Geotechnical Site Investigation

Geotechnical Site Investigation

Before a foundation is designed, before a grading plan is drawn, and before a contractor moves any dirt, someone needs to know what is actually in the ground. That is what a geotechnical site investigation establishes. For developers and project owners in California, the geotech report is not optional paperwork — it is the foundation of every cost estimate and design decision that follows.

At CaliChi Design Group, we coordinate geotechnical site investigations as part of our civil engineering services. We work with licensed geotechnical subconsultants to scope the investigation for each project, then we take the geotech report’s recommendations and translate them into the civil design — grading plans, retaining wall criteria, utility trench backfill specifications, and stormwater infiltration feasibility. The civil engineer does not prepare the geotech report, but we are the ones who have to make the design work with what the report says. That gives us a practical perspective on what a good investigation needs to include.

What a Geotechnical Report Includes

A standard California geotechnical report covers subsurface conditions across the project site based on field borings or test pits, laboratory testing of retrieved samples, and engineering analysis. Here is what you should expect in a complete report for a Bay Area commercial or multifamily project:

• Site description and project background — location, existing conditions, proposed use
• Boring and test pit logs — soil descriptions using USCS classification, blow counts, groundwater observations
• Laboratory test results — grain size distribution, plasticity index, expansion index, consolidation, shear strength as applicable
• Seismic hazard evaluation — site class per ASCE 7, liquefaction potential, fault setback requirements, peak ground acceleration
• Engineering recommendations — allowable bearing capacity, grading and compaction specifications, retaining wall design parameters, pavement section design, utility trench backfill, expansive soil mitigation
• Groundwater depth — seasonal high and observed depth during investigation

Every one of those sections affects either the civil design or the project budget. If the expansion index is high, compaction specifications will require pre-saturation and moisture conditioning. If liquefaction potential is moderate to high, the foundation engineer may require ground improvement or deep foundations. If groundwater is shallow, utility installation depths and dewatering requirements change. Understanding how to read a geotech report is part of knowing how to read civil plans — if you want to understand what the numbers on a grading plan mean, reading the geotech report behind it helps. We have written a practical guide on how to read a grading plan that covers how geotech recommendations translate to the civil sheet.

When Is a Geotechnical Report Required in California?

California Building Code (CBC) Section 1803 requires a geotechnical investigation for commercial and multi-family projects, and for any project assigned to Seismic Design Category D, E, or F — which covers most of coastal California, including the entire Bay Area, Los Angeles Basin, and San Diego County. Most grading permits in Bay Area jurisdictions require a geotech report regardless of building type; the permit application itself will list it as a required submittal.

Projects in Alquist-Priolo Earthquake Fault Zones require a fault hazard investigation performed by a licensed engineering geologist (CEG) before the building permit can be issued. The Alquist-Priolo Act prohibits the construction of occupied structures intended for human habitation within 50 feet of an active fault trace. If your site is near a mapped fault, this adds both time and cost to the pre-design investigation scope.

Hillside and graded sites in the Bay Area also face additional scrutiny from cities like Oakland, Berkeley, and Los Altos Hills, which have their own geologic hazard overlay districts. These local programs may require a geologic report separate from the standard geotechnical report, prepared by a CEG who addresses landslide, debris flow, and erosion hazards in addition to foundation conditions.

How Soil Conditions Affect Grading and Foundation Design

The geotech report drives civil design in ways that are not always obvious until you have worked on projects where soil conditions created significant cost surprises. Here are the soil conditions we see most often in the Bay Area and what they mean for project design and budget:

Bay Mud

Bay Mud is a soft, highly compressible marine sediment found along the San Francisco Bay shoreline and in filled areas throughout Alameda, Santa Clara, and Contra Costa counties. Sites on Bay Mud typically require deep foundations — driven concrete or steel piles at $80 to $150 or more per linear foot — significant over-excavation and recompaction, and long-term settlement monitoring. Grading plans on Bay Mud sites need to account for the settlement that will occur under fill loads, sometimes over a period of years. We size detention basins and design site drainage on these sites with future settlement in mind, or the drainage will not function as designed.

Expansive Soils

Expansive soils are common in the East Bay hills and parts of the South Bay. The expansion index (EI) from the geotech report determines the required treatment — soils with EI above 130 typically require pre-saturation before fill placement, moisture conditioning to within 3 percent of optimum, and compaction to specific moisture ranges that need to be maintained during earthwork. The civil grading specifications need to reflect these requirements explicitly, and the contractor needs to sequence the earthwork to allow adequate time for pre-saturation. This adds both cost and schedule to the grading phase.

Liquefiable Soils

Loose, saturated, fine-grained sands are susceptible to liquefaction during earthquake shaking. The geotech report will assess liquefaction potential using standard penetration test data and site-specific seismic parameters from ASCE 7. If liquefaction is found to be a concern, the foundation engineer has three options: ground improvement (compaction grouting, stone columns, vibrocompaction), deep foundations that extend below the liquefiable layer, or a mat foundation designed to bridge through potential ground deformation. All three options have major implications for the civil grading design and the project budget.

Shallow Rock

Rock at shallow depth is common on hillside sites in Oakland, Berkeley, and parts of San Jose. Rock excavation costs $8 to $25 or more per cubic yard compared to $2 to $5 for soil excavation. If the geotech investigation does not reach rock, the contractor’s rock excavation bid will be based on unit price allowances that can become significant change orders. We recommend borings deep enough to characterize the rock surface across the grading footprint, not just at the building pad.

How the Geotech Report Affects Stormwater Design

For projects with LID requirements, the geotech report is also essential for stormwater infiltration feasibility. Bioretention cells that infiltrate to native soil require a minimum native soil infiltration rate — typically 0.5 inches per hour or better — and a minimum separation from the bottom of the bioretention cell to the seasonal high groundwater table, usually 10 feet. If the geotech report shows clay soils with low infiltration rates or a high water table, the LID design shifts to flow-through planters with underdrain systems, which treat runoff differently and have different sizing requirements. We use the geotech infiltration data from the beginning of the LID sizing process, not after the bioretention cells are already designed.

Geotech Due Diligence Before You Buy

For developers evaluating a site acquisition, a Phase I Environmental Site Assessment is standard practice — but a preliminary geotechnical assessment is equally valuable and often skipped. A desktop review of existing boring data from adjacent sites, review of historical USGS maps and aerial photos, and a site walk by a geotechnical engineer can identify red flags — Bay Mud, fill areas, slide scarps, drainage patterns — before the purchase contract is signed. If the site has a known geotechnical challenge, understanding the cost implications before closing is worth more than the cost of the assessment.

We can coordinate a preliminary geotech assessment as part of a civil due diligence package that also covers utility availability, infrastructure capacity, and regulatory requirements. If your project requires a will serve letter from the water or sewer district, we track that coordination simultaneously with the geotech scope so the due diligence package is complete before you commit to entitlement investment.

Our Role: Coordinating the Geotech and Using It in the Civil Design

The civil engineer does not prepare the geotech report, but we are often the ones who scope it, review the draft recommendations, and flag issues before the report is finalized. We have seen geotech reports that recommended compaction to 95 percent relative compaction without addressing moisture content — which is not implementable with expansive soils. We have seen reports that did not address stormwater infiltration rates when the project clearly needed LID design. We catch these gaps before they create problems in the permit review or the field.

During construction, we verify that actual field conditions match the report’s assumptions. If the contractor uncovers different soil conditions than what the borings showed, we work with the geotechnical engineer to issue revised recommendations rather than proceeding with a design that may no longer apply to the actual conditions.

If your project is in the early planning stages and you need a civil engineer to coordinate the geotechnical investigation scope, give us a call at (510) 250-7877 or reach out for a consultation. We will help you scope the right investigation for your site conditions and project type before you commit to the geotechnical contract.

Thanks for directing and addressing the site/traffic questions/discussion. The city reps clearly loved the site plan design. Great job!”

JASON SHEETS, MODA4 Design

Market sectors we serve:

[display_market_sectors]

Looking For a Partner?

With an agile team, innovative technology, and offices across the country, we are ready to take on your project. To learn more about our civil engineering and dry utility consulting services or get a quote,

Get In Touch

Frequently Asked Questions