Pump Station Excavation

Transcription

1 Pump Station Excavation SPONSORED BY THE KIEWIT CORPORATION A capstone project for the The Department of Civil & Environmental Engineering in The Ira A. Fulton College of Engineering and Technology Brigham Young University Prepared by Spencer Esplin, Braden Error, Philip Lemperle, and Conrad Smith 4/2015

2 Executive Summary The plans for the Pump Station at the Everist Reservoir site have been reviewed and a temporary excavation has been designed for the installment of the pump station. The excavation will consist of a 2:1 sloped portion and a soldier pile wall at the toe supporting the vertical portion. Thrust walls have also been designed for the tunnel boring machine to be used in connecting the Fort Lupton East and Hill-Oakley cells to the Golden cell. The two thrust walls consist of two drilled piles connected by a waler, for each tunnel. These designs for the excavation and the thrust walls are described herein. A risk analysis is also included in the appendix. 1 P a g e

3 Table of Contents Executive Summary... 1 Introduction... 3 General Overview... 3 Excavation Design... 4 Surface and Subsurface Geologic Information... 4 Design... 5 Thrust Wall Design... 7 Piles... 8 Waler Summary Appendix Idealized Soil Profile Excavation Design Calculations Thrust Wall Calculations Waler Shear and Moment Diagrams Risk Assessment P a g e

4 Introduction This report investigates and presents the results for the design of an excavation that will be completed near the city of Aurora, CO. Also presented in this report is the design of a thrust wall that will be utilized to provide the required reaction force in a pipe-jacking micro-tunnel operation. The "Everist Reservoir Golden Cell Pump Station Geotechnical Report" presented by Deere & Ault Consultants, Inc. dated March 13, 2009 is referenced extensively throughout this report. These designs have been completed by following general practices and guidelines from the Federal Highway Administration (FHWA) and the California Department of Transportation (Caltrans). General Overview The Everist Reservoir site is located in Weld County, Colorado on the bank of the South Platte River. The site consists of seven gravel pits: Golden, Hill-Oakley, Parker-Panowicz, Swingle North, Swingle South, Fort Lupton East, and Fort Lupton West. All seven of the cells are located on the alluvial terrace formed by the South Platte River. Soil-bentonite slurry walls have been built around the Fort Lupton East, Golden, and Hill-Oakley pits to prevent seepage (see Figure 1). Figure 1. The location of the gravel pits and the proposed pump station. The proposed construction consists of building a pump station connecting the Fort Lupton East, Hill-Oakley, and the Golden Reservoir cells through conduits. Other structures include a 3 P a g e

5 discharge conduit near the river and an electrical control building adjacent to the pump station (see Figure 2). Figure 2. Plan of the proposed pump station, excavation, and microtunnels. The main objective of the project is to build a pump station in a deep and open excavation on the northwest corner of the Golden Reservoir Cell. It is necessary for the excavation to be approximately 60 ft. below the ground level. The conduit into the Golden Cell will be constructed in an open bedrock excavation from the pump station into the southeast reservoir. The microtunnels to the Hill-Oakley Reservoir and East Fort Lupton cells will be constructed using a Tunnel Boring Machine (TBM). Significant aspects include the excavation design as well as the creation of the TBM thrust wall. The project will allow for the efficient flow of water within the Golden, Hill-Oakley and East Fort Lupton cells. Excavation Design Surface and Subsurface Geologic Information Within the immediate vicinity of the proposed excavation site, there are four (4) assumed geologic units: 4 P a g e

6 1. Overburden (0-5 ft) - Stiff, brown, sandy clays and medium dense clayey sands. It is assumed that all overburden has been removed from the immediate excavation site and is not considered in the calculations and analysis. 2. Alluvial Sand and Gravel (25-55 ft thick) - The sand and gravel is medium dense to dense and ranges from very sandy gravel to gravelly sand. These soils have an assumed moist unit weight (γ m) of 123 pounds-per-cubic-foot (pcf) and a friction angle (φ') of Alluvial Mud Lens (2-15 ft thick) - The mud lens lies almost directly in the middle of the sand and gravel and consists of stiff, gray to brown sandy clays or clayey sands. The mud lens has an assumed moist unit weight of 121 pcf, cohesion (c') of 100 pounds-per-squarefoot (psf), and friction angle of Laramie Formation Bedrock (depths at ft) - The bedrock mainly consists of claystone interbedded with clayey sandstone. The bedrock has an assumed moist unit weight of 121 pcf, cohesion of psf, and friction angle of 24. It is assumed that the inside of the slurry wall of the Golden Cell has been dewatered and groundwater will only be encountered in the bedrock formation. If groundwater is encountered at shallower depths, further analysis will be required and a more robust design will be likely. The idealized soil profile used in calculations and analysis is included in the appendix. Design The design for pump station construction excavation will consist of an open slope excavation paired with a vertical earth retaining system in the lower 14 feet of the excavation. The cut slope of the excavation will begin at a 15 foot horizontal distance from the slurry wall in an attempt to reduce slurry wall disturbance. The open slope will be graded at a slope ratio of 2:1 (Horizontal: Vertical) to a depth of 45 feet below the top of the cut slope. A horizontal path and drainage canal will be included at the end of the open excavation to facilitate water drainage. The final 14 feet of the excavation will be vertically supported using soldier piles and wood lagging. Calculations for design of the excavation can be found in the appendix. The soldier piles will be W 12 x 35 steel shapes, spaced 8 feet on center around the vertical portion of the excavation in 24 inch drilled shafts backfilled with 100-psi lean-mix concrete. Each pile will be a minimum of 28 feet long, 14 feet of which will be placed below ground and the remaining 14 feet will compose the retaining wall. A section view of the wall is shown in Figure 3. Lagging between piles will be no less than 3 inches thick. 5 P a g e

7 Figure 3. Section view of soldier pile. UTexas 4 was used in the slope stability analysis of the excavation. The unrestrained excavation, without external reinforcement, was modeled and the results are shown in Figure 4 and Figure 5. The factors of safety for the two failure planes were 1.2 and 1.3 as shown in the figures. While this would suggest that a soldier pile wall would not be necessary, the rapid deterioration and slaking potential of the claystone requires some form of reinforcement. Figure 4. UTexas results for failure plane 1. 6 P a g e

8 Figure 5. UTexas results for failure plane 2. Simplified Rankine pressure theory was used to determine the size and length of the piles. To avoid the unrealistic result of a tension crack assumptions, the maximum horizontal active pressure acting on the up-slope side of the wall was assumed to be 25H where H is the height of the vertical portion of the wall. The height, H, can be multiplied by factors varying between 25 and 40, depending on soil type and conditions. It was determined that due to the cohesion in the claystone, 25 would be an appropriate value. The excavation has been designed with a design life of two years to facilitate construction of the pump station. The support of excavation design is not intended to withstand forces developed by seismic activity. Thrust Wall Design Two thrust walls are needed to complete the tunnel boring for the conduits connecting the Fort Lupton East and Hill-Oakley cells to the pump station. The TBM will be stationed in the Golden cell and will exit into each of the other cells. The thrust wall will need to resist a force of approximately 700 kips applied by the TBM jack. This force is the yielding capacity of the pipe. A few design alternatives were considered including driven piles with an attached concrete block, but the following design was determined to be optimal. The reasons for this design are included in the following descriptions. Calculations can be found in the appendix. 7 P a g e

9 Piles Piles will provide the lateral resistance for the thrust wall. The piles will be steel W 24 x 146 shapes, 15 feet long, and placed in 36 inch drilled shafts. The drilled shafts will be back filled with lean mix, 100 psi concrete. Driven piles were considered but determined not to be feasible due to the hardness of the claystone. Two piles per thrust wall will be sufficient. Four- and six-pile groups were checked but proved to be excessive. The piles will be located 46 6 (for the Hill- Oakley tunnel) and 50 6 (for the Fort Lupton East tunnel) from the soldier pile wall, opposite of the tunnel openings, and spaced 5 feet apart on center, as shown in Figure 6. Figure 6. TBM and thrust wall plan. 8 P a g e

10 Three feet of unexcavated claystone will remain behind each pile, adding to the passive resistance of the soil. This means that the drilled shafts will extend 12 feet into the claystone, and 3 feet of the pile length will be free to restrain the TBM jack. The section view of one pile is shown in Figure 7. Figure 7. Section of thrust wall pile. 9 P a g e

11 Soil resistance was calculated using Rankine passive pressure theory. Arching effects, according to the 2011 California Trenching and Shoring manual, allows for a total width of 11 feet. To calculate the demand on the steel, the pile was modeled as a cantilever at a point 2 feet below the ground surface. These methods used the diagram shown in Figure 8. The critical factor of safety was that against yielding in the steel. The factor of safety against yielding is The piles should be stiffened at the connections to the waler to avoid local yielding. Waler Figure 8. Pressure diagrams for thrust wall pile. The waler is also a W 24 x 146 steel shape. It will be 7.5 feet long, centered between the pile pairs and connected 1.5 feet from the top of the pile (with respect to the waler s center). Two walers will be needed; one for each thrust wall. The factors of safety against bending and shearing are 6.87 and 2.01 respectively. A smaller shape could be used, but it was determined that it was more efficient to use the same shape as the piles. Also, the waler demanded less space than the concrete block alternative. The waler will need to be stiffened at the connections to the piles to avoid local yielding. Opposite the piles, the waler will receive the force from the TBM jack through a 3 x 7.5 backstop and a 4 x 8 plate. The waler and plate are shown in Figure 7 with the pile. 10 P a g e

12 Summary An open pit excavation will be completed in the north-west portion of the Golden cell with steepest slopes at 2:1 (Horizontal: Vertical) with a 14 foot vertical soldier pile wall at the base of the slope. The excavation will provide construction access for the building of the pump station. Full design and specifications can be found in the body of the report and appendix. The micro-tunnels connecting the Fort Lupton east and Hill Oakley cells with the Golden cell will require a tunnel boring machine for construction. A thrust wall was designed to withstand the force from the tunnel boring machine. The design and specifications for the thrust wall can also be located in the body of the report and appendix. 11 P a g e

13 Appendix Idealized Soil Profile Excavation Design Calculations Thrust Wall Calculations Waler Shear and Moment Diagrams Risk Assessment 12 P a g e

14 Idealized Soil Profile 13 P a g e

15 Excavation Design Calculations 14 P a g e

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18 Thrust Wall Calculations 17 P a g e

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20 Waler Shear and Moment Diagrams 19 P a g e

21 Risk Assessment The following is a preliminary analysis of possible safety risks for the Pump Station Excavation and Thrust Wall Design. All foreseeable control measures will be applied to prevent any compromise of safety. Risk: Laramie formation bedrock is lower than initially confirmed in the soils report. This is a substantial risk when considering the inconsistency in bedrock depths throughout the site. Cost: Any wrong information given in the soils report will result in supplemental time to accommodate a change in design and the implementation of said design. If the bedrock is at a location that is not in conformity with what was reported, the designed excavation will be compromised. Additional retention will be essential for a safe excavation. Risk: The existing sand and gravel alluvium and mud lens will not support the proposed excavation slope. Cost: The proposed excavation slope would be appropriately changed to accommodate a weaker soil. This change in design would require additional time but no immediate threat to personnel safety. Risk: Excess water presence in the excavation site from failure in the slurry wall, trapped water deposits resting above the mud lens, discharge from the bedrock, or any other source. Because of discrepancies in the amount of water discharge from the bedrock (anywhere from a few gallons per minute to 50 gpm), risk from water is probable. Cost: The presence of excess water can pose potential danger from compromised excavation. The necessity for a change in excavation would create additional cost. Risk: Foundations for the pump station, the electrical control building, and the discharge structure fail because of errors in the soils report. Cost: The cost for failure in the mentioned foundations would include additional design to accommodate the lower bearing capacity of the soil as well as construction time. Risk: Laramie formation bedrock is exposed too long and deteriorates. It is suggested to have the bedrock covered in shotcrete or a concrete mud slab upon exposure to air or water. Failure to cover the bedrock will cause it to slake and deteriorate rapidly, thus weakening its baring capacity. 20 P a g e

22 Cost: Bedrock that has slaked and deteriorated will compromise the strength of the design. Additional design and construction time will be required. Risk: The reported soils along the paths of the microtunnels and their accompanying load capacities are incorrect. Cost: If the reported soils for the microtunnels are false, possible detours in drilling might occur. Also, the strength of the tunnels could be compromised leading to a potential failure or collapse of a tunnel. Such a failure creates a significant safety risk personnel working on the site, specifically those operating the tunnel-boring machine (TBM). Additional time, design and construction as well as damage to the TBM would also produce more cost. 21 P a g e