Geotechnical Engineering:

Geotechnical reports provide data on surface and subsurface conditions at a site. The study may include review of available published and unpublished data about the site, a site visit to determine surface conditions and mark boring locations, and a subsurface investigation involving borings, cone penetration testing and/or test pits to determine the subsurface soil conditions. Laboratory testing will provide information regarding the suitability of the soil for the proposed development.

Due Diligence is used to review issues that may impact the proposed development. During this phase, the following geology/geotechnical issues may be evaluated:

• Location of active fault in relation to the site and their impact on the proposed development
• Anticipated depth to groundwater and bedrock (may be based on literature review or field investigation)
• Secondary effects of earthquake, such as liquefaction, landslide, etc.
• Presence of unsuitable soils (such as expansive or collapsible soils)
• Soil corrosivity
• Excavatability of the site soils

To address these issues, limited field investigation such as drilling borings or excavating test pits may be conducted.

Geotechnical foundation investigation is an expanded scope from the due diligence phase. Besides the issues listed above, design and construction recommendations are presented. Geotechnical foundation investigations are used to prepare grading plans and foundation designs. Investigations are generally conducted using the following techniques:

• Borings using hollow stem auger or bucket rig
• Test Pits using a backhoe equipped with 18-24 inches wide bucket
• Seismic Refraction Survey (in case of shallow bedrock and to determine excavability of the subsurface soils
• Cone Penetration Tests (CPT) - For liquefaction evaluation

Geologic and geotechnical issues are generally the following:

Unsuitable Soil Conditions are types of soils that require removal or the application of mitigation measures to make the site soils usable for the proposed construction. There are three basic types of soil: sand, silt and clay.

Expansive soils generally are clayey soils that tend to expand (increase in volume) as it absorbs water and it will shrink (lessen in volume) as water is drawn away.

When expansive soils supporting a foundation are allowed to become moist, cracks may appear, windows and doors may stick and floors may slope as the foundation becomes progressively more out of level.

When expansive soil becomes very wet, the concentrated weight of the foundation and structure will inhibit the soil's upward expansion, and outward expansion can occur. As the soil dries and shrinks, the footings may not be returned to their original position. Instead, they can move to a slightly lower level. This process can accumulate if the wetting and drying is allowed to continue season after season, year after year.

Collapsible/Compressible soils are soils with various amounts of clay, and voids and are susceptible to settlement when wet.

Collapsible soils are distinguished by their potential to undergo large decreases in volume upon increase in moisture content without increase in external load. Weakly cemented sands and silts collapse where cementing agent is soluble. Deposits of collapsible soils are usually associated with regions of low moisture.

Compressible soils are distinguished by their potential to undergo large decreases in volume upon increases in external load.