A soil investigation report is the document your structural engineer reads first. Translating its jargon into project consequences saves money and avoids over-engineering.
What's in a typical report
- Site location plan and history
- Trial pit or borehole logs
- Soil descriptions by stratum
- SPT (Standard Penetration Test) values
- Laboratory test results (plasticity, sulfate, pH)
- Groundwater observations
- Recommendations for foundation type and bearing depth
Key parameters explained
SPT N-value
Number of blows to drive a sampler 300 mm. Indicates soil density/stiffness:
- 0–4: Very loose / very soft (poor bearing)
- 5–10: Loose / soft
- 10–30: Medium dense / firm
- 30–50: Dense / stiff
- >50: Very dense / hard (good bearing)
Plasticity Index (PI)
How much the soil swells and shrinks with moisture. Critical for clay soils:
- PI < 20: Low shrink-swell
- PI 20–40: Medium
- PI > 40: High (heave risk, foundation depth typically 1.5m+ minimum)
Sulfate class (DS-class)
Risk of sulfate attack on concrete. Determines cement type and design class:
- DS-1: Low (standard concrete OK)
- DS-3+: Significant (sulfate-resisting cement required)
Made ground
Fill material from previous development. Unpredictable bearing capacity — usually requires deeper foundations to reach natural strata.
Consequences for design
- Shallow strip foundations on good ground: R3,600–R5,200/m³ excavation
- Deep trench-fill on clay: R8,000–R14,000/m³
- Piled foundations on poor ground or made ground: R300,000–R900,000 for typical domestic project
- Raft foundations on heave-prone or variable ground: R160,000–R400,000+
When to commission
Before serious design work. Foundation costs swing by 5–10× depending on ground conditions; designing without a soil report is gambling. Typical cost: R24,000–R70,000 for a domestic-scale investigation.
MCFAR reviews soil reports and designs foundations to suit your site.
Need expert engineering on your project?
MCFAR GROUP has been delivering structural engineering, building, and plumbing services since 1998. Talk to our team about your build, retrofit, or renovation.
Request a QuoteFrequently Asked Questions
Do I always need a soil investigation?
For extensions on known stable ground (away from trees, no history of subsidence), trial pits by your engineer may suffice. New builds and complex sites always need formal investigation.
What's the difference between trial pits and boreholes?
Trial pits go 2–4m deep, give visual inspection. Boreholes go deeper (10m+), needed for piled foundations or deep basements.