What is Soil Investigation and Why is it Mandatory?

Soil Investigation: The Foundation of the Invisible Foundation

The permanence of every structure depends on the characteristics of the ground it is built on. Foundation systems designed with incorrect or insufficient soil knowledge can lead to settlement, cracking, tilting, and in the worst case, structural collapse. Soil investigation is a technical necessity that eliminates these risks from the outset. In this guide, we address in detail what soil investigation is, how it is done, and why it is of vital importance.

What is Soil Investigation?

Soil investigation (geotechnical investigation) is the entirety of the scientific research process carried out to determine the physical, mechanical, and chemical properties of the land on which a structure will be built. It covers drilling, laboratory experiments, and field tests; as a result, a soil report providing input for structural design is obtained.

Legal Requirement

In Turkey, since 2007, submitting a soil investigation report in building permit applications has been a legal requirement (Regulation on Buildings to be Built in Earthquake Zones and Turkish Standards). Starting construction without having a soil investigation done both prevents obtaining a permit and can create serious legal liability.

How is Soil Investigation Done?

Stage 1: Preliminary Field Survey

The geotechnical engineer visits the plot; observes surface conditions, neighboring structures, existing data, and possible risks. At this stage, drilling points and depths are planned.

Stage 2: Drilling and Sample Collection

Holes are drilled to certain depths (usually 10–20 m, deeper for some projects) in the ground using mechanical drilling machines. Soil samples are collected from each depth. Rotary drilling is the most common drilling method.

Stage 3: Field Tests

During drilling or separately, the following tests are performed:

• SPT (Standard Penetration Test): Determines soil hardness and bearing capacity; the most common field test.
• Pressuremeter test: Measures soil deformation modulus.
• Groundwater level determination

Stage 4: Laboratory Tests

Collected samples are analyzed in accredited laboratories:

• Granulometry (grain size distribution)
• Atterberg limits (liquid limit, plastic limit)
• Unconfined compression and triaxial shear strength
• Consolidation (settlement estimation)
• Chemical analysis (sulfate, chloride content — affects concrete)

Stage 5: Report Preparation

A soil report is prepared by evaluating all data. The report includes: soil profile (layer sequence and characteristics), soil bearing capacity (safe foundation pressure), groundwater condition, settlement estimates, recommended foundation type and depth, earthquake parameters (local soil class, amplification factors).

How Does the Soil Investigation Report Affect Foundation Design?

Soil Condition	Recommended Foundation Type
Hard/rocky soil, high bearing capacity	Isolated or strip foundation (shallow foundation)
Medium-hard soil	Raft foundation (mat foundation)
Soft/fill soil, low bearing capacity	Pile foundation or ground improvement
High groundwater	Waterproofed basement + drainage system

What Happens if Construction Starts Without Soil Investigation?

• If there is an unexpected soft layer under the foundation, the structure settles and cracks.
• Basements built without knowing the groundwater level get flooded.
• A structure built with the wrong foundation type sustains much more damage in earthquakes.
• The building remaining as an unlicensed structure faces legal problems and demolition risk.

Soil Investigation Cost and Duration (2026 Turkey)

• For a single villa plot: ₺25,000–₺60,000 (depending on number of borings and depth)
• Duration: Field work 1–3 days, laboratory + report 1–3 weeks
• Important note: Soil investigation cost is generally 0.1–0.3% of the total construction budget. Such a small investment prevents much larger costs such as foundation renewal.

Conclusion

Soil investigation is the first and most critical step in placing your villa on a correct foundation. Skipping soil research to save money invites much more costly problems in the coming years. As Tunahanyeniçeri, we treat soil investigation as an integral part of the standard process in all our villa projects and design the safest and most economical foundation solution based on the results.