We know foundation sometimes rest on compacted fill. In case of questionable soil after sufficient test foundation engineer sometimes suggest to remove this soil and replace the excavated soil with fill material providing sufficient compaction. Dear reader we have used the term problematic soil which is defined in IBC as questionable soil.
In IBC seismic site classification, site class F is defined by this type of soils. The questionable soils are expansive soil, collapsible soil, soluble soil etc. we have published many post about these types of soil.
When a shallow foundation is supported on compacted fill having thickness more than 305 mm (12 inch), the geotechnical investigation is essential and which include the following requirements:
a. Specification to prepare site before placing fill material and subsequent compaction.
b. Specifications for fill material that will be left for compaction.
c. Dear reader we know any compaction process should have accountability and we have two parameters to measure performance
- Optimum moisture content
- Maximum dry density
The test methods for determining these parameters should be defined in investigation.
d. Allowable maximum thickness of the each layer of fill materials
e. Test method to determine in-situ dry density in the field
f. Minimum allowable in-situ dry density which is generally represented as percentage of maximum dry density from item (c) known as MDD.
g. Required frequency and number of in-situ tests to check minimum allowable in-situ dry density.