength increase due to incomplete pore-pressure dissipation. If such evidence was found, then two further cycles were done at the ASTM rate. Because of the nature of claystone and bedding-plane shear material tested, irregular, nonplanar shear surfaces fre-
quently developed during testing and resulted in ramping. The effects of ramping on the test data were treated as an asperity correction, as described by Patton (1966) and Hoek and Bray (1977). Selection of representative values Residual angles of shearing resistance measured in the laboratory for a given bedding-plane shear may be expected to show a distribution of values. The scatter of results is a reflection of the variation of characteristics along a bedding-plane shear, including the clay content and the index properties of the fillings. In the assessment of shear strength along the beddingplane shears, the peak shear strength measured in the laboratory is referred to as the in situ shear strength for that particular sample. This in situ shear strength recognizes that the maximum shear strength measured in a bedding-plane shear is likely to be considerably less than the peak value that existed prior to shearing during the geological past. For each continuous bedding-plane shear, the following relationships were plotted using the laboratory data: (i) residual shear stress versus effective normal stress, (ii) in situ shear stress versus effective normal stress, and (iii) residual angle of shearing resistance versus in situ angle of shearing resistance. Typical plots are shown in Fig. 4. Preliminary results of direct shear tests on samples from the 22 continuous bedding-plane shears indicated that five of the shears would have no significant effect on the design of the dam, spillway, and other structures. The remaining 17 shears were subjected to more intensive sampling and testing. Based on a thorough and critical assessment of direct shear test data, a representative residual angle of shearing resistance for each of the 17 bedding-plane shears was selected from the range of measured residual values. This value is taken to represent the residual angle of shearing resistance for a given bedding-plane shear. It was considered
CAN. GEOTECH. .I.VOL. 28,
1991
0
,
10
20
30
40'
50
60
70
U Q H O UWT
LEGEND: Intact Claystone Sample Sheared Along Bedding 50 mm x 50 mm Direct Shear Test
rusnaw
0
Bedding Plane Shear Sample Obtained From Drillhole 50 mm x 50 mm Direct Shear Test
+
Bedding Plane Shear Sampk Obtained From Shaft or Trench 100 mm x 100 mm Direct Shear Test
WMX
(c)
FIG. 5. Index properties versus 4;. (a) Clay content versus residual angle of shearing resistance. (b) Liquid limit versus residual angle of shearing resistance. (c) Plasticity index versus residual angle of shearing resistance. unduly conservative to design on the basis of the minimum residual value measured. On the other hand, because of uncertainty, the value selected was not excessively above the minimum measured residual angle of shearing resistance. Table 1 gives the representative residual angles of shearing resistance for the 17 selected bedding-plane shears. Also included are the range and mean values for the residual angles, the difference between the representative residual
angle and the minimum measured residual angle, the percent of measured residual angle values that are less than the respresentative residual angle, and the total number of residual angles measured. The selected representative residual angles were such that the number of measured values of residual angle less than the representative residual angle were usually between 15 and 35%.
ET AL.
General findings Correlations between residual angle of shearing resistance, blenderised (Heley et al. 1971) clay content, liquid limit, and plasticity index are shown in Fig. 5. It can be seen that only a lower bound curve can be drawn. Generally, with increasing clay content, liquid limit, and plasticity index, the residual angle of shearing resistance decreases, which agrees with Skempton (1964, 1985). It was also found that the silty clay and clay materials in bedding-plane shears have the same residual angle of shearing resistance as adjacent claystones of similar mineral composition. As a consequence of the natural shear deformation that has occurred, the bedding-plane shears exhibit considerably lower peak angle of shearing resistance and higher natural moisture contents than in the adjacent unsheared claystone. This observation was used to advantage where bedding-plane shears in drill core were too disturbed to be tested in the shear box. A test on the intact rock above or below the bedding-plane shear provided an indicator of the residual shear strength of the bedding-plane shear material. Additional components In situ state component The peak shear strength measured in the direct shear test is an indicator of actual in situ strength, provided that no displacement of rock on either side of the bedding-plane shear in the test specimen has occurred. For some bedding-plane shears, the in situ angle of shearing resistance is at its residual value. If one or more in situ angles of shearing resistance measured were at the residual value, an in situ state component of zero was used. For bedding-plane shears where all in situ angles of shearing resistance measured were higher than the residual values, an in situ state component equal to 0.5" was added to the representative residual angle of shearing resistance. This recognizes that, in a rock mass which has been subjected to shear deformation, not all bedding-plane shears have been sheared to the same degree, nor to the residual value. Geological components 1. Roughness Roughness measurements of bedding-plane shears were used to assess the roughness component of the design angle of shearing resistance. Roughness was assessed by mapping several shears in a number of continuous rock exposures along a construction access road, in the test tunnel and diversion tunnels, and at both diversion tunnel portals. These measurements indicated that roughness of beddingplane shears, as measured over base lengths varying between 10 and 160 m, ranged between 0 and 1.6" and averaged approximately 0.5". These measurements were considered representative of the geometry of all bedding-plane shears at the dam site over moderate to large distances. Some very detailed measurements were also taken of wallrock roughness, i.e. roughness of the surfaces separating the filling and neighboring intact wall rock. These measurements were made along bedding-plane shears greater than 10 mm thick and along very short base lengths, i.e., less than 500 mm. In the great majority of cases, the wall-rock roughness was found to range between 1 and 16". It was noted, however, that the actual plane of shearing rarely followed the wall rock and that usually it was more planar and centrally located in the filling.
,\
705
In selecting the app;opriati angle bf roughness to be used in determining design angle of shearing reshtance, consideration was given to the scale of the potential failure surface being analyzed. The roughness estimates based on long base lengths, not short base lengths, were considered most appropriate for large rock mass movement. Hence, a roughness component of the angle of shearing resistance was conservatively estimated to be 0.5". This was considered applicable to all design shear planes. 2. Thickness The thickness of bedding-plane shears was considered a factor in determining design angle of shearing resistance. All other conditions being equal, shears characterized by mated rock-to-rock contact with no fillings or thin fillings were judged to be of higher angle of shearing resistance than those shears with less rock-to-rock contact and relatively thick fillings. Thickness of bedding-plane shears varied considerably; the majority (76%) ranged from 3 to 34 mm thick, 9% ranged between 1 and 2 mm, and 15% were in excess of 60 mm thick. Thicker shears often comprised several shear planes in close proximity. For the purpose of evaluating the thickness component of design angle of shearing resistance, the bedding-plane shears were classified as thin if their thickness was of the order of 1-2 mm. A 0.5" angle of shearing resistance component was applied to these bedding-plane shears. Other bedding-plane shears were classified as thick, and no thickness component of the angle of shearing resistance was applied in these cases. The approach of using a 2 mm thickness criterion to determine a 0.5" thickness component of the angle of shearing resistance was based on consideration of the mechanics of shearing along the bedding-plane shears over considerable distances. In view of the data on which the assessment was based, the 0.5" value was considered reasonable and conservative.
Design angle of shearing resistance The design angle of shearing resistance is calculated as the sum of the representative residual angle of shearing resistance plus the in situ state component, plus roughness component, plus thickness component. From the 17 continuous bedding-plane shears investigated in detail, only 14 were considered to be critical for the design of the dam, spillway, and other structures. This was decided on the basis of a comparative assessment of elevation and angle of shearing resistance of neighbouring bedding-plane shears. The design angle of shearing resistance values determined for the 14 design bedding-plane shears are given in Table 2. Shear strength across beds The results of laboratory shear-box and triaxial tests on intact, unsheared mudrocks and the sandstones were used to select the other parameters for design. To ensure a conservative design, these values tend to be slightly on the low side of the average of the range of measured shear-strength parameters. They are presented here to give an indication of the available shear strength in the rock not directly affected by shearing. For the massive sandstone units an effective angle of shearing resistance of 60" with zero effective cohesion was considered representative. The use of zero effective cohesion
CAN.
GEOTECH. J. VOL. 28,
1991
, \
TABLE 2. Design angle of shearing resistance Effective angle of shearing resistance, Elevation (m)
Representative residual
R18 R17 R16 R14 R13 R9 R8 R7 R6 R1
1115 1107.7 1105 1100 1095.5 1088.5 1087.5 1086.5 1084.5 1068.2
11.5 16.0 12.5 15 15 11 12 12 12.5 10.5
V18 V16 V12 V10
1049.4 1046.8 1030.7 1025.6
15.0 15.5 12.0 13.5
Bedding-plane shear No.
+I(")
Thickness component
In situ state component
Design value
Riverbanks 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
0 0 0 0 0 0 0 0.5 0 0
0.5 0 0 0.5 0 0 0 0 0.5 0.5
12.5 16.5 13.0 16.0 15.5 11.5 12.5 13.0 13.5 11.5
Valley bottom 0.5 0.5 0.5 0.5
0.5 0.5 0 0
0 0.5 0 0.5
16.0 17.0 12.5 14.5
Roughness component
was consistent with the extensive vertical jointing that is found in all sandstone units. An effective angle of shearing resistance of 35" was used for the cross-bedding shear strength of all mudrock units. The effective cohesion in cross-bedding was varied to reflect the degree of weathering and stress relief and to account for the relative amount of siltstone and claystone in each unit. For the weathered, brecciated, slickensided, and highly fractured mudrock layers, the effective cohesion in crossbedding was taken to be zero. For unweathered intact mudrock, effective cohesion in cross-bedding shear equal to 200 kPa was used. For the slightly weathered mudrocks, intermediate values were assigned. Influence of bedding-plane shears on design The low shear strength available along continuous bedding-plane shears had a major effect on design and layout of the project structures. The earth and rockfill dam required relatively flat upstream and downstream slopes (Fig. 3). All potential sliding surfaces passed through bedding-plane shear V12. Bedding-plane shear V16, which has a higher shear strength but is closer to the base of the dam, does not control stability, as it has been eroded away in parts of the dam foundation and is therefore discontinuous. The wide base of the dam, in turn, determined the length of the diversion tunnels and the spillway and its approach channel. The presence of bedding-plane shears had a less pronounced effect on stability of the concrete spillway headworks which is governed by bedding-plane shear R9. This is due to the requirement that it was designed to discharge the probable maximum flood. This resulted in the structure being located in a 35 m deep excavated channel where any potential sliding surface through a bedding-plane shear would mobilize high cross-bedding shear strength where it passes up to the surface at each side of the structure. Conclusions Bedding-plane shears frequently occur at dam sites in the western Canadian plains. Their characteristics, namely, position, orientation, continuity, fillings, roughness, and thick-
ness, are very important in thg assessment of foundation shear strength. Proper shear-strength assessment requires (i) thorough collection of surface and subsurface exploration data and interpretation of same, and (ii) extensive sampling, direct shear testing, and data analysis. The assumption of continuity of bedding-plane shears does not require conclusive evidence of their presence in all boreholes and surface exposures. Evidence may be missing because of such factors as frequency of boreholes, core recovery, thickness of bedding shears, logging error, regularity of bedding dip, and maximum trace lengths observed in exposures. Judgement is required to decide on the relative percentage of evidence of continuity necessary, which must take into account the structure dimensions. Only bedding-plane shears identified as continuous over structurewide base lengths need to be considered in stability analyses. A representative residual angle of shearing resistance must be selected for each bedding-plane shear from a range of measured residual values. The design angle of shearing resistance for a given bedding-plane shear can be calculated by summing the representative residual angle of shearing resistance, the primary component, and three lesser components of the angle of shearing resistance, i.e., its in situ state, roughness, and thickness. Although considerable effort was given to studying the character and properties of bedding-plane shears, no means were found to quantify the in situ state and thickness components of the angle of shearing resistance. It became necessary to resort to the intuitive selection of conservatively low values. The actual shear strength available along beddingplane shears in excess of the residual value is an area that needs further study and research along the lines of Patton (1966) to rationalize the selection of shear-strength parameters for design. Acknowledgements The authors are grateful to Alberta Public Works, Supply and Services, for permission to publish this paper. Prelimi-
DAVACHI ET
AL.
707 \
nary geological and geotechnical investigations in the diversion tunnels area were done by Golder Associates under the supervision of G.E. Rawlings and in all other areas within the project site by Thurber Consultants Ltd. under the superof K'W' Savigny' Most of the laboratory testing referred to in the paper was done by Thurber Consultants Ltd. under the D.Wand J.M. ASTM 1986. Standard method for direct shear test of soils under consolidated drained conditions. ASTM D3080. American Society for Testing and Materials. BAGGOTT, B.L., BAKER,D.G., and WADE,N.H. 1985. Development of equipment and methods for wall sampling in exploratory shafts. Presented at Annual General Meeting, Canadian National Committee on Large Dams, Fort St. John, B.C. FERGUSON, H.F. 1967. Valley stress release in the Allegheny Plateau. Bulletin of the Association of Engineering Geologists, 4: 63-71. FERGUSON, H.F., and HAMEL,J.V. 1981. Valley stress relief in flat lying sedimentary rocks. Proceedings, International Symposium on Weak Rock, Tokyo, Japan, pp. 1235-1241. HELEY,W., and MACIVER, B.N. 1971. Engineering properties of clay shales. Report 1. Development of classification indexes for
clay shales. Report 4U 7 5 6 - 1 2 3 . ' ~ . A ~ .m y ~ngineerWaterways Experiment Station* Vicksburg~MS: '. HOEK,E., and BRAY,J.W. 1977. Rock slope engineering. 2nd ed. The Institute of Mining and Metallurgy, London, pp. 83-86. MATHESON, D.S., and THOMPSON, S. 1973. Geological implications of valley rebound. Canadian Journal of Earth Sciences, 961-978. MORGENSTERN, N.R. 1988. Recent experience with dam foundations on clay shale in western Canada. Contributions to a Stateof-the-Art Paper presented at 12th International Conference on Soil Mechanics and Foundation Engineering, Rio de Janeiro. PATTON,F.D. 1966. Multiple modes of shear failure in rock. Proceedings, 1st Congress, International Society of Rock Mechanics, Lisbon, Vol. 1, pp. 509-513. SKEMPTON,A.W. 1964. Long-term stability of clay slopes. GCotechnique, 14: 77-101. -1985. Residual strength of clays in landslides, folded strata and the laboratory. GCotechnique, 35: 3-18. THURBER 1986. Foundation exploration. In Oldman River Dam Preliminary Engineering Report. Appendix D. Thurber Consultants Ltd., Edmonton, Alta. UMA-SNC, 1979. Dickson Dam preliminary engineering report. UMA Engineering Ltd., Edmonton, Alta., and SNC Consultants Ltd., Calgary, Alta.
