CASE G5 - Expansive Soils, M. Jones
Expansive clay subgrades.
Heaved floor slabs.
Collection of Facts: Expansive clay soils, if not properly controlled, can
swell and cause significant damage to structures. In one project, which was
"over the money", the lime stabilization of the subgrade was deleted as a cost
reducing measure. The decision was made without review of the geotechnical
engineering study. Soon after construction, after an extended rainfall, floor
slabs heaved and were substantially damaged. The cost of repair was many
times the amount saved by deleting the lime stabilization.
Solution/Alternative: Identify expansive characteristics with Atterburg
Limits, soil activity, and consolidometer tests during the geotechnical
investigation. For limited areas such as directly under building slabs and
shallow foundations, the most effective control is through removal and through
replacement of the expansive soil with compacted granular fill. The depth of
removal can be determined by plotting a profile of the percent expansion
against depth (see DM 7.1). For areas of wide extent, such as highway and
airfield pavements, lime stabilization is the preferred method of treatment.
The tendency to swell can also be partially controlled by increasing the soil
moisture to near saturation and reducing the compactive effort.
CASE G6 - Sheetpiling/Bulkhead Replacement, A. H. Wu
Problem: Sheetpile bulkheads are extensively used in the Navy's shoreline
environment. Many bulkheads have markedly deteriorated. Some of them are
beyond repair, while in other cases, the deterioration has been more moderate.
When a bulkhead shows extensive corrosion and deterioration, remedial work
is required. Repair could be accomplished by either welding steel plates to
the corroded sheetpiles, or covering them with cast-in-place concrete panels.
However, installing a new sheetpile in front of the existing one, and joining
the two by a tie-rod, is probably more cost effective. This bulkhead
replacement method can be employed if the existing bulkhead has moderate
deterioration, and the structure is in stable condition.
In the NAVFAC DM-7.2 , Chapter 3, Analysis of Walls and Retaining
Structures, a free-earth support method is described. Fig. 1 shows the
free-earth support method adopted by the Navy.
Until now, the free-earth support method has been generally used in
bulkhead replacement design. However, there are some difficulties with this
approach: (a) it commonly assumes that the existing bulkhead has neither
usable value nor supporting capacity even though the structure is to remain
in-place, (b) values of the soil strength parameters are conservatively
assumed, with the common result of an overdesign calling for excessive
sheetpile length, unnecessarily large steel section modules, and an anchor
spacing closer than necessary. If the soil is a weak soil, replacement of
bulkhead by the free-earth method is impractical and uneconomical.