COMPREHENSIVE FOUNDATION REHABILITATION AT BEAR CREEK DAM. Conrad H. Ginther 1 John E. Charlton 2 ABSTRACT INTRODUCTION

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1 COMPREHENSIVE FOUNDATION REHABILITATION AT BEAR CREEK DAM Conrad H. Ginther 1 John E. Charlton 2 ABSTRACT TVA s Bear Creek Dam is a high hazard embankment dam in northwest Alabama that provides water supply, flood control, and recreation benefits. Since its initial filling, the Dam has experienced significant seepage through its karst limestone foundation. After experiencing limited or temporary success at controlling seepage using supplemental grouting programs and downstream seepage collection systems, TVA elected to embark on an extensive rehabilitation effort. Paul C. Rizzo Associates, Inc. (RIZZO) was hired to design a robust, permanent solution for the Dam s issues, which include PMF overtopping problems in addition to the excessive foundation seepage. This solution will consist of a downstream RCC reinforcement structure and a composite seepage barrier consisting of a two line grout curtain with cutoff wall panels at select locations. INTRODUCTION Bear Creek Dam is a 1,385 ft long homogenous fill embankment dam constructed in the late 1960 s and first filled in The Dam s crest elevation is 618, has a maximum height of 85 feet, and is equipped with an ogee crest overflow chute spillway (crest el. 602 ) and a gated intake tower to a 9 diameter sluiceway tunnel and stilling basin used to control lake levels under normal conditions. Figure 1 shows the typical maximum section of the embankment dam. Figure 1. Typical Section of Embankment Dam 1 Assistant Project Engineer, Paul C. Rizzo Associates, 101 Westpark Blvd Suite B, Columbia, SC 29210, conrad.ginther@rizzoassoc.com 2 Senior Project Geologist, Southeast Region Manager, Paul C. Rizzo Associates, 101 Westpark Blvd Suite B, Columbia, SC 29210, john.charlton@rizzoassoc.com

2 The Dam was constructed with a single line grout curtain and well cleaned key trench for approximately two thirds of the embankment foundation. During the initial construction, numerous solution features and voids were encountered and backfilled, large volumes of extremely weathered rock were removed, and large grout takes were common. Upon first filling in 1969, seepage was discovered along the toe of the embankment and has been the subject of various studies and treatment programs since, with sustained foundation seepage flows measured on the order of 800 gpm at normal summer pool levels. Subsequent grouting programs were successful at temporarily reducing flows to around half of the historical maximum, however the grout curtain was never brought to closure and over time, flow reductions were lost. In December of 2004, a high headwater event resulted in the appearance of numerous boils, small sinkholes, and new seepage flows from the toe. A study consisting of piezometer installation, coring of the foundation rock, and cpt drilling in the embankment performed after the flood identified the left abutment foundation as the source of the majority of the seepage. Photo 1 is an aerial photo of Bear Creek Dam at high flood stage in Photo 1: 2004 Flood Event HW at El. 604 The Dam provides flood control, water supply, and recreational benefits to the area. In order to preserve these benefits, TVA elected to embark on an extensive rehabilitation effort. Paul C. Rizzo Associates was hired to design a robust, permanent solution for the Dam s issues. Following an exploratory drilling program and preliminary design phase, an RCC reinforcement structure downstream of the existing embankment dam and a composite seepage barrier consisting of a two line grout curtain with cutoff wall panels at select locations were chosen as the best solutions for remediation of Bear Creek Dam. Figure 2 shows the design maximum section for the RCC reinforcement structure and cutoff.

3 Figure 2: Design Maximum Section for RCC berm RCC REINFORCEMENT STRUCTURE FOUNDATION EXCAVATION AND DENTAL TREATMENT Initial Exploratory Program Bear Creek Dam is located in southwest Franklin County, Alabama, and lies at the contact of the Cumberland Plateau and the Fall Line Hills of the Coastal Plain Physiographic Provinces of Alabama. The site vicinity is characterized by stream valleys incised into Mississippian rocks of the Parkwood and Bangor Formations and ridges and plateaus capped by Cretaceous sediments of the Tuscaloosa Group. The site consists of Mississippian Age ( Ma) rocks of the Bangor Formation. In summary, from rock surface to depth, the geologic cross section of the foundation consists of the upper Bangor Limestone (cherty crystalline limestone and fossiliferous packstone), the Bangor Shale, and the lower Bangor Limestone (fine grained oolitic packstone). The various rock characteristics within the Bangor formation have proven significant to rehabilitation of the foundation. Specifically, the packstone member of the

4 Bangor, which occurs approximately between elevations 538 and 560 at the site has proven, through field observations and in lab results, much more susceptible to solution activity. The packstone has proven the most challenging zone of the subsurface with respect to grouting and foundation preparation due to large solution features and weathered zones that are not present to the same extent in overlying and underlying crystalline and cherty limestone layers. The initial subsurface exploration program for the Bear Creek Dam Reinforcement Structure included core drilling, borehole pressure testing, limited soil sampling, geophysical borehole logging, surface geophysics, field and lab testing, and groundwater flow analysis. Site investigation began in June of 2007 and was significantly completed in September, The main objectives of the site investigation were to: Determine characteristics such as rock quality designation (RQD), unconfined compressive strength, bedding thickness and composition, degree of solution feature development, and weathering of the upper and lower units of the Bangor limestone and the Bangor shale unit C. This information was be used to define the extent to which a grout curtain and cutoff wall are the appropriate methods of serving as a long term fix for seepage through the Upper Bangor Limestone, and to identify the excavation surface of the RCC berm foundation. Determine hydraulic properties of the rock mass including extent and nature of karst development. Determine the thickness and characteristics of residual soil, fill material, and alluvium in and upstream of the proposed foundation to assist in de-watering system design, and to estimate excavation quantities. Twenty four core borings were advanced through overburden, the upper Bangor limestone, Shale Unit C, and into the lower Bangor Limestone and water pressure tested to determine hyrdaulic properties of the rock mass. Surface geophysical investigations including surface assessment of seismic waves (SASW), refraction, and microgravity in multiple lines in strike with the proposed foundation of the RCC berm were completed. Resistivity and ground penetrating radar were used in limited locations in combination with additional core borings to better define weathered zones near the rock surface. In addition to site investigation field activities, TVA construction records and previous remedial grouting program records were reviewed and incorporated in development of the overall site model. Data collected were used in order to assess the suitability of grouting and cutoff wall options for a seepage barrier and performance of the rock mass as a RCC berm foundation.

5 Rock Excavation and Cleaning of Features The Foundation Design Criteria for the RCC Berm foundation at Bear Creek is defined as competent rock as initially determined by the 2007 subsurface investigation and verified during foundation preparation. Foundation acceptance criteria is partially weathered rock (USBR, 2001), rock mass rating of good (60 or higher), and treatment of discontinuities according to the industry rule of thumb of a minimum excavation to depths of three times the width of the feature or (0.3)(width) + 5 ft. for features greater than two feet in width. Foundation rock was shaped to remove overhangs and steep surfaces. High rock surfaces were cut back to remain stable during construction and to maintain a smooth continuous profile to minimize differential settlement and stress concentrations within the RCC berm. Treatment of the exposed rock surface after removal of overlying soils depended on the type of rock and the irregularities present. The presence of weathered zones and solution features along geologic discontinuities has the potential to affect both the stability and the deformation modulus of the foundation. In cases where weathered rock and detritus in open cavities had to be removed and cleaned to depths as great as eighteen feet below the foundation grade, dental concrete was used to fill excavated deep weathered zones. The configuration of exposed hard rock surfaces was controlled by stratigraphic and structural characteristics. Specifically, a packestone layer containing up to 96% CaCO 3 bounded above and below by a cherty limestone containing 52% CaCO 3 exhibited significant karst karst features. In addition, joints associated with bedding planes, and two steeply dipping regional fracture sets striking roughly N30E and N55W acted as zones of accelerated weathering. Photo 2 shows an overview of the foundation with exposed N30E weathered zones. Depending on discontinuity orientations, these features sometimes resulted in horizontal surfaces, vertical surfaces, benches, deep depressions, and overhangs. Generally, the foundation surface was shaped adequately by conventional excavation of track hoe and hoe ram. Over-excavation was appropriate in zones of weathered rock along the aforementioned discontinuities.

6 Photo 2: Cleaning of N30E features The bedrock foundation for the RCC Berm was cleaned to provide acceptable conditions of contact between the body of the Dam and its foundation, and to provide for observation and documentation of details of foundation conditions at that interface. Exposure of potentially adverse conditions during cleanup provided the chance to undertake remedial activity. Photo 3 displays the primary method used to achieve initial bedrock foundation cleaning after excavation. Photo 3: Foundation Cleaning with high pressure air and water

7 After excavation, all loose or objectionable (weathered) material was removed by handwork, water jetting, and/or air jetting. Accumulated water and debris from washing operations was typically removed by a hydro-excavator or vacuum truck. Loose or unsuitable material, in cavities, fractures or seams, was removed. The rock surface and all pockets or depressions were carefully cleaned of soil and rock fragments before dental concrete could be placed. Dental concrete was also used to fill or shape holes, grooves, and extensive areas of vertical surfaces created by fractures, buried karst features, and other irregularities. Thin areas of dental concrete over rock projections on a jagged rock surface are likely places for concrete cracking and were avoided by using a sufficient thickness of dental concrete. The rock surface was thoroughly cleaned as described above and moistened prior to concrete placement to obtain a good bond between the concrete and the rock surface. When overhangs were filled with dental concrete, the concrete was well vibrated and forced into the opening by keeping the head of the concrete higher than the upper surface of the overhang. Dental concrete was typically cured with water and operations were not permitted over the dental concrete until 48 hours of curing. Final Cleaning and Foundation Approval Final foundation cleaning was achieved by use of pressure washers and a vacuum truck (see Photo 4). Photo 4: Final cleaning with vacuum truck and pressure washer The rock surface was cleaned so that partially weathered to fresh rock was exposed for dental concrete placement. Once cleaned, a geologist mapped the foundation, describing degree of weathering, hardness, lithology, and locating and describing discontinuities per the USBR Engineering Geology Field Manual. Once mapped, photographed, and

8 verified as suitable for foundation surface, the geologist completed a foundation acceptance form for the prepared area in order to track the approval procedure. Data acquired in the geologic mapping process were then converted to maps in AutoCad in order to prepare a final record of foundation conditions. Figure 3 provides an example plan view foundation geologic map. Figure 3: Geologic Map of Foundation Dental concrete was placed in approximately one foot lifts using a pump truck in order to prevent segregation. Each concrete truck was tested for slump and a set of cylinders was prepared for each fifty cubic yards of concrete in order to provide concrete strength data. Once dental concrete was placed in cleaned solution features and crevasses, an as built survey of the top of concrete was completed in order to track the foundation treatment progress. Additional assessment of the foundation cleaning and dental placement was completed using data from the grouting program which followed foundation excavation and dental treatment process. Water pressure tests of core holes through the dental concrete and visual inspection of core through the concrete rock interface yielded evidence of no water takes and clean concrete-rock contacts. Preparation of the foundation required excavation of approximately 40,000 cubic yards of residual soil, 25,000 yards of alluvium, 6,000 yards of fill, and 10,000 yards of moderately to intensely weathered rock. 5,500 yards of minimum 3,000 psi dental concrete were placed in irregularities in the foundation, and an additional 600 yards were placed in order to prepare a working surface for grouting rigs. DRILLING AND GROUTING PROGRAM Design and Implementation of Drilling and Grouting Program The Bear Creek Drilling and Grouting Program was designed with three objectives: first, to effectively seal groutable (ie, relatively clean and open) fractures and voids in the

9 solutioned and weathered rock mass under the foundation of the RCC reinforcement structure, second, as an exploration and design tool to determine the necessary extents of the cutoff wall panels at locations where ground conditions such as clay infill and intense weathering would limit the effectiveness of a grout-only barrier, and third, to act as a preliminary treatment to facilitate the possible construction of cutoff panels. In order to meet these objectives, the program was designed to provide the maximum amount of subsurface information possible in real time to increase understanding of the foundation conditions, to create a treatment spacing with enough resolution to limit the possibility of leaving untreated windows in the seepage barrier, and to provide grout mix properties that would facilitate treatment of the foundation. Real Time Monitoring, Data Collection, and Reporting Computer controlled real time data monitoring of water pressure test and grouting parameters was required for all stages of testing and grouting. Real time data logging proved invaluable to effective treatment of the foundation, whether by indicating the type of flow condition in a water pressure test, or as a tool to guide grout mix changes to effectively react to the ground conditions being encountered in a particular stage during grouting. To ensure that all available information was accessible as a resource to guide the grouting program, foundation preparation activities, and selection of cutoff panel sections, daily updates of the drilling, water pressure test results, and grout results were required. Results of daily activities were plotted on a color coded subsurface profile showing the existing geology, borings, stages, and takes during pressure tests and grouting. An example of the water pressure testing and grouting is shown in Figure 4. Figure 4: Example Subsurface Profile

10 The use of a color coded scale for water pressure test results made identification of areas of high conductivity in the foundation simple and intuitive, guiding the selection of higher order treatments and helping to further develop details in the site geologic model. Subsurface Exploration In order to develop understanding of the foundation conditions, a comprehensive system of logging both Exploratory (HQ size core) and Production (rotary percussive drilling) borings including geophysical methods was enacted for the drilling program. A total of 34 exploratory HQ size core holes were placed on 80 centers on both lines of the grout curtain. These borings were logged conventionally by a geologist in the field as the core was recovered, then subjected to geophysical logging after being washed thoroughly when the coring was completed. Geophysical logging included photographic logging of the walls of the core hole with a downhole optical televiewer camera capable of identifying bedding features and fractures and producing a 360 degree view of boring sidewalls, gamma logging to assist in delineating bedding features (primarily shale lenses), and caliper logging to measure spatial deviations in the sidewalls of the core hole. Figure 5 shows examples of typical logs for downhole camera and gamma and caliper logging. Upon completion of coring and geo-logging of the exploratory borings, the borings were water pressure tested using 5 step Houlsby type tests and grouted when necessary. Figure 5: Typical Optical Televiewer and Gamma/Caliper Logs Upon completion of the exploratory borings in a given area, production drilling starting with primary holes and proceeding to higher orders was performed. Production drilling was performed using a rotary percussive drill rig with water used as the flushing medium. In order to gain information from these destructive drilling techniques, a Drilling Parameter Recorder (DPR) was installed on the drill rig, which recorded drilling rate, thrust pressure, drilling torque, and water flow through the drill string for every boring performed. Through the course of the project, the DPR logs generated proved to be a valuable resource for identifying areas of fractured rock, clay infill, and changes in

11 stratigraphy, particularly as the driller s understanding of site specific ground conditions improved. Figure 6 shows a typical DPR log output. Grout Mix Properties Figure 6: Typical DPR log output Grout mix properties such as viscosity, pressure filtration, and bleed to a large extent dictate the effectiveness and durability of grout injection treatments. For example, very low viscosity (highly flowable) grouts are suitable for treatment of relatively fine fractures, while higher viscosity grouts are typically necessary to effectively seal more open fractures. In cases of large voids and or flowing groundwater, Low Mobility Grout may be required to arrest flow and fill the void. Due to the extreme variability of the subsurface conditions at Bear Creek Dam, both High Mobility Grout (HMG), for relatively small, open features and fractures, and Low Mobility Grout (LMG) for large voids and subsurface flows that cause washout of HMG. A suite of three balanced, stable HMG mixes with the following parameters was required in the design phase: Table 1: High Mobility Grout Parameters Parameter (unit) Mix A Mix B Mix C Purpose of requirement Bleed (%) Low bleed prevents voids caused by grout settlement (stability) K pf (min -1/2 ) Low K pf corresponds to low formation of filter cake, promotes long distance penetration into fractures Viscosity (sec) Provide range of viscosities to adjust

12 Stiffening Time (hr) as appropriate to subsurface conditions Provide enough time for mix, injection, travel prior to set In addition to the required suite of mixes, the contractor performing the drilling and grouting program elected to add a medium mobility grout (MMG) mix to the suite, consisting of Mix C batched with the addition of fly ash. This mix proved very useful in bringing large take holes to closure through the course of the project. Because of the particular nature of LMG injection, a mix and method was not directly specified, but left at the discretion of the Contractor to propose a method according to his proprietary equipment and experience. Grout Curtain Resolution and Closure In order to provide a continuous seepage barrier and to clearly characterize the subsurface under the foundation of the RCC berm, the two line grout curtain extends from the area downstream of the left abutment of the existing embankment, across the existing spillway structure, and terminates at the right abutment of the RCC berm to the north. Figure 7: Drilling and Grouting Program Layout To provide a tight enough treatment spacing to limit the possibility of leaving untested or untreated windows in the foundation, two grout lines, 10 feet apart, of opposing holes inclined 15 from vertical were planned. The A line parallels the RCC centerline to the upstream, while the B line parallels the RCC centerline to the downstream. Primary and secondary borings on each line were set at 20 foot center to center spacing, with tertiaries and higher order borings split spaced as indicated by the results of lower order holes. To provide a clear means of identifying the order of performance and location of borings, a system employing the boring order, grout line, and RCC stationing at the location was developed, as shown below: For a primary boring on the B line at RCC Station 8+52, boring label is: PB 8+52

13 Figure 8: Typical Grout Hole Layout Primary borings were extended into the Bangor Shale layer, while the depth of higher order borings were generally selected based on available information such as stratigraphy, previous water pressure tests results, etc. In light of the highly variable nature of the karst terrain at the site and the need to provide fine resolution of the curtain, completion of drilling and grouting activities through second order borings was required as a minimum regardless of the results of primary borings. The criteria for achieving closure of a given portion of the curtain lines was a zero take water pressure test in the next higher order borings after having had takes in lower order holes. For example, after takes in adjacent primary and secondary borings, a tertiary boring would be drilled and water pressure tested. A test result less than 5 Lugeons (the criteria for closure of the curtain) in the tertiary boring would stop the progression to higher order borings. As both lines in a given section of curtain were brought to closure, verification borings (HQ size core) were performed between the A and B lines at areas of interest as indicated by previous grout results. A multistage water pressure test result of 5 Lugeons or less in the verification borings was the criteria used for acceptance of closure of the grout curtain. Sluiceway Treatment Program The headwater levels at Bear Creek Dam are controlled via an intake tower and sluiceway tunnel through the right abutment of the embankment dam, with a stilling basin and outlet works just downstream of the toe of the embankment. At the time of construction, the sluiceway tunnel intersected several large solution features or areas of significant clay infill. An original contact grouting program consisting of circumferential drilling and grouting at numerous sections along the sluiceway alignment was performed from inside of the tunnel at the time of construction. In the course of foundation excavation and dental treatment, a large solution feature was discovered crossing the sluiceway tunnel, partially within the planned footprint of the RCC berm. Based on inspection of the surrounding rock during dental cleaning and backfill of this feature, it was determined that the rock mass surrounding the sluiceway tunnel could represent a perforation of the seepage barrier that the originally planned two line grout curtain might not treat effectively. To reduce the potential of the sluiceway

14 acting as a seepage path, a two part treatment program was prescribed, including a contact grouting program consisting of vertical holes at the tunnel liner/rock contact and a rock mass treatment program consisting of angled holes targeting the rock around and below the tunnel. Figure 9 shows a plan view layout of the Sluiceway Treatment Program. Figure 9: Layout of Sluiceway Treatment Program Results of Drilling and Grouting Program Two Line Grout Curtain Pressure grouting using balanced, stable mix designs and closely monitored injection methods was an effective seepage treatment method for the majority of the footprint of the RCC reinforcement structure. The combination of carefully controlled grout mixes with predictable rheological characteristics combined with real time data allowing the contractor and engineer to respond directly to grouting conditions, ensuring quantifiable results and guiding the progress of further treatments. Proof of the effectiveness of grouting the karst foundation at Bear Creek Dam include the results of the verification testing performed between the grout lines at locations of high pre-treatment permeability, and visual observations of downstream flows into the spillway tailrace and in posttreatment excavations performed in the dry within the foundation. One example of a visual expression of the grout curtain s effectiveness is discussed below. Prior to the performance of grouting operations, significant and continuous seepage flows were collected by two steel pipes installed into the bedrock in the spillway tailrace at the downstream face of the spillway chute. Flows from these pipes were enough to support aquatic life under normal conditions, and caused violent boils and outflows during flood conditions.

15 Photo 5: Seepage flow from collector pipes eliminated post-grouting As shown in Photo 5, after the completion of both lines of the grout curtain in the left abutment area, flows in the collector pipes stopped completely, even during high water events. Sluiceway Treatment Program The Sluiceway Treatment Program treated the rock mass around the sluiceway tunnel from the tunnel headwall to approximately 70 feet upstream of the headwall, at the first bend of the tunnel. Rock mass treatment was performed via angled holes penetrating under the tunnel, and generally consisted of low or no-take test results in the borings. Closure of the rock mass treatment was verified based on the test results of higher order borings. The contact grouting portion of the Sluiceway Treatment program encountered several instances of deleterious materials in the area around the interface between the tunnel liner and foundation rock, including form wood and thin clay lenses. A total of ten borings, 5 on each side of the tunnel contact were drilled and tested at or inside the intersections of the two line grout curtain and the sluiceway tunnel, two of which required grouting. The final results of the contact grouting program indicate that any potential flow paths are sealed, however, due to the existence of the clay lenses and embedded wood, a piezometer pair will be installed nearby to monitor long term performance of the seepage barrier. Indications for Cutoff Wall Panels At several locations, ground conditions prevented the grouting program from providing a robust seepage barrier that would be effective in the long term. These ungroutable conditions correspond to significant subsurface clay infill encountered at two locations

16 near the left abutment, the existence of very weathered zones in the Bangor Shale geologic unit in the old river channel. Clay infill conditions were encountered in the drilling and grouting program between RCC Stations 7+00 to 7+40 and 8+00 to 8+67 from depths as shallow as 5 feet up to depths of 25 feet. Clay infill conditions at these locations were determined to be caused by crossing solution features or caves concealed by very hard cherty caprock opposed to the larger solution features, which had surface expression. These connecting caves were full of clay and detritus to varying degrees, with some large voids with significant underflows encountered. Locations where void and underflow activity was encountered required the injection of around 50 cubic yards of Low Mobility Grout (LMG) to arrest the seepage flows. In the area of the old river channel, much less limestone cover exists over the underlying Bangor Shale layer. As a result of being less protected from weathering, the shale layer that at other locations on the site acts as a near-continuous water barrier was found to have several very weathered zones through it. Where slightly weathered, the shale typically would demonstrate hydraulic conductivity in water pressure testing, but exhibit very low grout takes. Where very weathered, grout takes were high and often grouting operations resulted in grout connections to other borings and the rock surface in the surrounding area. In all, four cutoff wall panels were prescribed in light of the results of the drilling and grouting program and a subsequent exploratory drilling program consisting of destructive borings on 2 or 3 foot centers. These panels, their limits, and the reason for the panel are outlined below: Cutoff Panel Number Station Extents Table 2: Cutoff Panel Information Expected Max. Depth (ft) Reason for Panel Panel to Clay infill/void activity at depths up to 30 Panel to Clay infill at depths up to 30 Panel to Panel to Cutoff very weathered zones in the Bangor Shale at the maximum section of the new structure. Cutoff the continuation of sluiceway solution feature, act as test panel for construction method. Construction of these Cutoff Panels is currently ongoing and is planned to be completed before the end of 2008 in preparation for the construction of the RCC reinforcement structure.

17 CONCLUSION TVA s Bear Creek Dam is a high hazard embankment dam built in the late 1960 s. The dam s historical problem with dangerous volumes of foundation seepage and vulnerability to overtopping in the PMF induced TVA to procure a robust, long term solution to the dam s issues an RCC reinforcement structure with an effective foundation seepage barrier downstream of the existing embankment. The principal solution to the foundation seepage problem consists of provision of a well cleaned foundation, installation of a two line grout curtain, and construction of several cutoff wall panels at areas indicated by the results of the drilling and grouting program. To date, foundation cleaning and dental treatment and the two line grout curtain have been completed and have had immediate results on foundation seepage. Currently, four cutoff wall panels are being constructed and verified in preparation for construction of the RCC berm in the winter of