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SETTLEMENT OF EMBANKMENT DAMS – DON’T FORGET ABOUT THE BEDROCK

By: Robert J. Huzjak, P.E. (1), and Adam B. Prochaska, Ph.D., E.I. (2)

Abstract

Predicting deformation of embankment dams and ancillary facilities during and post construction is a critical design consideration. Total and differential settlements of these structures are generally evaluated based on consideration for immediate settlement and consolidation of the embankment materials and the soil foundations, with minimal consideration of bedrock settlement. However, bedrock in many parts of the world, and particularly in the western United States, consists of geologically young (less than about 100 million years old) fine grained sedimentary units. Under loads from moderate to large dams, settlement of this type of bedrock can become a critical design condition.

This paper presents the analyses, field and laboratory data, and design features that were incorporated to accommodate bedrock settlement below a 200-foot-high embankment dam that is being constructed in two phases on a claystone bedrock foundation.

Construction and post construction settlement were computed using one-dimensional hand and two-dimensional finite-element methods. Data obtained from settlement sensors, piezometers, and outlet conduit surveys during and after the first phase of construction were used to calibrate laboratory-obtained settlement properties to observed behavior. These calibrated properties, and the time since completion of the first construction phase, were used to predict the ultimate settlement for the foundation of the completed embankment. Features incorporated into the embankment and the ancillary facilities to accommodate the predicted construction and post construction bedrock settlement will be described. Finally, recommendations for predicting construction and post construction bedrock settlement will be presented.

Introduction

Predicting deformation of embankment dams and ancillary facilities during and post construction is a critical design consideration. For dams constructed on soft bedrock, settlement of the bedrock must be considered in addition to settlement of the embankment fill and foundation soils. Rueter-Hess Dam is currently being constructed near Parker, Colorado and is founded on soft claystone bedrock. This paper presents bedrock deformation analyses that were performed to support design of the embankment dam and outlet works conduit. Bedrock deformation properties were developed based on unconfined compression tests, triaxial compression tests, and consolidation tests performed on samples of bedrock core recovered from subsurface explorations. Construction and post construction bedrock settlements were computed using one-dimensional and two-dimensional analyses. Instrumentation and survey data were used to calibrate deformation properties to observed behavior. This paper also discusses features incorporated into the embankment and outlet works to accommodate the predicted construction and post construction bedrock settlement.


General Overview of the Project

Early in project planning it was recognized that the dam site had a storage capacity of about 72,000 acre-feet (ac-ft) if a 200-foot-high dam was constructed and that ultimately, a reservoir of that size would provide a significant asset to the region. However the Owner, Parker Water & Sanitation District, could not show purpose and need for a reservoir of that size without participation from other local water districts. Without adequate purpose and need for a reservoir of that size, it was not possible to obtain a U.S. Army Corps of Engineers 404 permit for the ultimate size project. Therefore, the Owner decided to develop the project in two phases.

The initial phase (Phase I) would include a 135-foot-high earthen dam to create a reservoir that would have an active storage volume of 16,100 ac-ft. One of the key design criteria for the initial phase was that the dam and ancillary facilities needed to be designed to accommodate construction of a future raise up to the ultimate reservoir capacity of 72,000 ac-ft without significant lowering of the reservoir pool or interruptions to reservoir operations.

The dam raise (Phase II) was initially anticipated to be completed several decades after completion and filling of Phase I. However, shortly after the start of construction of Phase I several local water districts requested storage space in the reservoir and planning and design for the raise began.

A general plan of the Phase II dam is provided on Figure 1. The primary components of the dam include:

• EMBANKMENT: A 200-foot-high zoned earth embankment with a crest at Elevation (El.) 6220. The crest is 7,700 feet long and the embankment contains about 14.4 million cubic yards of earth. The dam includes a central/upstream sloping clay core, upstream and downstream stability berms, internal drainage system, and upstream erosion protection.

• OUTLET WORKS/SERVICE SPILLWAY: Combined outlet works/service spillway along the right side of the valley floor that includes: o A reinforced concrete low-level intake structure. o A 78-inch-diameter steel conduit encased in reinforced concrete from the intake

structure to a gate tower. o A 185-foot-tall reinforced concrete gate tower. The gate tower includes gates to

allow selective withdrawal from five reservoir elevations and an un-gated service spillway with a crest at El. 6210.

o Two 78-inch-diameter steel conduits encased in reinforced concrete from the outlet tower to the terminal facilities.

o Terminal facilities at the end of the downstream conduits to regulate flows and to dissipate energy.

o An access bridge from the dam crest to the gate tower. • AUXILIARY SPILLWAY: An excavated approach channel, a reinforced concrete labyrinth

crest control structure with a crest at El. 6213.6, a soil-cement upper discharge channel, and an unlined lower discharge channel.


Figure 1. Embankment Plan.


General Geology and Foundation Conditions

The Rueter-Hess Dam is located within the Colorado Piedmont subdivision of the Great

Plains Physiographic Province, which is generally characterized by an ancient, elevated erosional surface. Remnants of this ancient surface are evident by flat mesa tops that surround the site. The site is located within a broad valley that cuts through the surrounding mesas in a southwest to northeast trend.

Bedrock below the dam consists of 70 million-year-old (Cretaceous age) to 45 million-year-old (Eocene age) sedimentary rocks mapped as the Lower Dawson formation and the Upper Dawson formation. The 34 million-year-old (Oligocene age) Castle Rock conglomerate overlies the Upper Dawson formation and forms resistant mesa tops west and south of the site. A general bedrock geologic map is provided on Figure 2.

The Lower Dawson formation at the dam is predominately interbedded claystones and sandstones with localized lenses of strongly cemented conglomerate. Generally the unit is about 60 percent claystone, 25 percent sandstone, and 15 percent conglomerate. The claystones are generally medium to highly plastic with up to about 30 percent narrowly graded fine sand. The sandstones are generally moderately to well cemented, and comprised of narrowly graded, fine to medium grained sand with up to about 35 percent medium to highly plastic fines. The conglomerates are mostly strongly cemented and consist of fine to coarse grained gravel with a fine to coarse sand material. The lenses of strongly cemented conglomerate are present at depths below about El. 5990 in the valley.

Sediments that comprise the Lower Dawson formation consist of materials that were eroded from the Rocky Mountain Front Range uplift and were deposited in non-marine alluvial channels, meandering stream, and floodplain environments. Prior to erosion of the valley, the top of the Lower Dawson sediments were consolidated by an average of about 260 feet of overburden along the maximum section of the dam.

The Upper Dawson formation is predominately sandstone, with local basal conglomerates and interbedded claystone lenses. The sandstones are generally weakly to very weakly cemented and consist of widely graded, fine to coarse sand with up to 15 percent gravel to 3 inches, and less than 20 percent low to medium plastic fines.

The Upper and Lower Dawson formations are separated based on an ancient erosional surface located below the contact between the two formations. This erosional surface is identified by the presence of intensely weathered and altered bedrock characterized by red or orange sandstone and variegated claystones. The contact between the Upper and Lower Dawson Formations along the dam axis is at about El. 6065 on the right abutment, and El. 6080 on the left abutment. The valley floor is at about El. 6030. The erosional surface between the Upper and Lower Dawson formations dips downward at about 1 degree to the southeast.

In the valley bottom the dam is founded on about 35 to 40 feet of alluvial soils overlying the Lower Dawson formation bedrock. The right abutment of the dam is founded on about 20 to 45 feet of Upper Dawson formation overlying the Lower Dawson formation bedrock. The left abutment of the dam is founded on about 20 to 60 feet of Upper Dawson formation overlying the Lower Dawson formation. The general geology along the axis of the dam is shown on Figure 3.


Figure 2. General Bedrock Geology.

Figure 3. Geologic Section.


Settlement of the predominantly sandstone Upper Dawson formation was not a primary concern for embankment deformation analyses because settlement was anticipated to be minor and to occur as elastic deformation simultaneously with construction. Settlement of the predominantly claystone Lower Dawson formation was a design concern because settlement was estimated to be significant and because consolidation settlement was expected to continue after completion of construction. About 70 percent of the anticipated consolidation of the Lower Dawson formation was estimated to occur after completion of construction.

Although the Upper and Lower Dawson formations are geologically considered to be rock, they generally consist of weakly cemented, over-consolidated sediments and exhibit properties intermediate between those of soil and rock. Similar to the predominantly fine grained Lower Dawson formation, Figure 4 shows areas in the United States where weak mudstones are present near the ground surface. These types of rock are present in the Rocky and Appalachian Mountains, Great Plains, Midwest, and Ohio River Valley.

Figure 4. Weak Rock Locations.

Data Collection

A series of site investigations were completed during both phases of design within the general area of the proposed dam and reservoir footprint and borrow areas to evaluate stratigraphy, perform in-situ tests, and collect soil and rock samples. A total of 18 boreholes within the general footprint of the dam and 10 boreholes in the general area of the outlet works penetrated into the Lower Dawson formation. These boreholes extended from about 8 to 100


feet into the Lower Dawson Formation. The general location of these boreholes that extended into the Lower Dawson formation is shown on Figure 5.

Borings were advanced through rock using NQ- or NX-sized continuous coring equipment. Core samples selected for laboratory testing were wrapped in plastic to help preserve the in-situ moisture content and then wrapped with tape axially to maintain the samples in compression. These procedures were used to reduce sample disturbance of the specimens before laboratory testing was performed.

Figure 5. Subsurface Exploration Locations.


Table 1 summarizes the laboratory tests performed on selected specimens of the Lower Dawson materials to support development of material properties for use in evaluation of deformation. Table 1. Summary of Laboratory Testing.

Borehole Number

Approximate Elevation of

Sample

Test Completed

Unconfined Compression(ASTM 2166)

CU Triaxial (ASTM 4546)

Consolidation (ASTM 4546)

Consolidation(ASTM 2435)

B-405 5983.7 X B-406 5976.3 X B-407 5987.8 X B-602 6024.3 X B-603 6013.8 X B-605 6003.8 X B-607 6008.3 X B-702 6053.3 X B-704 6091.0 X E-701 5987.0 X X E-701 5977.5 X F-402 6103.6 X F-406 6033.6 X F-406 6033.2 X

Initial Model Development

Deformation Model Stratigraphy Two of the most critical parts of developing a representative deformation model are definition of the subsurface stratigraphy and selection of the material properties. Two issues were critical to development of the deformation model of the Lower Dawson formation:

• Identification of a depth where the bedrock below could be reasonably considered to be “incompressible.” This was needed to provide a boundary condition for the model.

• Deciding if the bedrock within the zone above the “incompressible” material should be subdivided into various layers with different material properties or if consistent material properties should be used for the entire layer.

We recognized that impacts from a) weathering of the upper part of the Lower Dawson formation prior to the deposition of the Upper Dawson formation, and b) from vertical stress changes caused by erosion of the bedrock and deposition and subsequent erosion of some of the alluvium would decrease at greater depths into the formation. However, limited defined criteria are available for selection of these two critical model inputs.

For two-dimensional stress-deformation modeling, it is critical to bound the problem by defining a bottom edge of the model geometry as a zero displacement boundary, to behave as a reactionary force to the applied loading. A minimum lower boundary of El. 5990.0 for incompressible claystone was selected based on the borehole records. The weathering and fractures tended to decrease and rock quality designation (RQD) tended to increase based on data from the boreholes at about El. 5980 to El. 5990 in the valley section. A plot of the RQD data is shown on Figure 6. The hardness of the bedrock above this range in elevation


generally varied from very soft (H7) to moderately hard (H4), and below this range in elevation the bedrock is predominately moderately soft (H5) to moderately hard (H4).

Figure 6. Elevation versus Rock Quality Designation.

For deformation analysis of the embankment at the maximum section (STA 25+00) a

conservative lower boundary of El. 5955 was selected to correspond with the installation depths of proposed settlement sensors. For the deformation model of the outlet works, the bedrock upstream of OW Station 17+00 below El. 5990 was considered incompressible, and downstream of OW Station 18+00 the bedrock below El. 5960 was considered incompressible. The top of the incompressible bedrock transitioned linearly between OW Stations 17+00 and 18+00. The gate tower and dam centerline are at about OW Stations 10+00 and 14+50, respectively. These models are shown on Figure 7. Elastic Properties

The two primary methods for evaluating settlement of rock foundations are the elastic method and the finite element method. Both of these methods require that the Modulus of Elasticity (or Deformation) (E) and Poisson’s ration (μ) be selected for the rock.

A major limitation of these two methods is the approximation of the value of E. The value of E can be estimated from either field testing or laboratory testing. Field testing, such as plate load tests, is a preferable method for estimating an applicable value of E, but can be time-consuming and expensive to perform. Field testing was not considered feasible for this project because of the deep excavations that would be required to test the Lower Dawson formation at critical locations.


Figure 7. Deformation Model Geometry.


The Modulus of Elasticity can be computed based on data from the following laboratory tests:

• Unconfined Compression Tests • Triaxial Shear Tests • Consolidation Tests

One of the primary concerns related to developing the Modulus of Elasticity from laboratory tests is that laboratory tests are performed on relatively small specimens that are usually intact and devoid of discontinuities and sample disturbance. Based on published information, modulus values computed using data from laboratory tests are generally higher than field tests because the settlement of rock is strongly influenced by large-scale in-situ properties such as joints, fractures, and faults. Modulus values computed from laboratory tests will also vary because the stress-strain response of rock is not linear. Additionally the modulus values are dependent upon the portion of the stress-strain curve used for computation. The Modulus of Elasticity computed using data from unconfined compression tests is usually considered as an upper bound (USACE, 1994). Using results from unconfined compression tests usually overestimates the value of E, with the overestimation increasing as the RQD decreases. It is important to obtain a significant number of samples to establish this upper bound. Computing modulus from triaxial tests can be more accurate because the specimen can be tested at confining stresses that simulate in-situ conditions.

The value of modulus may also be obtained from consolidation testing. For an applicable range in stress, the coefficient of volume compressibility (mv) can be estimated, with the reciprocal of mv regarded as the constrained or stress-strain modulus (Es). In most cases, actual settlements are expected to be somewhere between settlements computed using E from consolidation testing and compression testing. During design, the following laboratory test data was used to develop the secant elastic modulus for claystone:

• Three Unconfined Compressive Strength Test (ASTM D 2166) • Two Consolidated Undrained Triaxial Tests (ASTM D 4767) • Nine Consolidation Tests (ASTM D 2435 and D 4546)

Secant elastic modulii were computed from the peak deviator stress from each unconfined compression test. This method generally underestimates stiffness at low stresses and overestimates stiffness at high stresses. The corresponding axial strains at peak deviator stresses were generally less than about 2 percent. Based on three unconfined compressive tests, the computed secant elastic modulii varies by about one order of magnitude and generally increases with increasing effective in-situ stress. Unconfined compressive strength tests generally yielded higher computed elastic modulii than triaxial and consolidation tests, which agrees with most published literature (USACE, 1994).

The secant elastic modulii computed from triaxial tests were generally between the values computed from unconfined compressive tests and consolidation tests. The secant elastic modulii were computed from peak deviator stresses similar to the procedure used for the confined compressive tests. The corresponding axial strains at peak deviator stresses were also less than about 2 percent.

Most of the available deformation data was obtained from one-dimensional consolidation tests. Secant elastic modulii were computed from the constrained modulii (Bardet, 1997) and were the lowest of the computed values, which was expected.

The modulus of elasticity was then selected based on the 80 percent exceedance value, which is the point where 80 percent of the laboratory results had an elastic modulus of at least


the design value. The computed secant elastic modulus and the 80 percent exceedance value for the claystone are plotted on Figure 8, which illustrates various computed secant modulii versus the in-situ vertical effective stress. Rock discontinuities were not explicitly considered when selecting an elastic modulus value for the claystone. However, the selected value was conservative with respect to the values obtained from laboratory samples, and in our opinion, was representative of the rock mass. Elastic modulus was assumed to be constant with stress to simplify calculations and because the data did not show a definitive trend with stress (Figure 8).

We developed Poisson’s ratio based on empirical correlations and published data on similar materials (Salgado, 2008). We considered that the claystone bedrock would behave between a saturated to an unsaturated clay, which could range in μ = 0.1 to μ = 0.5. A typical range of Poison’s ratio for a sound intact shale is μ = 0.25 to μ = 0.33. We selected a Poisson’s ratio of μ = 0.27 for the claystone bedrock because it represented a conservative value within recommended range.

Figure 8. Secant Elastic Modulus vs. Vertical Effective Stress.

Consolidation Properties

Initial void ratio and consolidation properties for Lower Dawson claystone were based on results of five consolidation tests. The coefficient of compression (cc) and coefficient of recompression (cr) were developed from a plot of void ratio versus vertical effective stress. The preconsolidation pressure (σ'p) was estimated using Casagrande constructions from the


consolidation tests to be about 65,000 psf. The vertical loading of the claystone was not expected to exceed this pressure, cr was used for foundation deformation analysis. The computed range of cr for the various tests is presented on Figure 9.

Figure 9. Coefficient of Recompression vs. Vertical Effective Stress.

To estimate the rate of dissipation of pore pressures or rate of settlement of the

claystone bedrock, a value of the coefficient of consolidation (cv) was developed. The value of cv = 68 ft2/year was estimated by the square-root-of-time method from five consolidation tests. The computed range of cv is presented on Figure 10. We developed the value of cv as a constant, which is often not the case in the field. A factor that greatly affects the development of cv is sample disturbance, which can greatly reduce the value, especially when calculating the value from the recompression of the specimen. Another factor is the difference in temperature from the laboratory and the field. The laboratory temperature increases is generally higher than the field and the computed value of Cu increases with temperature.

The selected value of the coefficient of recompression (cr) was calculated as the arithmetic mean of the available data and was assumed constant with stress because apparent trends were identified in the available data. The coefficient of consolidation (cv) appears to decrease with stress as shown on Figure 10, but we did not have sufficient data to justify this trend. Therefore, we conservatively used about 15 percent below the arithmetic


mean of the values for design. Values of the consolidation properties, void ratio, and elastic deformation parameters selected for analysis are provided in Table 2.

Figure 10. Coefficient of Consolidation Vs. Vertical Effective Stress.

Table 2. Properties for Initial Deformation Analyses.

Material Type

Deformation Parameters

RecompressionIndex

(cr)

Coefficient of Consolidation

(cv) (ft2/year)

Initial Void Ratio (eo)

Modulus of Elasticity

(E) (psf)

Poisson’s Ratio

(μ) Lower Dawson Claystone 0.049 68 0.72 3.5 x 105 0.27

Initial Analyses of Bedrock Deformation

The initial analyses of bedrock deformation were based on properties presented in

Table 2. Analysis of foundation deformation was a critical part of design of the embankment and outlet works. Deformation of the embankment was used to design the camber, which was based on settlement of embankment soils and foundation soils. The embankment soils were estimated to almost fully consolidate during construction; however, only about 30 percent of the total of the claystone foundation settlement was estimated to occur during construction.


Vertical deformation of the foundation was estimated using one-dimensional analysis and two-dimensional linear elastic finite-element modeling techniques. One-dimensional analyses were completed using Terzaghi’s one-dimensional theory of consolidation. Two-dimensional analysis was performed using the SIGMA/W module of the GeoStudio 2004 software package.

The analyses for the embankment were completed under maximum sections (STA 25+00 and STA 30+00) of the embankment and along the centerline of the outlet works, which crosses the embankment at about STA 21+00. Analyses were performed at these locations because it would allow for direct comparison of predicted deformation and recorded deformation using instrumentation data that would be installed during Phase I.

For the two-dimensional analysis under Phase II embankment loading, we considered that the foundation is saturated and the rate of settlement is controlled by consolidation, not elastic properties.

Results of the predicted foundation deformation for the Phase I and Phase II embankment at the maximum section (STA 25+00), and along the outlet works are shown on Figures 11 and 12, respectively.

Instrumentation and Surveying Data Instrumentation was included in the Phase I dam to monitor the behavior of the embankment, foundation, and appurtenant structures during construction and throughout the life of the project. The instrumentation was designed to monitor critical elements of the dam for use in evaluation of embankment performance and to provide an early warning of changing conditions that could impact the safety of the facility. The following instrumentation and monitoring was established to collect data related to bedrock deformation:

• Vibrating wire piezometers • Settlement sensors • Surveying of the outlet works conduit

The general locations of the vibrating wire piezometers and settlement sensors in the valley section of the embankment are shown on Figure 13.

Readings from the piezometers installed within the bedrock foundation were reviewed, but the data did not display expected behavior of the bedrock pore water pressures under loading. Therefore, the readings were not used for calibration of design parameters.

Settlement instruments S1 and S2 are located below the core along the Phase I dam centerline and record settlement in bedrock. Instruments S3 and S4 are located about 100 feet downstream of the Phase I dam centerline and monitor settlement in the alluvial foundation soils and bedrock. The subsurface profile at sensor S3 consists of about 15 feet of clayey alluvium, underlain by 15 feet of granular alluvium, underlain by 45 feet of claystone. At sensor S4, the subsurface profile consists of about 23 feet of granular alluvium underlain by 52 feet of claystone. Data from Sensor S1 was not used because the data was considered not to be reliable. Data from these settlement sensors are shown on Figure 14.


Figure 11. Total Initial Predicted Deformation at Maximum Section.

Figure 12. Total Initial Predicted Deformation for Outlet Works Conduit.


Figure 13. Locations of Phase I Instrumentation.


Figure 14. Phase I Settlement Sensor Data.

Calibration from Instrumentation

To directly compare the data from the settlement sensors, a time-rate of consolidation

analysis needed to be performed. Using the laboratory value of cv = 68 ft2/year, approximately 22 percent of the total deformation in the bedrock was estimated to occur at completion of the Phase I construction, which was a time length of about 24 months. This percentage of consolidation was then compared to the settlement readings taken at the end of Phase I construction. Predictions versus instrumentation readings generally correlated well and are presented in Table 3. Table 3. Comparison of Instrumentation to Predicted Settlement.

End of Phase I Total Foundation Settlement Instrument

Identification Approximate Location Instrumentation

Data (inches)(1)

1-D Analyses (inches)(2)

S2 Centerline Phase I Dam Crest (STA 25+00) 2.7 3.2 S3 100 Feet Downstream of Phase I Dam Crest

(STA 30+00) 23.3 29.1

S4 100 Feet Downstream of Phase I Dam Crest (STA 25+00)

7.8 10.3

Notes: 1. Total recorded settlement at end of Phase I, includes bedrock and alluvium. 2. Total computed settlement in alluvium and 22 percent of total predicted consolidation settlement in bedrock.


The predicted settlements from the one-dimensional analyses, using values for compression index (cc), recompression index (cr), and initial void ratio (eo) for the claystone developed from laboratory data, correlated well with the recorded data from the settlement sensors.

Monitoring results from the settlement sensors were additionally reviewed to calibrate a value of the Modulus of Elasticity. Based on the settlement at each sensor, initial and final stresses, and the previously described soil/rock thicknesses along the length of the sensors, we solved for E or cc using three equations. The computed E value from this analysis for claystone was 2.2 x 106 psf, which is higher than the E value computed using the laboratory testing of 3.5 x 105 psf. Calibration of E Based on Outlet Works Conduit Survey

The vertical alignment of the outlet works conduit was surveyed during construction and the first post-construction survey was performed in July 2007, which was about 300 days after the end of the Phase I construction. The conduit alignment elevations were established to generally correspond to the maximum estimated foundation settlements and to provide a positive downstream slope along the entire conduit. The survey of the conduit was an optimal method to check construction elevations and to check the predicted deformation behavior of the foundation materials.

The vertical alignment of the conduit based on the July 2007 survey, and the computed alignment of the conduit from the two-dimensional model from SIGMA/W using the laboratory value for E, were compared for the Phase I dam. Based on analyses completed for the embankment, consolidation of the claystone under Phase I loads was estimated to be about 30 percent complete at the time of the July 2007 survey. The survey data was then adjusted to represent total anticipated settlement (consolidation at 100 percent complete). These elevations are provided on Figure 15.

The two-dimensional analysis of the outlet works conduit was re-computed varying the Modulus of Elasticity of the claystone bedrock in the finite-element model from the initial value of 3.5 x 105 psf until the predicted settlement profile for the Phase I embankment approximated the data from the adjusted July 2007 survey. We concluded that a modulus of elasticity in the claystone bedrock of 5.0 x 105 psf resulted in a predicted profile that most closely matched the survey data and was adopted for design. The computed two-dimensional vertical settlement of the outlet works conduit using and E of 5.0 x 105 psf and the survey data from July 2007 is presented on Figure 16.

Final Analyses

Final deformation analyses to support design of the Phase II dam and outlet works and to confirm adequacy of the outlet works components constructed during Phase I were completed using the adjusted Modulus of Elasticity of 5.0 x 105 psf. The computed maximum deformations for the Phase II dam below the maximum section of the embankment and along the outlet works for both the initial analyses and final analyses are presented on Figures 17 and 18, respectively.

The revised deformations in the bedrock for the Phase II dam using the calibrated modulus values are generally about 25 percent less that originally predicted. The predicted deformations were used to a) compute the forces in the conduits to confirm that the previously constructed components were adequate for the applied loads, b) to design the additional


Phase II components of the outlet works, and c) to design the camber for the Phase II embankment.

Design Features to Accommodate Settlement

Total and long-term consolidation of the bedrock impacted design of both the embankment and outlet works. The following features were included in the design of the project to mitigate the impacts from deformation of the bedrock.

The primary impact to design of the embankment was the amount of additional camber needed to accommodate bedrock settlement in addition to settlement of the embankment fill and foundation soils. Table 4 presents the required camber for the Phase II dam that was required based on the results of the initial and final deformation analyses. Table 4. Crest Elevations and Required Camber.

Station

Elevation

Total Camber (inches)(1)

Camber for Bedrock(2)

Right Abutment -17 + 05.8 6220 0 0

8 + 00 6221 12 3 20 + 00 6222 .24 14 34 + 00 6222 24 14 46 + 50 6221 12 3

Left Abutment 59 + 69.7 6220 0 0 Note: 1. Crest of dam slopes uniformly between identified stations. 2. Represents the amount of camber to compensate for bedrock consolidation.

The magnitude and variable deformations of the foundation bedrock below the outlet works presented a greater design challenge than deformation below the embankment. The deformations along the conduit and between the conduit and the gate tower are not uniform because the gate tower is a relatively large (76 feet in the upstream/downstream direction) and is located at the downstream end of the flat upstream berm. The geometry of the embankment is not uniform because of the variation between the upstream and downstream slope and the downstream berm. Typically, the connection between the outlet conduits and the gate tower is designed as a rigid connection. One result of these variable deformations between the gate tower and conduits was that the use of this typical design connection concept would not be possible because the resulting moment from a rigid connection was extreme. The following elements were included in the design of the outlet works to accommodate expected short- and long-term deformation of the bedrock:

• The design elevation of the conduit included a non-uniform slope (camber) that was based on the computed deformations. The design slope was selected to maintain a positive downstream slope along the entire conduit for both the Phase I and Phase II dam. This camber should maintain the conduits in compression.

• The connection between the gate tower and conduits was designed as a pinned connection instead of a rigid connection.

The connection was designed to allow axial movement of the joint and limited rotational movement. The joint was designed to allow up to 1.5 inches of lateral movement and up to 1 to 2 degrees of rotation. The joint detail is illustrated on Figure 19.


Figure 15. Outlet Works Conduit Elevations and Survey Data.

Figure 16. Comparison of Predicted Vertical Deformation for Phase I Outlet Works Conduit.


Figure 17. Predicted Phase II Embankment Deformation.

Figure 18. Predicted Phase II Outlet Works Conduit Deformation.


Figure 19. Gate Tower and Conduit Connection.


Conclusions

Evaluation of deformation is required to support design of any type of dam. However

caution needs to be used when analyses are based on an assumption that deformation of bedrock does not need to be considered. This paper described deformation analyses that were completed to evaluate bedrock settlement to support design of a large earth dam and how settlement of the bedrock impacted design of the project. Similar types of bedrock exist in many parts of the United States and it is important early in the design of a new dam to identify what type of data is needed to investigate not only the strength but the deformation properties of the foundation bedrock.

Typically large-scale field tests are not performed to support deformation analyses needed to design dams because of cost and other practical considerations to collect this data. Also it is rare for a dam to be designed to be enlarged and to include instrumentation and monitoring that is designed to collect data to allow calibration of the analyses models. We offer the following recommended guidelines related to development of material properties for a bedrock deformation analyses.

The depth of influence can be estimated from the site investigation to help determine an incompressible boundary for computing deformation by finite element analysis, because stresses within the foundation rock from foundation loads tend to decrease with depth. In cases where the foundation consists of multi-layered rock masses, the depth of influence should be considered because each layer may have different elastic properties. Attention should be given to changes in intensity of fractures, weathering and values of RQD.

Instrumentation data is not usually available during initial design stages, therefore laboratory data will likely be the initial or only method used to estimate material properties for predicting deformation.

For development of consolidation properties of claystone bedrock, consolidation tests are a preferable method to obtain design values. Selecting the average value for the coefficient of compression or recompression and coefficient of consolidation yield good representative values for predicting deformation. A number of consolidation tests should be performed to have sufficient data for selection of a representative value. A good check of the value of coefficient of consolidation can be performed by comparison to settlement sensors installed within the bedrock. Although the vibrating wire piezometers installed for this project did not provide reasonable data within the bedrock, readings of dissipation of the excess pore water pressure from construction may additionally provide a calibration of cv.

Using laboratory data for selecting a modulus of elasticity has proven to be more difficult. Depending on the method of estimation of modulus, the value will vary and most tests yield a wide range of values for selection. We offer the following observations and opinions for consideration of testing programs and selection of modulus of elasticity for similar geology:

• Perform a practical number of unconfined compression tests to establish an upper bound value and to evaluate the variability of the rock.

• Perform a number of triaxial compression tests. Derivation of modulus from triaxial tests may still yield an upper bound value, but the value should be closer to an appropriate design value and should compare reasonably to other test methods. This test can be compared to consolidation testing and help indicate outliers from the various test methods.


• Perform a higher number of consolidation tests. The value of modulus derived from these tests is likely to be within the closest range to a representative design value. Using the 80 percent exceedance value appears to underestimate the average modulus of the claystone by about 30 percent, resulting in over predicting deformation by about 25 percent. However, without calibration of modulus from survey data or instrumentation, selecting the 70 to 80 percent exceedance value from this test method provides a reasonable conservative and practical value for design.

• Install monitoring instruments (when applicable) in the early stages of construction to review design assumptions and monitor deformation, because the time for weak claystone bedrock to achieve full consolidation will likely occur after construction of the project is complete. A reasonable investigation and testing program was executed; however, upon

examining the deformation of soft bedrock, additional sampling and testing should have been performed within the compressible bedrock. Specifically additional consolidation testing should have been completed to provide more data for the selection of modulus of elasticity. The initial development of modulus provided a value that generated a safe and conservative design; however, using survey data to calibrate the modulus value resulted in a more value-engineered design.

Bedrock settlement can be a significant issue for medium to large earth dams and the potential deformation of the bedrock should be evaluated early in design.

References

Bardet, Jean-Pierre (Bardet) (1997). Experimental Soil Mechanics. Prentice Hall, Upper Saddle River, NJ.

Day, Robert W. (2006). Foundation Engineering Handbook (pp7.34-7.40). McGraw-Hill, New York, NY.

Salgado, Rodrigo (2008). The Engineering of Foundations (p 285. McGraw-Hill, New York, NY.

U.S. Army Corps of Engineers (USACE) (1994). Rock Foundations, EM 1110-1-2908, November 30.

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