Arma-84-0922_foundation Studies for a Roller-compacted Concrete

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Chapter96FOUNDATION STUDIES FOR A ROLLER-COMPACTED CONCRETE

GRAVITY DAM

by Gregg A. Scott

Civil Engineer, Bureau of ReclamationDenver, Colorado

ABSTRACTUpper Stillwater Dam s to be a roller-compacted concrete gravitystructure, founded on nearly horizontally bedded sandstone andargillite rock. An overview of the testing and analyses used toevaluate the adequacy of the foundation relative to deformation,seepage, and stability is presented.

INTRODUCTIONUpper Stillwater Damwill be the Bureau of Reclamation's firstroller-compacted concrete gravity dam. It is located on the southflank of the Uinta Mountains in northern Utah. The maximumheight ofthe dam is 82 m, and the crest length is 812 m at elevation 2492 m.An ungated overflow spillway will be constructed near the center ofthe dam. Water is diverted to Stillwater Tunnel or regulated to RockCreek through a single intake structure. The general features of thedam are shown in figure 1.The damwill be founded on interbedded sandstone and argillite ofthe Precambrian Uinta Mountain group. The bedding structure isnearly horizontal at the site. The bedrock has been subdivided intofive rock units for mapping purposes as follows: (1) an uppersandstone unit near the top of both abutments, (2) a middle sandstoneunit with numerous nterbeds of argillite and siltstone, (3) a thickargillite designated unit M extending to near the base of both abut-ments, (4) a lower sandstone unit which forms most of the foundation,and (5) a small but continuous argillite interbed designated unit Lwithin the lower sandstone unit. A construction contract was per-formed to strip surficial materials, exposing seven minor faults

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ROLLER-COMPACTEDCONCRETEGRAVITY DAM 923

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PROFILE ALONG AXIS

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Fgue 1. - nea] design and geologyOER SADSTETYPIGL

desJgnated F-[ through F-4 and F-7 through F-9. They ae nealyvertical, coss the foundation fom upstream to downstream, andgenerally consJst of a gouge zone several mJllJmetes wJde wJthJn afactued zone of ock about [ m wde. OoJntJng, othe than beddingjoints, Js 1JmJted to nea vertical ses pedomJnantly oientedparallel to the faults. OoJnt spacJngs average between 0.5 and 3 mdependJng on the set and 1ocatJon. Thee Js no dJstnct weatheMngpofJle, but most joJnts ae fJlled wJth sand and oganJc mateMalsto depths aveagJng 6 m. An eosJonal channel foms a bedrock lowabout [4 m deep nea the Mght side of the foundatJon. ThJs channel,temed the tough, should elJeve any lage hoMzontal stresses.AddJtJonal detaJls of the geology ae shown Jn fJgue [.DEFORRATION STUDIES

Initial attempts to estimate in situ foundation deformation moduluswere based on correlations with RMR Rock Mass Rating, Bieniawski,1978) and geophysical shear wave frequency. However, neither methodwas found to be totally acceptable for the rock at the UpperSti 1 water damsite.In situ jacking tests were performed at the site utilizing theGoodman orehole jack. The data were reduced according to the methoddescribed by Goodman,Van, and Heuze (1968). Adjustment factors werealso considered. However, the measured modulus values were low


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924 ROCK MECHANICS N PRODUCTIVITY, PROTECTIONenough that corrections were considered unnecessary. The resultsfrom argillite were used directly because they were consistent andindicated a modulus parallel to the bedding consistently 1.5 timesthat perpendicular to the bedding. The sandstone results requiredfurther analysis. Goodman1982) suggests evaluating the dataaccording to the following equation:

1/Erm : 1/E + 1/(KnS' ) (1)where Erm s the rock massmodulus rom the jacking tests, E is themodulusof intact rock, Kn is an effective joint stiffness, and S' isthe effective spacing of joints. S' can be calculated from RQD RockQuality Designation) using the relationship proposed by Priest andHudson (1976).

The value of Kn is determined from the jacking tests and should befairly constant. However, the best behaved Goodman ack tests insandstone indicate that Kn is not constant as shown n figure 2.Modulus values calculated from the Kn vs. RQD elationship usingequation (1) are also plotted in figure 2. The results indicate thatthe modulus is relatively insensitive to changes in RQD. This is inpart due to the fact that S' (or RQD) s a measure of fracturing inthe direction of the drill hole, and the Goodman ack tests rock in adirection perpendicular to the drill hole. The small volume of rocktested by the Goodmanack probably also contributes to the observedresults. Therefore, empirical relationships between RQDand labora-tory modulus reduction factors were used.

Two-dimensional plane strain finite element studies were performedat five sections perpendicular to the dam axis. A typical mesh isshown in figure 3. The sections were located so as to be aboutevenly spaced and to take advantage of the available drill hole data.Element boundaries were chosen to correspond to boundaries of equalRQD for a given rock type, and appropriate modulus values wereassigned to each element. The foundation was considered to bemassless in the models, because it has already deformed as much as itwill under its self-weight. Loads consistent with constructing thedam and filling the reservoir were applied to each model. The non-uniform foundation modulus values were then replaced with a uniformvalue, and the analyses were repeated for various values of uniformmodulus. The calculated deformation at the base of the dam wasplotted for each case as shown n figure 4. Deformation patternswere compared for cases of uniform and nonuniform modulus, and anequivalent uniform modulus was estimated for each section. Thedeformation pattern and modulus distribution were determined to beacceptable across the foundation.

The deformation modulusof the unit M argillite is considerablyless than that of the surrounding sandstone, and differential defor-mation of the abutments was considered worthy of study. Three-dimensional finite element mesheswere developed as shown n figure


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ROLLER-COMPACTED CONCRETE GRAVITY DAM 925

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Figure 2. -Results fromGoodman Jack tests.NODAL POINTS AT BASE OF DAM

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Erin 6895 Po.Non-uniform modulus -

. ... --. __C)..._..0______4. )Erin-S448 MPoDsplocementcaleFigure 4. - Deformations atbase of dam.

Figure 3. -Typical two-dimensionalfinite element mesh.

Figure 5. - Three-dimensionalfinite el ement mesh.

5, because wo-dimensionalanalyses were not consideredappropriate.Comparisons were made between various ratios of sandstone toargillite modulus,and a uniform abutmentmodulus. Deformationpat-terns on the abutmentwere examinedand determined to be acceptable.Maximum ensile stresses in the dam structure at the abutment arenearly horizontal on the upstream face. These 'stresses result fromwater load bending and are actually reducedby the presenceof softerrock units in the abutments.SEEPAGE STUDIES

Upper Stillwater Damwill contain a drainage gallery from which adrainage curtain will be drilled into the foundation. The uplift atthe line of drains was calculated using the equations presented byCasagrande 1961). As an approximation, it was assumedhat the


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926 ROCK MECHANICS N PRODUCTIVITY,PROTECTIONtotal head at the drains is constant and equal to tailwater elevationplus the uplift at the line of drains. This total head was assignedto a line of nodal points to represent the drains in finite elementmodels.

Two-dimensional finite element seepage studies were conducted atthe five study locations previously discussed. Element boundarieswere chosen to also correspond to those of equal permeability, iden-tified by results from packer pump-in tests. Equipotential lineswere constructed from the results of the studies, as shown in figure6. Several sensitivity studies were conducted to examine the effectsof length and orientation of drains, isotropic or anisotropic per-meabilities, and other uplift assumptions. The results indicate thatwater forces acting on foundation planes are relatively insensitiveto the assumptionsof the seepage studies, provided the drains areaccounted for and extend below any potentially critical slidingplane.

Figure 6. - Equipotentiallines. Figure 7. - Direct sheartest results.STABILITY STUDIES

A number of direct shear tests were conducted on open beddingjoints in sandstone and argillite. A portable direct shear devicewas used to perform tests in the field on samples of the unit Largillite under in situ moisture conditions. Tests were conductedon50- and 100-mm-diameter ore containing sandstone oints. Theresults, shown n figure 7, indicate that the strength of theargillite is somewhatess under in situ moisture conditions thandry. The size of core had little effect on the strength of sandstonejoints. Because he bedding joints do not daylight and other jointsare near vertical, instability of the foundation requires movementthrough someportion of intact rock. Therefore, triaxial tests were


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ROLLER-COMPACTED ONCRETEGRAVITY DAM 927conducted on intact rock, cored at various angles to the bedding, toestablish friction angles and cohesion values for the materials.

The concept of an active and passive block, as shown in figure 8,was used to evaluate the stability of the foundation. This was con-sidered appropriate based on the orientation of jointing at the site.Side plane resistance was not considered but could be included ifresults from conservative two-dimensional analyses indicated moreresistance was required. Water forces acting on the foundation pla-nes were calculated by integrating pressure heads from the seepageanalyses. Solution of the two-block problem then involves thesesteps: (1) a factor of safety, FS , is assumedor the active block;(2) a value for the interblock force, P, is then calculated; (3) afactor of safety, FSp, is computedor the passive block; and (4) ifthe calculated value-of FSp is not equal to the assumed alue ofFS , an adjustment is made-to FS and the process is repeated.Analyses of this type were performed at each time step (0.01 S)during a Richter M6.0 design earthquake located at a 2-km faultdistance. Forces from the dam were calculated from response historyfinite element analyses which included hydrodynamic interaction, andvertical and horizontal componentsof ground motion. Inertia forcesfrom the two components of ground motion were also included.

Sensitivity studies indicated that, aside from shear strength, theresults were most sensitive to the assumed value of the interblockforce angle, e. The value of e should approach the friction angle ofthe block interface. However, the orientations of principal stressesnear the toe of the dam were also evaluated at each time step tostudy the potential value of e. The minimumvalue of e estimated inthis manner was 33 However, to be conservative, 15 was used for ein the analyses. The value for a friction angle of the active planewas selected from figure 7 by examining maximum ormal stresses. Twocases were examined relative to the shear strength of the passiveblock. In one case, the strength results from the triaxial testswere reduced by an appropriate factor to account for jointing. Inthe other case, the empirical criterion proposed by Hoek and Brown(1980) was used to evaluate the limit strength of the rock mass.Several depths of potential sliding planes were considered, includingthat of the unit L argillite, at the five study sections previouslydiscussed. The factor of safety was plotted for each time step asshown in figure 9, and was found to be acceptable in all cases.

To study the possibility of localized overstressing of the foun-dation, leading to progressive instability, studies were performedusing the structural and seepage finite element analyses previouslydiscussed. Mass was included in the foundation for these studies toobtain the appropriate stress distribution. A case was also examinedwhich included a temperature load in the dam structure. Local fac-tors of safety were computed for each element considering waterpressures, normal and shear stresses on the bedding, and theappropriate shear strength from figure 7. The potential for


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928 ROCK MECHANICS IN PRODUCTIVITY, PROTECTION

p iwp o,4CTIV BLOCK

Figure 8. - Two-blockpotentialmode of instability.

0.00 1.25 2.50 3.75 5.00 6.25 7.50 8.75 10.00TIME - SECONDS

Figure 9. - Factor of safety vs.time.

progressive instability was then evaluated in the following manner:(1) for elements with local factors of safety less than 1.0, theexcess shear force was transferred to the adjacent element; (2) a newshear stress and local factor of safety was computed for this adja-cent element; (3) if the new local factor of safety was less than1.0, the excess shear force was transferred to the next adjacent ele-ment; and (4) the process was repeated until stability was reached.An adequate margin of safety was found for all five study locations.Buckling of a tabular zone near the toe of the damwas evaluatedaccording to the Euler formulation. A critical buckling load perunit width was calculated. This load was compared to the actual loadas estimated from finite element studies. Several lengths of poten-tial buckling zones were evaluated at the five study locations pre-viously discussed and an adequate margin of safety was found in allcases. The Euler formulation assumespinned ends and neglects theweight of overlying materials, which are both conservative assump-tions. However, it ignores the effects of cross jointing.

FOUNDATION TREATMENT AND INSTRUMENTATIONFoundation treatment for Upper Stillwater Damwill follow standardBureau of Reclamation practice. Controlled blasting will be utilizedto excavate below the majority of fracturing and fracture fillings.Consolidation grouting will be performed over the entire foundation,and a grout curtain will be installed from the gallery. The groutingwill be monitored using a computerized system. A drainage curtainwill also be installed from the gallery as discussed earlier. Thegrout and drainage curtains will extend below the unit L argillite.Several three-dimensional finite element studies were performed toevaluate the effects of fault treatment. Deformations on both sidesof the faults, with and without various dental treatment concrete


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ROLLER-COMPACTED CONCRETE GRAVITY DAM 929alternatives, were examined. Principal and shear stresses were exam-ined in the treatment concrete and in the dam adjacent to the foun-dation. No adverse conditions were found, largely due to the factthat the loading is relatively uniform on both sides of the faults.Exit gradients were also calculated at the toe of the dam for varioustreatment depths. Extra treatment depths were not required to keepexit gradients to an acceptable level. Therefore, fault treatmentwill 'consist of excavating the fault material to refusal using mecha-nical methods. The zones will then be backfilled with dental treat-ment concrete and grouted.

Piezometers will be installed at the base of the dam, in the abut-ments, and near the depth of the unit L argillite. Seepage flowswill also be monitored. Foundation movements will be monitored withextensometers anchored deeply in vertical and angled holes. Theextensometers will be installed after completion of the gallery tomoni ot deformati OhS during constructi on.ACKNOWLEDGMENTS

Many Bureau of Reclamation personnel contributed to these studies,including several at the Uinta Basin Construction Office in Duchesne,Utah.REFERENCES

Bieniawski, Z. T., 1978, Determining Rock Mass Deformability:Experience from Case Histories, Int. J. Rock Mech. Min. Sci. andGeomech. Abstr., vol. 15, pp. 237-247.Casagrande, A., 1961, Control of Seepage through Foundations andAbutments f Dams, Geotechnique,vol. 11, no. 3, pp. 161-182.Goodman, R. E., Van, T. K., and Heuze, F. E., 1968, The Measurementof Rock Deformability in Boreholes, Proceedings, loth Sj/mposiumnRock Mechanics, Austin, Texas.Goodman, R. E., 1982, RecommendationsConcerning FoundationInvestigations for Upper Stillwater Damsite, USBRContractNo. 2-07-DV-00162.Hoek, E., and Brown, E. T., 1980, Empirical Strength Criterion forRockMasses, Proceedings,ASCE . Geotech. Enqr. Div., GT9,pp. 1013-1035.Priest, S. D., and Hudson, J. A., 1976, Discontinuity Spacings inRock, Int. J. Rock Mech. Min. Sci. and Geomech. Abstr., vol. 13,

pp. 135-138.


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Arma-84-0922_foundation Studies for a Roller-compacted Concrete