January/February 2025 - 28

Very Soft Subgrades
Most working platforms are constructed
on fine-grained or " cohesive " soils since
coarse-grained or granular soils like
sand often possess sufficient shear
strength to support piling rigs without
the need of a working platform layer. In
rarer cases of platform design on loose
granular subgrades, the T-value method
may also be used but with alternative
relationships between T and the friction
angle of the subgrade soil. The strength
of cohesive soils is expressed as an
undrained shear strength. It can be measured
both in situ and in the laboratory
on undisturbed samples. It is not a fundamental
parameter and varies with
moisture content. It may change due to
construction of the platform and changing
weather conditions. This variation
during the design life of the working
platform should be considered in the
selection of undrained shear strength.
Existing guidance on working platform
design applies only to undrained
shear strengths above about 20 kPa or
420 psf. This is primarily since assumed
failure modes such as punching shear
may not apply on such soft subgrades.
This is less of an issue for the T-value
method, derived from FEA parametric
study where there are no predetermined
failure modes. Constructing and compacting
platforms on such soft materials
is also challenging and the same density
may not be achieved as on firmer subgrade
soils. Furthermore, this method's
parameters show very high sensitivity
of T to small changes in the undrained
shear strength at the low end of the
subgrade strength scale.
A correction to the T-value, in use for
5 years by the author, was proposed at
the recent DFI Annual Conference.
Rather than the undrained shear
strength alone, it is based more appropriately
on the dimensionless ratio of
undrained shear strength to platform
effective self-weight. At values of this
ratio below 1.25, which corresponds
approximately with the 20 kPa (420 psf)
undrained shear strength cutoff of existing
guidance, the T-value is progressively
reduced below the values derived
from the main derived equations.
Summary
The T-value method to calculate twolayer
bearing capacity has been used
successfully in the design of working
platforms around the world in the 5
years since its introduction. It allows
alternative aggregate sources to be
introduced straightforwardly, including
those mechanically stabilized by geogrid.
This has led to thinner, optimized
working platforms on hundreds of
projects with significant time and cost
savings and a reduced carbon footprint.
Andrew Lees, Ph.D., MICE, C.Eng., is the global
application technology manager at
Tensar, a division of CMC and is the originator
of the T-value method. He has 25 years of experience
of geotechnical analysis and design.
28 * DEEP FOUNDATIONS * JAN/FEB 2025

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January/February 2025

Table of Contents for the Digital Edition of January/February 2025

TOC
January/February 2025 - Intro
January/February 2025 - 1
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January/February 2025 - TOC
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