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The 5 most common errors in deep excavation design.

May 26th, 2017

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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

50ft deep excavation with sheet pile wall design model

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:

Sample active pressure calculation for 50ft deep excavation
And at 75ft depth, where the wall bottom is assumed:
Sample active pressure calculation for 50ft deep excavation
The active pressures can be illustrated as:
Active and passive pressures for 50ft deep excavation

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:

Apparent earth pressure calculations for FHWA method

Apparent earth pressure calculations for FHWA method

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:

Net loading diagram for 50ft deep excavation

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

Blum's method is illustrated below:

Blum's method for 50 deep excavation

FHWA Simple span method:

FHWA Simple span method for analyzing a deep excavation

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

FHWA beam method with Blum's assumptions for fixity

Caltrans Trenching and Shoring Manual Approach

CALTRANS Trenching and shoring manual example for 50ft deep excavation

CALTRANS Method with 20% negative moments

Caltrans method with negative wall moments for 50ft deep excavation

How the different analysis methods compare

Comparison of different analysis methods for deep excavations

Non-linear analysis comments

In the non-linear analysis, the range of values represent how wall friction was assumed. On the low range of moments, wall friction was assumed on both wall sides whereas on the high range of results wall friction was assumed only on the resisting side.

Sample results for non-linear analysis methods for a 50ft deep excavation

Some things are for free - others are not

These free webinars are really great, but if all we did was for free I would be sleeping in a deep excavation! We make our living selling deep excavation software, and not just any software. I have spent the last 15 years adding all this knowledge and more expertise in our deep excavation design software DeepEX. Please take a look and try it out, let me know if you would like us to arrange an online demonstration.

Looking forward to talking with you,

Dimitrios Konstantakos, P.E.

 

Optimize and design your excavations like never before!