By Topic Intermediate

Soil Survey Results: How to Adapt Your Foundations

12 min read

What Is a Soil Survey?

A soil survey, or geotechnical study, is a scientific analysis of the ground on which you plan to build. It is carried out by a specialist firm that performs boreholes, sampling and laboratory testing. The result is a technical report describing the soil layers, their strength, the presence of water and geological risks.

The most common study for individual house construction is the G2 AVP study (Preliminary Design), which corresponds to the geotechnical design mission under French standards.

flowchart TD A{Soil type?} A -->|Rock| B[SHALLOW FOOTINGS] A -->|Compact sand| C[STRIP FOOTINGS] A -->|Clay| D{Shrink-swell risk?} A -->|Fill / Silt| E[PILES] D -->|Low| F[RAFT FOUNDATION] D -->|High| G[DEEP FOUNDATIONS] style A fill:#0F4C81,stroke:#0F4C81,color:#fff style D fill:#FDFCF9,stroke:#C67A3C,color:#0F4C81 style B fill:#56C6A9,stroke:#56C6A9,color:#fff style C fill:#56C6A9,stroke:#56C6A9,color:#fff style F fill:#F58220,stroke:#F58220,color:#fff style G fill:#CD212A,stroke:#CD212A,color:#fff style E fill:#6B5876,stroke:#6B5876,color:#fff

The Different Levels of Geotechnical Study

Mission Full name Purpose Who commissions it
G1 ES Site study Identify major geological risks Land seller
G2 AVP Preliminary Design Define foundation principles Client / Owner
G2 PRO Detailed Design Precisely size foundations Structural engineer
G3 Execution Supervise construction Foundation contractor
G4 Supervision Monitor execution Client / Owner

💡 Tip — For an individual house, the G2 AVP mission is the essential minimum. It costs between €1,500 and €3,000 depending on site complexity and region.

How to Read a Soil Report

A geotechnical report generally contains the following sections:

1. Geological Context

The geotechnician describes the local geology from BRGM maps and site observations. You will find the expected geological formations (limestone, clay, alluvium, etc.).

2. Boreholes and Tests

Several types of investigation are used:

  • Auger borehole: mechanical drilling to observe soil layers
  • Pressuremeter test: measures soil strength and deformability at depth
  • Penetrometer test: drives a cone into the soil to measure resistance
  • Piezometer: measures the water table level

3. The Geotechnical Model

The report presents a cross-section of the soil showing the different layers identified, their thickness, nature and mechanical properties.

4. Foundation Recommendations

This is the most important part for you. The geotechnician recommends:

  • The suitable foundation type
  • The founding depth
  • The allowable bearing pressure
  • Special precautions (drainage, soil treatment, etc.)

⚠️ Warning — Do not try to interpret a geotechnical report on your own. Have it reviewed by your architect or structural engineer, who will translate the recommendations into concrete foundation plans.

Main Soil Types and Their Foundations

Clay Soil

Clay is the most problematic soil type in France and many parts of Europe. It swells with moisture and shrinks during dry periods (shrink-swell phenomenon). This causes ground movements that can crack foundations and walls.

Recommended foundations:

  • Deepened strip footings (80 cm to 1.20 m minimum)
  • Rigid raft with movement joints
  • In severe cases: piles or micropiles

Specific precautions:

  • Keep trees away from foundations (distance = mature height of the tree)
  • Install perimeter drainage
  • Maintain constant moisture around the foundations

Sandy Soil

Sand generally offers good bearing capacity but can cause settlement problems if the grains are fine and loose. In the presence of water, it can liquefy during significant vibrations.

Recommended foundations:

  • Standard strip footings (if compact sand)
  • Raft (if loose sand or high water table)

Rocky Soil

Rock is the ideal soil for building: it offers excellent bearing capacity and barely deforms. The main challenge is excavation, which may require a hydraulic rock breaker.

Recommended foundations:

  • Shallow strip footings
  • Direct anchoring into the rock if possible

Silty or Backfilled Soil

Silts are fine soils, often damp, with low bearing capacity. Backfill is even more problematic as its compaction is irregular.

Recommended foundations:

  • Ribbed raft
  • Piles if the poor soil layer is thick
  • Soil replacement over shallow depth

Summary Table: Soil vs. Foundation

Soil type Typical bearing capacity Recommended foundation Risk level
Sound rock > 5 bars Shallow strip footings Low
Compact gravel 2 to 5 bars Strip footings Low
Compact sand 1.5 to 3 bars Strip footings Medium
Firm clay 1 to 2 bars Deep strip footings or raft High
Silt 0.5 to 1.5 bar Raft or piles High
Backfill Variable Piles or soil replacement Very high

✅ Checklist: soil survey

  • G1 ES study provided by the seller (mandatory in clay zones in France)
  • G2 AVP study commissioned from a certified firm
  • Report sent to architect and structural engineer
  • Foundation recommendations integrated into plans
  • Foundation budget adjusted according to recommendations
  • Specific precautions identified (drainage, tree distances, etc.)

Summary

The soil survey is the founding document for your foundation project. It costs little compared to the problems it can prevent. Take the time to commission one before even finalising your house plans: the results may influence the choice between crawl space and raft, require piles, or necessitate a foundation budget far higher than initial estimates.