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Portfolio/Townhome Duplex Development
GeotechnicalResidential

Townhome Duplex Development

Garden Grove, CA

8 distinct soil layers in 50 ft; mapped liquefaction zone — analysis confirms low susceptibility and zero seismic settlement

Overview

Moment performed the geotechnical investigation for two duplex structures — four townhome units total — on a suburban lot. The investigation characterized an unusually complex layered stratigraphy, performed a liquefaction analysis (the site is within a mapped liquefaction zone), and confirmed that shallow foundations were feasible despite the subsurface variability.

The Challenge

The site revealed eight distinct soil layers in 50 feet, with soft clays sandwiched between granular deposits in a complex alternating sequence. The site is mapped within a seismically induced liquefaction hazard zone, which required a site-specific analysis even though preliminary indications were favorable. Groundwater at 37 feet added a constraint for any future subgrade work.

Our Approach

Moment advanced three borings to 50 feet across the 10,932 SF lot to resolve lateral variability and confirm consistent bearing conditions. Lab testing confirmed low expansion potential. The liquefaction analysis demonstrated that despite mapping, the site soils have low liquefaction susceptibility with zero anticipated seismic settlement — a finding that significantly simplified the foundation and stormwater design.

Results

The three-boring program resolved the complex stratigraphy and confirmed that shallow spread footings at 18-inch depth would adequately support the two-story structures. The liquefaction analysis cleared the site of seismic settlement concerns, allowing conventional foundations and potentially infiltration-based stormwater management.

Technical Summary

8 distinct soil layers in 50 ft requiring 3-boring programMapped liquefaction zone — analysis confirms LOW susceptibility, zero seismic settlementGroundwater at 37 ftLow Expansion Index5 ft over-excavation with remedial grading
Borings
3 borings to 50 ft across 10,932 SF lot
Soil Conditions
8-layer alternating stratigraphy: silty sands (shallow) over interbedded clays and granular deposits; groundwater at 37 ft
Foundation Type
Shallow spread footings — 18 in. depth (2-story); minimum 15 in. continuous, 24 in. square pads

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We deliver soil reports, foundation recommendations, grading plans, and stormwater compliance — on schedule and ready for permitting.