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Portfolio/Seven-Story Urban Multifamily
GeotechnicalResidential

Seven-Story Urban Multifamily

North Hollywood, CA

Mat foundation on liquefiable soils; 3.7 in. total settlement (static + seismic); infiltration prohibited

Overview

Moment performed the geotechnical investigation for a 7-story, 35-unit apartment building (32,100 SF) with two levels of on-grade parking (16 stalls) on a compact urban infill site. The project required a site-specific liquefaction analysis per CGS SP 117A, mat foundation design, and a critical determination on stormwater infiltration feasibility.

The Challenge

Liquefiable soils in the upper 20 feet created competing demands: the structure needed a rigid mat foundation to span the weak zones, while city stormwater regulations pushed for on-site infiltration — which would add water to the very soils susceptible to liquefaction. The liquefaction analysis projected 2.7 inches of seismic settlement at design-level ground motion and 3.8 inches at MCE — magnitudes that required explicit structural accommodation.

Our Approach

We advanced two borings to 50 feet using a truck-mounted drill rig with SPT sampling and performed a site-specific liquefaction analysis using GeoLogismiki software per CGS SP 117A. The analysis quantified seismic settlement at both design-level (2.712 in.) and MCE (3.818 in.) ground motions. Moment designed a mat (raft) foundation with a minimum thickness of 12 inches and a modulus of subgrade reaction (k) of 100 pci. Over-excavation extends 5 ft below existing grade or 2 ft below footing (whichever greater), with 5 ft lateral extension beyond the building footprint. Subgrade beneath the mat is compacted to 95% at slightly above optimum moisture over the upper 12 inches.

Results

The mat foundation and infiltration prohibition shaped the entire project design. Total anticipated settlement (static + seismic) of 3.712 inches with 1.856 inches differential was explicitly communicated to the structural engineer for design accommodation. The infiltration prohibition directed civil stormwater design to lined detention and biofiltration with underdrains — no water enters the ground.

Technical Summary

Liquefiable soils in upper 20 ft requiring structural mitigationStormwater infiltration prohibited due to liquefaction riskSeismic settlement: 2.7 in. (design-level) / 3.8 in. (MCE)5 ft over-excavation with 95% compaction to slightly above optimumNo groundwater to 50 ft
Borings
2 borings to 50 ft (truck-mounted drill rig, California Sampler)
Soil Conditions
Loose to moderately dense sand (0–20 ft, liquefiable) over dense to very dense sand and gravel (40–50 ft); Late Quaternary alluvium, San Fernando Valley
Foundation Type
Mat (raft) foundation, minimum 12 in. thick, with 5 ft over-excavation and recompaction

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