Geotechnical Considerations
The first step in the geotechnical analysis was to gather the pertinent data from the
geotechnical report. The geotechnical report provided the unconfined compression strength (qu) for
the clay layers and the Texas Cone Penetration (TCP) test results for the rock layers. The geotechnical
report also provided boring logs for various locations. These boring logs provided information about the
different soil layers that were present at various locations and the depth and thickness of the soil layers.
The unconfined compression strength is an important characteristic for determining the capacity of clay
layers. This characteristic can be used in many methods for determining the capacity of cohesive soils
including the “α method” and the “Mohr Coulomb Shear Stress Criterion” which were both considered
in the geotechnical analysis. The capacity of the rock layers were determined using the TCP data and
graphs published in the TXDOT Geotechnical Manual.
The geotechnical report that was available was not specific to our site but it was from the
surrounding area within Dallas TX. The lack of site specific data in the geotechnical report led to a
statistical analysis of the reported data aimed at determining representative soil characteristics that
could be applied to various soil profiles. The statistical analysis consisted of producing a 95% confidence
interval of all necessary soil characteristics for each different type of soil layer. The results of the
statistical analysis, and subsequent soil capacities, were presented in the geotechnical summary in the
form of an Excel spreadsheet.
Drilled Shaft Analysis
Drilled shafts were used in both the column and abutment foundations. The basis for the drilled
shaft analysis is the results of the geotechnical analysis, the loads on the shafts, and the methods used
to determine the capacity of each shaft. The soil strengths obtained from the geotechnical report and
subsequent statistical analysis were used in three different methods to determine the capacity of each
drilled shaft, and the determined capacity was compared with the applied load to determine the size
and adequacy of each shaft. The “α method” was the first method to be considered for the cohesive soil
layers as it was the most familiar method. The TXDOT geotechnical manual specifies and recommends
the “Mohr Coulomb Shear Stress Criterion”. The “Mohr Coulomb Shear Stress Criterion” produced
slightly higher values than the “α method” and was the value used in the final design.
Based on the soil capacities, applied loads and boring logs assigned to each shaft location it was
determined that the clay layers depth and strength were not sufficient to carry the applied loads. This
result necessitated that the drilled shafts be founded at sufficient depth to activate the capacity of the
underlying limestone layers which were determined to be sufficient to carry the applied loads. After the
size, depth, and founding soil were determined, the settlement of each shaft was calculated and
compared with allowable settlements and differential settlements, and determined to be adequate.
Mechanically Stabilized Earth Wall Analysis
The Mechanically Stabilized Earth Wall (MSEW) design also relied on the data produced in the
geotechnical report and analysis. The MSE wall is subject to five failure modes; sliding, overturning,
bearing capacity, global stability, and pullout. Many of these failure modes depend on the capacity of
the soil that the wall is founded on. The MSE wall is founded on a CH or CL clay, depending on the
portion of the wall, both of which were analyzed in the geotechnical report and geotechnical summary.
These failure modes were all analyzed for each section of wall and for all wall heights. The most critical
failure mode was the overturning failure mode which required some of the wall sections to have strip
lengths increased from the typical 70% of the wall height to as much as 100% of the wall height. The
failure modes for the MSE wall also depend on factors including the surcharge on the soil atop the wall,
the unit weight of the soil, and the internal friction angle of the soil. The global stability failure mode is
the most difficult failure mode to analyze by hand because of the almost infinite number of slip surfaces
possible. For our project we analyzed 20 slip surfaces on the highest wall section, all of which had
factors of safety well above that required.