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CASE G1 - Subsidence of Soil at the Naval Research Laboratory, M. Yachnis
Unexpected subsidence of soil during an excavation.
Symptoms:  Progressive settlement of soil under the moving load of a
Caterpillar D-6 Bulldozed.
Collection of Facts:  A geotechnical company was engaged to perform soils
investigation at the site of a proposed very sensitive building. The general
trend is to minimize the number of soil borings and spend as little money as
possible in geotechnical investigations. In this case, the depth of borings
was adequate but the number unreasonably small for the size and the
sensitivity of the building.  The subsurface conditions were described in the
soils report as favorable for pile foundation system.  The contractor
proceeded to perform a shallow excavation by using earth moving equipment,
The operation started smoothly, (see Fig. l), until a Caterpillar D-6
bulldozer started sinking due to settlement of the soil (see Fig. 2). A
detailed subsoil investigation concluded that, prior to the failure, a number
of cavities of various size existed at the proposed building site which were
not uncovered during the original soil investigation.  An extensive soil
boring program was undertaken and most of the cavities were uncovered.
Solution:  The cost of soil borings is minimal in comparison with the total
construction cost.  Particularly in special structures, where small
deflections and settlements are critical, extensive geotechnical evaluation
imperative.  The number of soil borings must be reasonable enough to insure
comprehensive knowledge of the subsurface conditions.
Fig. 2
Fig. 1

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