How to Read a Soil Investigation Report: What SPT N-Values Really Tell You

When rebuilding or constructing a new home, the Soil Investigation Report (SI) is one of the most important — and least understood — documents.

Many clients ask us:

• “What’s an SPT N-value?”

• “What kind of soil do I have?”

• “Will it affect my construction cost?”

This guide breaks it down in simple, visual language — and helps you spot red flags, especially if you’re building on soft ground or encountering unusually high resistance.

Key Sections of a Soil Investigation Report

A typical SI report includes:

Section	What It Shows
Borehole Log	Visual cross-section of the soil layers at each drilling point
SPT N-Values	Strength and density of each soil layer
Soil Description	Clay, sand, silt, gravel, or mixed
Groundwater Table	Depth at which water is encountered (affects foundation)
Recommendations	Suggested foundation method: raft, footings, bored piles, etc.

How to Read the Borehole Log

Imagine the borehole log like a layered cake sliced vertically. It shows:

• Topsoil (0–1.5m): Usually soft or mixed fill

• Subsoil Layers: May alternate between clay, sand, silty clay, etc.

• Depth markers every 0.5m or 1m

• SPT N-values indicated beside each depth

Each log entry will include:

Example: Silty CLAY (firm), greyish brown, moist, N=17

Interpreting SPT N-Values

SPT (Standard Penetration Test) is done at regular intervals to check soil strength. It shows how many hammer blows are needed to drive a steel sampler into the soil by 300mm.

N-Value Range	Soil Description	Engineering Implication
0–4	Very loose/soft	Poor support, needs piles
5–10	Soft clay / loose sand	Risk of differential settlement
11–30	Medium dense/firm	May be okay for footings
31–50	Dense/stiff	Good bearing strength
>50 or Refusal	Very dense / boulder / rock	May require socketed piles or excavation

When High N-Values Are a Problem

Most homeowners assume high N-values = strong ground = good.

That’s not always true.

Here’s why excessively high N-values (e.g. > 50) may indicate problems:

1. Possible Boulder or Obstruction

If refusal happens too early (e.g. 2–3m depth), the pile might not embed deep enough to safely support the house. You’ll need:

• Core sampling

• New pile layout or raft design

• Special drilling equipment

2. Shallow Hard Layer

If a hardpan or rock layer sits too shallow, the foundation may not gain proper anchorage.

This affects structural safety and cost.

Foundation Design Impacts

Depending on the SI report, your engineer may recommend:

Soil Type	Recommended Foundation
Very soft clay	Bored piles, micro piles
Loose sand	Raft with deep ground beams
Stiff clay/dense soil	Footings may be sufficient
Refusal layer	Socketed piles or rock anchoring

Poor Soil = Piling needed

Questions to Ask Your Builder/Engineer

1. Is the high N-value due to rock or obstruction?

2. What’s the depth of stable soil layer?

3. Will we need longer piles or special pile cap?

4. Does the soil support raft foundation instead?

🛠 Bonus: Red Flags in Soil Reports

• Sudden jump in N-value from 10 → 60 = potential boulder

• Constant N=0 for >1.5m = very soft compressible clay

• Groundwater table < 1.5m = waterproofing and piling cost increase

Summary Table: SPT Interpretation

Depth (m)	Soil Type	N-Value	Meaning
0.0–1.5	Fill	3	Soft, unsuitable for footing
1.5–3.0	Clayey SAND	12	Medium, needs reinforcement
3.0–5.0	Silty CLAY	25	Firm, possible for shallow footings
5.0–6.0	Dense gravel	55	Refusal likely — investigate further

Final Thoughts

Your Soil Investigation Report is not just a technical document — it’s the foundation of your foundation. Understanding it helps you:

• Avoid unnecessary rework

• Plan construction cost more accurately

• Choose the right builder and structural system

Always review your report with your builder and structural engineer before finalizing your construction budget.