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Solving Deep Excavation Design

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 July 15, 16, 2020

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50ft Deep Excavation Design Example - Sheet pile wall analyzed with FHWA Apparent Earth pressures

A 50 ft deep excavation is to be analyzed with different analysis methods. The purpose of this example is to illustrate different analysis methods and their differences. Ground water level is assumed at 35ft depth. Soil layers consist of the following:

0ft to 50ft: Sand with friction angle 30 degrees, unit weight 120 pcf

50ft and below: Sand with friction angle 36 degrees, effective cohesion c'= 100 psf, and a unit weight of 125 pcf

Water pressures are calculated with a simplified 1D flow analysis. In the FHWA approach we need to determine first the total active thrust above the excavation subgrade. Next, we need to multiply this thrust with a 1.3 multiplier. The next step is to distribute the total thrust in an apparent earth pressure envelope, with the top and bottom triangular portions established at 2/3 of the clear height to the first support, and 2/3 of the clear height from the excavation to the bottom support.

A sample active pressure calculation is presented, at El. -50:

And at 75ft depth, where the wall bottom is assumed:
The active pressures can be illustrated as:

By integrating the active earth pressures above the excavation the total active thrust is established. The total apparent thrust and maximum apparent earth pressure are then calculated:

Next, we need to establish the net loading diagram on the wall. In general, the net loading is determined by adding lateral earth pressures, surcharges, water pressures, and seismic:

Next step, analyze the wall beam with different analysis methods:

Blum's method is illustrated below:

FHWA Simple span method:

FHWA simple span method with virtual support established according to Blum's method

Caltrans Trenching and Shoring Manual Approach

CALTRANS Method with 20% negative moments

How the different analysis methods compare