A solid building starts underground, long before anything rises above it. A soil investigation (also called a geotechnical survey) uses scientific methods to examine the physical and mechanical properties of the land a structure will stand on, and it guides the design of the foundation. Whether you are building a home, a commercial property or an industrial facility, the safety, service life and earthquake performance of the project depend heavily on understanding the ground correctly. In this article we explain, in plain language, what a soil investigation is for, what happens on site and in the laboratory, how the report affects the foundation and where it fits in the permit process.
What Is the Purpose of a Soil Investigation?
The main goal is to know in advance how the ground will behave under the loads of the building. Even two plots in the same neighbourhood can have very different conditions: one may sit on solid rock a few metres down, while the plot next door has soft clay or loose fill. A soil investigation seeks answers to questions such as:
- Which layers make up the ground, and how thick are they?
- Can the bearing capacity of the soil safely carry the building loads?
- At what level is the groundwater, and how will it affect construction?
- Is there a risk of liquefaction, excessive settlement or ground motion amplification during an earthquake?
- Are there special hazards such as slope instability, landslides or swelling clay?
Without this information, a foundation design is either too weak, creating risk, or unnecessarily heavy, wasting resources.
Fieldwork: Boreholes and In-Situ Tests
The field stage usually begins with drilling. The number and depth of boreholes are set according to the size of the plot, the height of the building and the local geology. Disturbed and undisturbed samples taken from these boreholes are the most valuable evidence of what the ground is really like.
During drilling, the Standard Penetration Test (SPT) is commonly performed, giving a direct indication of the density and strength of the soil. Geophysical methods are added where needed: seismic refraction and surface wave measurements reveal shear wave velocities in the ground, which play a key role in determining the soil class used in earthquake calculations. The groundwater level is also measured in the boreholes and recorded.
What Do Laboratory Tests Reveal?
Samples from the site are sent to a laboratory, where the engineering properties of the soil are examined in detail. The exact tests depend on the soil type, but they generally fall into these groups:
- Classification tests: Grain size distribution, water content and consistency limits identify the type of soil.
- Strength tests: Shear box and triaxial tests measure how much load the soil can withstand.
- Consolidation tests: Especially in clay soils, these estimate the settlement that may develop over time.
- Chemical tests: When necessary, the soil and groundwater are checked for components such as sulphates that can damage concrete.
How Does the Report Shape Foundation Design?
Field and laboratory data are combined into a soil and foundation investigation report. This report is the most important reference for the structural engineer preparing the structural design. The recommended allowable bearing pressure, modulus of subgrade reaction, soil class and identified risks directly determine which type of foundation is chosen.
On strong, uniform ground, isolated or strip footings may be sufficient, whereas a raft foundation may be preferred on weak or variable soil. Where there is a liquefaction risk or very soft layers, piled foundations or ground improvement methods such as jet grouting come into play. On plots with high groundwater, basement waterproofing and drainage are also planned based on the report. In short, a well-prepared report is the key to a foundation that is both safe and sensible.
The Role of the Soil Investigation in the Permit Process
In Türkiye, the soil and foundation investigation report is one of the core documents required for a building permit application, alongside the architectural and structural designs. It is prepared by qualified engineers and reviewed and approved by the relevant authority. Structural calculations under the earthquake code also rely on the soil parameters defined in the report. Completing the investigation before the design drawings therefore prevents later revisions and delays.
Because regulations are updated from time to time, it is important to check the current rules and the documents your municipality requires when you apply. For conditions specific to your plot, we strongly recommend consulting experienced geological, geophysical and civil engineers.
Frequently Asked Questions
How long does a soil investigation take?
It depends on the plot size, the number and depth of boreholes, laboratory workload and the authority's approval process. Allowing enough time for the investigation at the planning stage keeps the project schedule comfortable.
Is a soil investigation needed for a small detached house?
Yes. A soil investigation report is required in the permit process regardless of building size. Small buildings can also be affected by settlement and earthquake risks caused by the ground.
What determines the cost of a soil investigation?
The cost depends on the number and depth of boreholes, the field and laboratory tests required and how accessible the site is. The best way to get accurate information for your project is to request a quote from a specialist team.
At Kırkhanlı İnşaat, we believe every project, from planning to turnkey delivery, needs a solid start. To learn more about soil investigations, project planning or your new building, or to request a quote, you can reach us through our Contact page.