Geotech & Civil · California
Geotech & Drainage for New Construction in California
How geotechnical investigation and drainage design actually drive a California new-construction project — from soils characterization through foundation recommendation, slope stability, retaining, grading, drainage, and stormwater compliance. Engineering decisions made early; not invented at plan-check.
At a glance
A buildable path starts with the right questions.
A geotechnical investigation (soils report) characterizes the lot's soils, bearing capacity, expansive soils, fill, groundwater, and slope-stability factors of safety — it drives the foundation recommendation, the drainage design, and the retaining design. Drainage and grading plans are produced by a civil engineer coordinated with the geotech and the structural design. Skipping or delaying either is the single most common way California hillside and complex-lot projects blow up at plan-check or in the field.
This is suited to
- Hillside owners in LA, the California, other California markets, California, or coastal slopes.
- Owners planning luxury homes with basements, pools, retaining, or complex sites.
- Teardown rebuild owners on lots with prior fill, expansive soils, or known drainage issues.
- Fire-rebuild owners where post-fire soils, debris, or slope conditions have changed.
- Owners of small multifamily infill lots with utility-capacity or stormwater constraints.
Project consideration 01
When a geotech report is required
Virtually every California hillside parcel, every parcel inside a Seismic Hazard Zone (liquefaction or earthquake-induced landslide), every parcel with known expansive soils or prior fill, and most projects with basements, pools, or significant retaining will require a current geotechnical investigation. Many flat-lot custom homes also require one — local jurisdictions, lenders, and structural engineers commonly require it even when the city does not strictly mandate one.
Project consideration 02
Soils investigation — what is in the report
Site reconnaissance
Geologist / engineer walks the lot and reviews published geologic and seismic maps.
Subsurface exploration
Borings, test pits, or CPT (cone penetration testing) to characterize subsurface conditions.
Lab testing
Soil classification, expansion index, shear strength, consolidation, and moisture content as appropriate.
Engineering analysis
Bearing capacity, settlement, lateral earth pressures, slope-stability factor of safety, liquefaction potential, seismic site class.
Recommendations
Foundation type, allowable bearing pressures, retaining-wall lateral loads, drainage and surface-water recommendations, fill placement protocols, special inspection requirements.
Project consideration 03
Expansive soils
Expansive clays — common across much of Greater Los Angeles — swell when wet and shrink when dry, generating substantial uplift and lateral pressures on foundations and slabs. The geotech report quantifies expansion index and drives foundation design (post-tensioned slab, drilled piers with structural slab, deepened footings) and site grading and drainage strategy. Ignoring expansive soils on a flat lot is one of the most common preventable failure modes in California residential construction.
Project consideration 04
Slope stability
On sloped parcels, the geotech analyzes static and pseudo-static (seismic) slope-stability factors of safety. The analysis can determine whether the lot is buildable as proposed, requires substantial cut/fill/retaining to stabilize, or requires deep foundations bypassing unstable material. Many California hillside cities require minimum factors of safety (commonly 1.5 static and 1.1 pseudo-static), and failing to meet them is a project-stopper, not a comment to address.
Project consideration 05
Seismic considerations and Seismic Hazard Zones
The California Geological Survey publishes Seismic Hazard Zone maps identifying liquefaction and earthquake-induced landslide zones, and Alquist-Priolo Earthquake Fault Zone maps identifying surface-rupture-prone fault zones. Parcels inside these zones require additional geotechnical analysis and may have restrictions on habitable structures. The structural design responds with seismic site class, ground motion parameters, and lateral system selection.
Project consideration 06
Foundation recommendations
The geotech recommendation drives the structural foundation design. Common California foundation types:
Conventional slab-on-grade
Flat lots with non-expansive soils and adequate bearing — the cheapest path.
Post-tensioned slab
Common where expansive soils are present and the structure can tolerate a stiffened slab.
Raised foundation with footings and stem walls
Older convention; still common on remodels and some custom homes.
Drilled caissons with grade beams
Standard on hillside lots — drilled to competent bearing, tied with grade beams.
Driven piles
Rarer in residential; appears on certain liquefiable or deep-soft-soil sites.
Basement walls / retaining foundations
Where the foundation also retains earth (basements, hillside cuts).
Project consideration 07
Retaining walls and deep foundations
Retaining walls — cantilever, restrained, soldier-pile-and-lagging, or shoring — are engineered against active, passive, surcharge, and seismic loads using the geotech's recommended lateral earth pressures. Hillside foundations and deep retaining are coordinated with the civil drainage design from the start, because most retaining failures are drainage failures in disguise.
Project consideration 08
Drainage plans — surface, subsurface, and discharge
Drainage on a California new-construction project covers three layers: surface drainage (roof, swales, area drains, ribbon gutters), subsurface drainage (French drains behind retaining walls, foundation drains around basements), and discharge routing to an approved outfall — gutter, public storm drain, or engineered on-site infiltration. The civil engineer produces the drainage plan; the geotech informs the subsurface and retaining drains; the structural design coordinates with both.
Project consideration 09
Stormwater, SWPPP, and BMPs
California construction sites that disturb one acre or more require coverage under the State Water Resources Control Board's Construction General Permit, with a Stormwater Pollution Prevention Plan (SWPPP) and Best Management Practices (BMPs) implemented during construction. Smaller sites still typically require erosion-control plans and city stormwater approvals. Plans are prepared by qualified SWPPP developers and implemented through construction by qualified SWPPP practitioners.
Project consideration 10
Erosion control and grading plans
Grading plans show cut/fill quantities, finished grades, slope ratios, and earthwork sequencing, and are submitted with the civil package. Erosion-control plans cover construction-phase BMPs — silt fence, fiber rolls, stabilized construction entrances, and protected drain inlets. Most California jurisdictions require both before grading or demo permits issue.
Project consideration 11
Hillside, coastal, and fire-area considerations
Hillside parcels combine slope-stability, drainage, and fire-access requirements. Coastal-bluff parcels add bluff-stability analysis and Coastal Commission appeal jurisdiction. Fire-area parcels add WUI envelope and defensible-space requirements. Each overlay layers onto the geotech and civil package — and each is best characterized in the feasibility note rather than discovered at plan-check.
Project consideration 12
How geotech affects cost, timeline, and design
A geotech recommendation can move a project's foundation cost by 100–300% (slab vs caissons vs deep basement walls), shift the construction schedule by months, and reshape the architectural plan when slope-stability or buildable-area constraints limit the footprint. Geotech is not an end-of-design checkbox — it is a feasibility-stage input.
Questions
What owners ask first.
- Do I need a geotech report?
- Virtually every California hillside parcel, every parcel inside a Seismic Hazard Zone, every parcel with expansive soils or prior fill, and most projects with basements, pools, or significant retaining require one. Lenders and structural engineers commonly require one even when the city does not. We confirm at feasibility.
- How long does a geotech report take?
- Typically 4–10 weeks from scope to final report, depending on access, drilling availability, and lab turnaround. Hillside parcels with difficult access or extensive boring requirements sit at the longer end.
- What does a geotech investigation cost?
- A typical residential geotech investigation in California costs in the low-to-mid five figures, with hillside or extensive-boring scopes pricing higher. The cost is small compared to the foundation design it informs.
- Will the geotech tell us what foundation to use?
- Yes — the report includes a foundation recommendation (slab, post-tensioned slab, drilled caissons with grade beams, or other), along with allowable bearing pressures, lateral earth pressures for retaining design, and drainage recommendations.
- How does drainage affect my project?
- Drainage design is coordinated with the geotech, the foundation, and the architectural site plan from the start. Surface drainage, subsurface drainage, and discharge routing are engineered as one package — and most retaining failures are drainage failures in disguise.
- Do I need a SWPPP?
- Construction sites that disturb one acre or more require coverage under California's Construction General Permit with a SWPPP. Smaller sites still typically require erosion-control plans and city stormwater approvals. We confirm at feasibility.
- Can geotech results stop my project?
- Sometimes — extremely poor slope-stability, very deep loose fills, active landslide features, or surface-rupture fault traces can make a project infeasible as proposed. The honest output is that information, early.
- When should we order the geotech?
- At feasibility — before schematic design closes — on any hillside, expansive-soil, or complex-lot project. On flat-lot infill projects, the order can sometimes wait until early design development, but not later.
Official sources
- California Geological Survey — Seismic Hazard Zones
California Department of Conservation · Statewide Seismic Hazard Zone (liquefaction, earthquake-induced landslide) maps.
- California Geological Survey — Alquist-Priolo Earthquake Fault Zones
California Department of Conservation · Surface-rupture fault-zone mapping that restricts habitable structures.
- State Water Board — Construction General Permit (SWPPP)
California State Water Resources Control Board · Statewide construction stormwater permit and SWPPP requirements.
- LADBS — Grading, Excavation & Hillside
City of Los Angeles · City of LA grading, excavation, and hillside ordinance reference.
- LA County Public Works — Geotechnical & Materials Engineering
Los Angeles County Public Works · Unincorporated LA County geotechnical review and standards.
- California Building Standards Commission — California Building Code
California DGS · CBC requirements for soils investigations, foundations, and retaining walls.
RSA location
Based in Beverly Hills, serving Greater Los Angeles.
Your next step