SCGeo
Geotechnical
- Jun 9, 2005
- 20
I have performed a geotechnical exploration for a proposed parking lot. The area planned for construction is currently a field and will require 3-4 feet of fill from present ground surface in order to meet required grades. The site is approximately 150' by 350' and was explored with test pits in 6 locations.
In one of the test pit locations, buried topsoil and organics (branches up to 3 inches in diameter) were encountered from 3.5 to 5 feet below existing surface elevation. In another test pit location, buried topsoil was encountered at a depth of 5 feet and was approximately 1 foot thick.
At the remaining locations, mixed soil and rock fill (boulders up to 2 feet in diameter) with some asphalt debris was encountered to depths of up to 8 feet below existing grade. The soil within the mixed soil and rock was near optimum moisture based on natural moisture testing and experience with similar materials.
My report stipulated that provided the owner was willing to assume some risk associated with undocumented fill, the existing mixed soil and rock fill across the majority (90%)of the site could remain in place if it was shown to be stable through proof-rolling.
I made it clear that at the two locations where it was encountered, the buried topsoil and organic debris should be over-excavated and "chased" laterally in order to completely remove it from the area planned for construction and backfilled with engineered fill. In my limited experience, if you find buried trash at one location, you can bet that it extends laterally from the point across the site.
The developer says that he needs more options for repair. He believes my recommendations are too conservative and wants options to stabilize the lot through bridging or with geogrid.
If it were simply weak soils, I would agree. But 1.5 feet of topsoil and branches as big as my wrist are another matter altogether. I can't think of any constructed geotechnical project where it is acceptable to fill on top of buried topsoil or wood.
Am I wrong?
In one of the test pit locations, buried topsoil and organics (branches up to 3 inches in diameter) were encountered from 3.5 to 5 feet below existing surface elevation. In another test pit location, buried topsoil was encountered at a depth of 5 feet and was approximately 1 foot thick.
At the remaining locations, mixed soil and rock fill (boulders up to 2 feet in diameter) with some asphalt debris was encountered to depths of up to 8 feet below existing grade. The soil within the mixed soil and rock was near optimum moisture based on natural moisture testing and experience with similar materials.
My report stipulated that provided the owner was willing to assume some risk associated with undocumented fill, the existing mixed soil and rock fill across the majority (90%)of the site could remain in place if it was shown to be stable through proof-rolling.
I made it clear that at the two locations where it was encountered, the buried topsoil and organic debris should be over-excavated and "chased" laterally in order to completely remove it from the area planned for construction and backfilled with engineered fill. In my limited experience, if you find buried trash at one location, you can bet that it extends laterally from the point across the site.
The developer says that he needs more options for repair. He believes my recommendations are too conservative and wants options to stabilize the lot through bridging or with geogrid.
If it were simply weak soils, I would agree. But 1.5 feet of topsoil and branches as big as my wrist are another matter altogether. I can't think of any constructed geotechnical project where it is acceptable to fill on top of buried topsoil or wood.
Am I wrong?
