Cutter Soil Mix Wall Excavation in Seattle

Analysis and Cost Estimation of a Cutter Soil Mix Wall in Seattle

In this section we reanalyze a 43ft deep excavation supported by a 68ft cutter soil mix wall and three levels of tiebacks. The purpose of this reanalysis was to examine the factors that influence lateral wall response and evaluate how the analysis compared to the measured wall response. The original excavation results were reported by Doug Lindquist, Ben Upsall, and Garry Horvitz (Hart Crowser) in their publication titled "Cutter Soil Mixing Excavation and Shoring in Seattle's Pioneer Square District" (Earth Retaining Structures Conference, 2010).

PROJECT DESCRIPTION

The excavation consisted of a 0.8m thich cutter soil mix wall supported by three levels of tiebacks in a reclaimed area. Soils at the site consist of 38ft of fill, over approximately 28 ft a sandy silt (aquitard), over a lower sand aquifer. The following figure presents the lateral pressure design envelopes for the project as presented by the authors.

Design lateral earth pressures and soil profile for cutter soil mix wall in Seattle

MODEL SIMULATION IN DEEPEX

Exact soil design strengths and properties were not available and had to be estimated. The properties of the soldier beams inserted in the CSM wall were also not presented in the original publication and were estimated based on experience. Three levels of tiebacks were simulated with lockoff loads calculated from the apparent earth pressure envelope in the previous figure. The reanalysis was conducted with DeepEX and included all construction stages for the excavation.

Based on optical surveys and inclinometer readings the cutter soil mix walls deflected from +0.8 inches towards the excavation to -1.5 inches into the retained soil. Typical displacements are illustrated below:

Wall displacements of cutter soil mix wall in Seattle

Three excavation models were performed in DeepEX:

a) Without any wall friction and initially estimated soil properties

b) With wall friction and initially estimated soil properties

c) With adjusted soil properties to give a reasonable estimate of pile E35

Given the unknowns in the analysis, based on the results we can see that models A & B appear to have reasonably captured a considerable range of observed wall movements. The inclinometers did not extend below the bottom of the wall, and as a result any possible tip movements could not be measured.

Modeled CSM wall displacements without including wall friciton

Case A: Lateral wall displacements and soil pressures for case without wall friction

Modeled soil mix wall displacements in Seattle with wall friction assumed

Case B: Lateral wall displacements and soil pressures for case with wall friction

Benchmarked wall displacements with DeepXcav for excavation in Seattle

Case C: Approximate benchmarked wall displacements

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COST ESTIMATION WITH DEEPEX

DeepEX estimated the project cost at $7.6 million. Authors reported cost at $8.5 million, including underpinning work for other sections.

DeepEX has an intelligent cost estimation module included. Without knowing much of the details, we were able to come back with a quick and relatively accurate cost of the excavation as reported in the original publication. The procude included selecting the City and State so that cost multipliers could be obtained. The perimeter of the excavation and support system was defined, and the cost for the pumping operations was estimated (including personnel).

Cost estimation dialog for estimating deep excavation cost in Seattle

Cost estimation for cutter soil mix wall excavation in Seattle