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

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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.

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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 ................................................................................................................................................. 10

Summary ............................................................................................................................................... 11

Appendix............................................................................................................................................... 12

Idealized Soil Profile ......................................................................................................................... 13

Excavation Design Calculations ....................................................................................................... 14

Thrust Wall Calculations ................................................................................................................. 17

Waler Shear and Moment Diagrams ............................................................................................... 19

Risk Assessment .............................................................................................................................. 20

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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).

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

Figure 1. The location of the gravel pits and the proposed pump station.

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discharge conduit near the river and an electrical control building adjacent to the pump station

(see Figure 2).

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:

Figure 2. Plan of the proposed pump station, excavation, and microtunnels.

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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 (γm) of 123 pounds-per-cubic-foot (pcf) and a friction angle (φ') of 32°.

3. 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-square-

foot (psf), and friction angle of 26°.

4. Laramie Formation Bedrock (depths at 30 - 50 ft) - The bedrock mainly consists of

claystone interbedded with clayey sandstone. The bedrock has an assumed moist unit

weight of 121 pcf, cohesion of 0-500 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.

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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 3. Section view of soldier pile.

Figure 4. UTexas results for failure plane 1.

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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.

Figure 5. UTexas results for failure plane 2.

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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.

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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.

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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 1.72. The piles should be

stiffened at the connections to the waler to avoid local yielding.

Waler

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.

Figure 8. Pressure diagrams for thrust wall pile.

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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.

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Appendix

Idealized Soil Profile

Excavation Design Calculations

Thrust Wall Calculations

Waler Shear and Moment Diagrams

Risk Assessment

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Idealized Soil Profile

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Excavation Design Calculations

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Thrust Wall Calculations

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Waler Shear and Moment Diagrams

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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.

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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.