6.3 Soils, Compaction, and Erosion/Sediment Control
Key Takeaways
- IBC Table 1806.2 presumptive bearing values: bedrock ~12,000 psf, sandy gravel ~3,000 psf, clay/silt ~1,500 psf.
- Footing capacity = allowable bearing (psf) × footing area (sq ft); size footings to keep load at or below capacity.
- Standard Proctor is ASTM D698; modified Proctor is ASTM D1557; structural fill is usually 95% of modified Proctor.
- Compact fill in 6–12 in lifts near optimum moisture; too wet or too dry soil will not reach target density.
- Disturbing 1 acre or more requires an NPDES Construction General Permit and a written SWPPP with BMPs like silt fence.
Soils, Compaction, and Erosion/Sediment Control
The bearing capacity and stability of a building depend on the soil beneath it. The International Building Code (IBC) Table 1806.2 gives presumptive load-bearing values when no geotechnical report is available. Crystalline bedrock carries about 12,000 psf, sand and gravel about 3,000 psf, and clay/silt about 1,500 psf. Expansive (high-clay) soils swell and shrink with moisture and are flagged under IBC 1803.5.3 for special investigation.
Presumptive load-bearing values (IBC Table 1806.2)
| Material class | Allowable bearing (psf) |
|---|---|
| Crystalline bedrock | 12,000 |
| Sedimentary/foliated rock | 4,000 |
| Sandy gravel / gravel (GW, GP) | 3,000 |
| Sand, silty sand, clayey sand (SW, SP, SM, SC) | 2,000 |
| Clay, sandy clay, silty clay (CL, ML) | 1,500 |
These values use the Unified Soil Classification System (USCS). Quick check: a 2,000 psf soil under a 2 ft × 2 ft isolated footing (4 sq ft) supports 8,000 lb before exceeding bearing — a typical worked footing-sizing question.
Compaction and the Proctor test
Fill is compacted in lifts and verified against a laboratory maximum dry density.
- Standard Proctor — ASTM D698 establishes the baseline maximum dry density and optimum moisture content.
- Modified Proctor — ASTM D1557 uses higher compactive energy for heavier loads (roads, structural fill).
- Field density — ASTM D6938 (nuclear gauge) or sand-cone confirms percent compaction.
Structural fill is typically specified at 95% of modified Proctor; landscaped or non-structural fill may accept 90%. Compact in 6–12 in lifts and keep moisture near optimum — soil too wet or too dry will not reach target density, a frequent inspection-failure scenario.
Erosion and sediment control (NPDES / EPA)
Sites disturbing 1 acre or more need a Construction General Permit (CGP) under the federal NPDES stormwater program and a written Stormwater Pollution Prevention Plan (SWPPP).
Common Best Management Practices (BMPs):
- Silt fence — perimeter sediment barrier on the down-slope edge; trench in the toe.
- Stabilized construction entrance — crushed stone to knock mud off tires.
- Inlet protection — keeps sediment out of storm drains.
- Sediment basin — required for 10+ acres draining to a common point.
Controls must be inspected after rain events and maintained until the site is permanently stabilized with vegetation or pavement.
A geotechnical report is unavailable, so the contractor uses IBC presumptive values. A 3 ft × 3 ft isolated footing bears on sandy gravel rated 3,000 psf. What maximum column load can it support?
Under the federal NPDES Construction General Permit, what minimum area of land disturbance triggers the requirement for a SWPPP?
Soil Types and Bearing
Soils are classified for bearing capacity and behavior. Gravels and sands drain well and bear well; clays have high bearing when dry but shrink/swell with moisture (expansive soils heave foundations); silts and organic soils are weak and often must be removed and replaced. The presumptive bearing values in IBC Table 1806.2 (e.g., ~1,500 psf for clay/silt, ~3,000 psf for sand/gravel, higher for rock) guide footing sizing when no geotech report exists.
Compaction and the Proctor Test
Fill is placed in lifts (typically 6–12 in) and compacted to a specified percentage of maximum dry density from a Proctor test (ASTM D698 standard / D1557 modified). Common spec: 95% of modified Proctor under structural fill and pavement. Optimum moisture content is the water content at which maximum density is achieved — too wet or too dry and the soil will not compact. A nuclear density gauge verifies field compaction.
Erosion and Sediment Control (SWPPP)
Sites disturbing 1 acre or more need an EPA NPDES stormwater permit and a SWPPP. Controls include silt fence, inlet protection, stabilized construction entrances (rock pad to knock mud off tires), sediment basins, and quick seeding/mulching of disturbed areas. The goal is to keep sediment on site and out of storm drains and waterways; the Clean Water Act drives the requirement and fines.
Common Exam Traps
- Trap: Clay always bears poorly. Dry clay can bear well but is expansive with moisture.
- Trap: One thick lift compacts as well as several thin lifts. Compact in 6–12 in lifts.
- Trap: 95% compaction needs no moisture control. Optimum moisture is essential.
- Trap: SWPPP threshold is any disturbance. It triggers at 1 acre.
Structural fill must be compacted to 95% of maximum dry density. Which test establishes that maximum dry density and the optimum moisture content?
Dewatering, Shrink/Swell, and Bearing Verification
Where groundwater is high, dewatering (well points, sumps) lowers the water table so the excavation stays stable and concrete is not placed in water. Account for shrink and swell: soil bulks (swells) when excavated (loose) and compacts (shrinks) when placed, so bank-cubic-yards, loose-cubic-yards, and compacted-cubic-yards differ — a common earthwork takeoff trap. Before footings, the geotechnical engineer or special inspector verifies bearing at the subgrade; soft spots are undercut and replaced with engineered fill.
Geotextiles, Subgrade Stabilization, and Frost
Geotextiles separate and stabilize: a woven fabric between soft subgrade and aggregate base prevents the stone from punching into the soil, while a non-woven fabric filters fines from a drain. Frost-susceptible silty soils heave when they freeze and trap water; designers cut frost-prone soil to the frost depth or replace it with free-draining granular fill. Erosion controls must be installed before clearing begins and maintained after every storm until the site is permanently stabilized — installing them late is a frequent SWPPP violation.
Worked Compaction Lift Calculation
Example: A 4-ft-deep structural fill must be placed in 8-in lifts compacted to 95% modified Proctor. Number of lifts = 48 in / 8 in = 6 lifts, each tested before the next is placed. If the field density gauge reads only 91% on a lift, the crew must re-roll (and adjust moisture toward optimum) until it passes — it cannot be buried under the next lift. This lift-by-lift discipline is exactly what a special inspector documents, and skipping a failing lift is the classic earthwork exam trap.