Soil Reports for Retaining Walls

1. What Is In a Geotechnical Report?

A geotechnical report for a retaining wall typically includes:

• Soil classification — AS 2870 site classification (A, S, M, H1, H2, E, P) for reactive soil behaviour
• Soil profile description — what materials exist at what depth (clay, sand, fill, rock)
• Soil parameters — friction angle (φ), cohesion (c), unit weight, and bearing capacity for use in wall design
• Groundwater — depth to groundwater and seasonal variation
• Recommendations — specific advice for foundation type, drainage requirements, and construction method

2. AS 2870 Site Classifications

The AS 2870 classification describes the reactivity of soil to moisture changes. For retaining wall design it matters because reactive soils exert variable lateral pressure depending on season:

• Class A — most stable (sand, rock). Minimal moisture-related movement
• Class S — slightly reactive clay. Some seasonal movement
• Class M — moderately reactive clay. Noticeable seasonal movement
• Class H1 / H2 — highly reactive clay. Significant seasonal movement; retaining walls in H1/H2 soils need specific design attention
• Class E — extremely reactive. Specialist design required
• Class P — problem site (fill, soft ground, collapsing soil). Individual assessment required for every structure

Large parts of urban Australia — particularly Melbourne, Adelaide, and parts of Brisbane — have M, H1, or H2 classified soils. If you don't know your site classification, a soil report will tell you.

3. When Is a Soil Report Required?

A geotechnical report is required for retaining wall design when:

• Wall height exceeds 1.5m (most councils and engineers will require soil data)
• The site is on a slope with any indication of previous movement or soft soils
• The wall is near a building foundation (differential settlement risk)
• Surcharges from pools, buildings or driveways are involved
• The soil is visibly variable — soft patches, fill, or organic material is evident

For walls under 1m in straightforward conditions (known soil type, no surcharges, no slope), experienced engineers may design using assumed soil parameters. This is acceptable in low-risk situations but the engineer is carrying the risk of those assumptions.

4. Cost and Process

A residential geotechnical report for a retaining wall project typically costs $800–$2,500 depending on:

• Number of borehole or test pit locations required
• Depth of investigation required (deeper for taller walls)
• Laboratory testing (sieve analysis, Atterberg limits, etc.)
• Urgency — expedited reporting adds cost

The investigation typically takes half a day on site and 1–2 weeks to report. Commissioning the geotechnical investigation early — before or alongside the engineering design engagement — avoids delays later.

Retaining wall cost guide →