Hillside ADU on Steep Terrain

Beverly Hills, CA

Slopes up to 1:1 (H:V); 24-in. drilled piles to 20 ft into bedrock; 80 pcf soil-creep lateral load

Overview

A homeowner wanted to add a 1,200 SF accessory dwelling unit on a steep, canyon-facing hillside behind their existing home. Moment performed a full geotechnical investigation — including review of nine historic reports from adjacent parcels — and designed a deep drilled-pile foundation system to support the structure on slopes as steep as 1:1.

The Challenge

The ADU sits on descending slopes ranging from 4:1 to locally steeper than 1:1, with fill of unknown compaction history (sandy silt to clayey silt with shale fragments, up to 8 ft thick in the ADU area and 10–15 ft near the canyon edge) overlying Modelo Formation shale/siltstone bedrock. An adjacent parcel had a documented shallow slope failure, and the upper 8 feet of the slope are subject to long-term soil creep — meaning the foundation had to resist sustained lateral pressures equivalent to 80 pcf fluid pressure, not just gravity loads.

Our Approach

Moment excavated two test pits to 5–10 ft using hand auger and California Sampler, and reviewed nine historic geotechnical reports from neighboring properties to build a continuous subsurface profile along the ridgeline. We designed a 24-inch-diameter drilled cast-in-place pile and grade-beam system that derives 100% of its capacity from bedrock — with zero reliance on the overlying fill for either skin friction or end bearing. Each pile extends a minimum of 20 ft total length with at least 10 ft of embedment into competent bedrock (or 2 pile diameters, whichever is greater). Grade beams are physically separated from the slope surface to avoid soil contact and creep loading.

Results

The deep foundation system was designed and approved for permitting, allowing the ADU to proceed on a site that would otherwise be unbuildable with conventional foundations. Estimated settlement is less than 1 inch total and less than 0.25 inches differential over 30 feet — well within structural tolerances.

Technical Summary

Slopes up to 1:1 (H:V) with 40 ft vertical descentAdjacent parcel with documented shallow slope failureSoil creep zone — sustained lateral load design at 80 pcf equivalent fluid pressureFill of unknown compaction over bedrock — zero bearing or friction capacity assigned to fillSite Class C per CBC

Borings

2 test pits (5–10 ft depth, hand auger + California Sampler) plus 9 historic reports from adjacent parcels

Soil Conditions

Sandy silt to clayey silt fill with shale fragments (8 ft in ADU area, 10–15 ft at canyon edge) over Modelo Formation shale/siltstone bedrock

Foundation Type

24-in. drilled cast-in-place piles with tied grade beams; minimum 20 ft length, minimum 10 ft embedment in competent bedrock

Working on a similar project?

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

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