G2 Soil Survey Before Buying: Understanding Foundations and Preventing Cracks (French Riviera & Var)

Buying land to build on? Noticing cracks in your house that keep getting worse? Before you sign a preliminary sales contract or commission underpinning works, one factor is decisive above all others: the nature of the soil. That is exactly what a G2 geotechnical survey reveals — a standardised assessment (French standard NF P 94-500) that characterises the ground and its bearing capacity.

Unlike a structural building survey — which analyses visible defects, deformation and construction pathologies —, a G2 soil survey answers the question the building itself cannot ask: what’s happening beneath the foundations? This data is essential for designing suitable foundations, informing a purchase decision, or supporting a drought-subsidence insurance claim (RGA natural disaster declaration).

What is a G2 soil survey?

The design-stage geotechnical survey, known as a G2 assessment, is defined by French standard NF P 94-500. It comes in two complementary phases:

• G2 AVP (preliminary design): geotechnical characterisation of the site, identification of risks (clay shrink-swell, groundwater, cavities, fill), and general principles for adapting the project.
• G2 PRO (detailed design): precise foundation sizing, detailed construction recommendations (foundation type, embedment depth, seismic provisions where applicable), and a geotechnical calculation report.

When buying land or a house, the G2 AVP phase is often enough to inform the decision (presence of swelling clay, risk of differential settlement, projected cost of special foundations). The PRO phase then follows, during the design stage of the building project, to size the foundation works.

Note: a G2 survey does not replace a building survey. The geotechnical engineer characterises the ground; the independent building surveyor analyses the existing structure, interprets the geotechnical findings in light of observed defects (cracks, subsidence, deformation) and coordinates the interventions. These are two complementary missions, not competing ones.

What does a G2 AVP/PRO assessment actually include?

A G2 soil survey relies on on-site investigations and laboratory testing:

1. Boreholes and samples

Mechanical boreholes (auger, dynamic penetrometer, core drilling) are carried out on site to identify the different soil layers at depth (typically 5 to 10 metres, sometimes more depending on the context). Samples are taken for analysis.

2. Laboratory testing

The samples undergo standardised tests:

• Identification tests: particle size distribution, Atterberg limits (to characterise clay content), and natural moisture content.
• Mechanical behaviour tests: oedometer tests (settlement under load), shear tests (bearing capacity), and shrinkage limits (sensitivity to drought).
• Chemical tests where necessary (aggressiveness of groundwater towards concrete).

3. Site characterisation

The G2 report brings together:

• The stratigraphy (sequence of soil layers, thicknesses, nature: clay, sand, marl, limestone, fill, etc.).
• Any groundwater present (depth, estimated seasonal variation).
• Geotechnical parameters: bearing capacity (allowable pressure), expected settlement, internal friction angle, and cohesion.
• The soil’s shrink-swell risk classification (low, medium, high — based on the BRGM national clay-risk mapping).

4. Foundation recommendations

Based on the soil characteristics and the project, the geotechnical engineer recommends:

• The appropriate foundation type: shallow (strip footings, isolated pads), semi-deep (piers), or deep (piles, micropiles).
• The minimum embedment depth (below frost level, and below the drought-influence zone for clay soils).
• Construction measures: horizontal and vertical tie beams, movement joints, perimeter drainage, and root barriers (if vegetation is nearby).
• Execution precautions: trench shoring, groundwater lowering if needed, and inspection of the excavation base.

Who carries out a G2 soil survey?

A G2 survey is carried out by a geotechnical engineering firm, led by a qualified geotechnical engineer. This professional specialises in soil mechanics, laboratory testing and geotechnical engineering, with the drilling equipment (rig, penetrometer) and expertise needed to interpret the results.

The independent building surveyor does not carry out the soil survey. Their role is different, and complementary:

• They coordinate the geotechnical assignment: recommending the scope of investigation and checking that the commissioned survey actually matches the need (AVP or PRO, number of boreholes, relevant tests).
• They interpret the results in relation to the building: correlating observed defects (cracks, settlement) with the soil’s characteristics (swelling clay, heterogeneity, compressibility), and assessing whether the defects are consistent with the geotechnical conclusions.
• They put forward overall recommendations: the appropriate underpinning technique (micropiles, grouting, soil replacement), management of the soil/structure interface, and ongoing monitoring.

At Cassini Expertise (based in Antibes, covering the Alpes-Maritimes and Var), with 15 years’ experience in public works and project management, we handle this coordination and interpretation role with complete independence — we have no financial ties to construction firms or engineering practices. We use professional measuring instruments (Trotec) to diagnose the building, and we align our conclusions with the geotechnical data.

When should you commission a G2 soil survey?

1. Before buying land or a house

Buyers have every interest in including a G2 soil-survey condition precedent in the preliminary sales contract. This clause allows you to:

• Check that the soil can support the building project you have in mind.
• Estimate any extra cost linked to special foundations (micropiles for compressible soil, deep embedment for swelling clay).
• Withdraw from the purchase with full knowledge of the facts if the findings reveal a major geotechnical risk (a cavity, uncompacted heterogeneous fill, or shallow groundwater).

This approach protects your investment. The survey is usually commissioned before the final deed is signed, during the cooling-off period or while the conditions precedent are being fulfilled.

2. When cracks are getting worse

When a house has cracks that widen with the seasons, a building survey identifies the structural pathologies, but a G2 soil survey provides objective proof of how the ground is behaving: clay shrink-swell (RGA), differential settlement, scouring, and so on. This data is essential to:

• Build a case for official recognition of a drought-related natural disaster (cat-nat order, insurance compensation).
• Design the underpinning works (micropile embedment depth, number and layout).

3. Before foundation repair works

Any work on foundations (underpinning, an extension, adding a storey) requires precise knowledge of the bearing soil. The G2 PRO survey provides the calculation parameters the structural engineering firm needs.

4. Before selling a property (protecting the seller)

Since the 2018 ELAN law, selling buildable land in a medium- or high-risk clay shrink-swell zone requires a preliminary G1 geotechnical survey to be attached to the preliminary contract. But nothing stops a seller from providing a more thorough G2 survey instead, to reassure the buyer and smooth the transaction.

How much does a G2 soil survey cost?

The cost of a G2 soil survey varies depending on several factors:

• The number of boreholes required (depending on the plot’s size, its suspected heterogeneity, and the type of project).
• The required investigation depth.
• The laboratory tests to be carried out (mechanical, chemical, clay-content measurement).
• Site accessibility (terrain, vegetation, room for equipment to manoeuvre).
• Local geological complexity (karst areas, man-made fill, former quarries).

There is therefore no standard price. Each assignment is quoted individually by the geotechnical firm after a site visit and a precise definition of the need. As building surveyors, we help you scope the geotechnical assignment and put you in touch with competent, independent geotechnical engineers.

4 frequently asked questions about the G2 soil survey

What’s the difference between a G1, G2 and G5 survey?

French standard NF P 94-500 sets out several levels of geotechnical assignment:

• G1 (preliminary geotechnical survey): an initial-approach assessment at the scale of a site or area. It identifies major geotechnical risks but cannot be used to size foundations. Mandatory when selling buildable land in a medium- or high-risk clay zone (ELAN law).
• G2 (design-stage geotechnical survey): a project design assessment. It precisely characterises the soil beneath the project and provides foundation recommendations. This is the reference survey before construction or foundation repair.
• G5 (geotechnical diagnosis): an assessment carried out on an existing building, following damage or as part of a modification project. It combines analysis of the defects with supplementary geotechnical investigation (boreholes, tests) to identify the causes of the damage.

In short: G1 to sell or buy land (initial approach), G2 to build (foundation design), and G5 to diagnose damage (existing building).

Is a G2 survey mandatory for construction?

Yes, in most cases. Since the ELAN law and its 2020 implementing decree, a G2 design-stage geotechnical survey is mandatory before building any single-family house, whether it’s new construction on bare land or an extension with new foundations. This obligation falls on the client (the private individual having the house built), and the builder (design-and-build contractor, architect, or project manager) must ensure it is carried out before work starts.

In addition, the structural damage insurer may require a G2 survey to cover the project. Without a compliant soil survey, coverage exclusions can be invoked if damage related to the foundations occurs.

Who pays for the soil survey in a property purchase?

It depends on the negotiation between seller and buyer:

• For the sale of buildable land in a medium- or high-risk clay zone, the seller must provide a G1 survey attached to the preliminary contract (a legal obligation, at their own cost). The buyer can, if they wish, commission a G2 at their own expense to go further.
• For the sale of a house that already has cracks, nothing obliges the seller to provide a soil survey. But a well-informed buyer has every interest in making the purchase conditional on a G2 or G5 survey, at their own expense, with a condition precedent allowing them to withdraw or renegotiate the price depending on the findings.

In practice, buyers looking to protect their purchase often bear the cost of the G2 survey themselves, viewing it as an investment to avoid unpleasant surprises — unsuitable foundations, or underestimated repair costs.

Does a G2 survey guarantee there will be no cracks in future?

No. A G2 soil survey provides a snapshot of the site’s geotechnical characteristics at a given moment and foundation recommendations suited to the conditions found. It significantly reduces the risk of defects, but it is not an absolute guarantee, for several reasons:

• Boreholes are localised: they don’t cover the entire plot (local variation is possible).
• Future climate conditions (exceptional droughts, fluctuating groundwater) can fall outside what the survey anticipated.
• Performance depends on the geotechnical recommendations actually being followed on site: a workmanship defect (insufficient embedment depth, missing tie beams) can still cause damage even with a well-executed G2 survey.
• Later changes to the surrounding land (nearby trees being felled, altered drainage, neighbouring earthworks) can upset the existing balance.

A G2 survey is therefore a design tool, not an all-risks insurance policy. It should be backed up by careful foundation execution (a G3 geotechnical execution-monitoring assignment is recommended) and vigilant upkeep of the building (managing rainwater, keeping an eye on nearby vegetation).

Cassini Expertise supports you across the Alpes-Maritimes and Var

As an independent building surveyor based in Antibes, Cassini Expertise works throughout the Alpes-Maritimes and Var on every kind of technical building assessment: structural cracks, defective workmanship, latent defects, damp-related pathologies, drought damage (RGA), and pre-purchase surveys.

With 15 years’ experience in public works and project management, we guarantee complete independence (no ties to construction firms, insurers or developers) and use professional measuring instruments (moisture meters, Trotec thermal camera) to back up our findings with hard data.

For any geotechnical issue — buying land, worsening cracks, or a planned underpinning project — we coordinate the G2 soil survey, interpret its findings in light of the building’s defects, and put forward coherent, proportionate work recommendations.

Contact:
Phone: 04 22 46 06 04
Email: [email protected]
Service area: Alpes-Maritimes (06) and Var (83)

Related articles

• Pre-purchase crack audit: the checklist
• RGA crack diagnosis 06-83
• Buying a cracked house in the Alpes-Maritimes

📅 Mis à jour le 25/09/2026 — Cassini Expertise, expert bâtiment indépendant (06·83·13·40).