1 Geotechnical Assignment

General information

The project will be a mixed use development 5 -6 levels above grade, two levels below grade will take excavations to approximately 28 feet below grade.

The building footprint will occupy the entire site (this would indicate that all sides of the excavation will require soil retention systems e.g. sheeting and shoring.)

The site is located in a seismically active region of California. This does not normally affect means and methods of construction. However, permitting authorities may require overdesign of retention systems.

Soils

Soils consisted of very stiff to hard sandy clay at depths down to 7 - 17 feet and dense to very dense sandy gravel and gravelly sand, which extended to depths of 21 1⁄2 to 28 feet. (The geotechnical engineer has classified the soil as OSHA Class C which requires 1 ½ horizontal to 1 vertical slopes.

An obstruction was noted at Boring B 4 about 3.5 feet below the surface (indicates the possibility if existing foundations within the site)

Groundwater

Chemical testing of ground water samples determined that there is contamination from volatile organic compounds, and non organic constituents such as mercury, arsenic, antimony etc. Additional sampling and testing should be completed to determine if treatment is required.

Permits will be required to discharge ground water either to sanitary or storm lines. Treatment may be required prior to discharging. (Spoil (soil) removed from the site may need to be remediated.)

Free groundwater was observed in Borings B-1, B-2, B-3, and B-4 at depths of approximately 18 1⁄2 to 23 feet.

The report recommend that a groundwater depth of 14 feet be assumed. Unless the 2-level below-grade structure is drained, the structure must be designed to resist the hydrostatic pressure including uplift. (This is an important consideration during construction as you do not want the building to float or slabs to uplift)

It was recommended that the below-grade slabs be designed either to...