Information on soil for builders

A soil investigation is an analysis of the properties of the soil in the construction area. The investigation results will be used to draw up a foundation plan or a statement on the type of foundation.

Depending on the soil and future structures, the soil investigation usually includes drilling, sampling, test pits and groundwater level observation. The most common types of tests include weight sounding, cone and ram penetration tests, vane shear tests, wing drilling and motorised dynamic sounding.

The number of soil investigations required depends on how demanding and large the site is. Even at the construction sites of detached houses, tests should be carried out at least at the corners of the future building. Before conducting the investigation, we recommend that you ask the City of Vantaa’s customer service about existing soil investigation data.

The City of Vantaa only conducts new investigations for its own construction sites. The City’s Geotechnical Services unit is not involved in the planning of private projects or their soil investigations.

According to the Ministry of the Interior's building code, the quality of the foundation soil should typically be determined in advance for each construction project requiring a building permit.

The type of foundation can be reliably determined only with a proper soil investigation at the site and the resulting foundation plan. In practice, this requires a new soil investigation used as the basis for a plan prepared by a competent party.

The type of foundation is usually defined by a structural engineer specialised in foundations. A list of qualified engineers is available on the FISE website.

The purpose of a foundation is to transfer the load from the structures to the earth or rock so that the structure can withstand and function without damages for the intended service life. Where building foundations are concerned, you shouldn’t use risky solutions or save on investigation costs. A poorly designed foundation plan can come as a unwelcome surprise in the construction phase, or a failed foundation can cause damage to the structures.

The most common foundation types include a foundation wall, ground or rock-supported structures and pile foundations. A pile foundation is usually the best options for clay and weak soil areas where the load-bearing soil layer is deep. Steel or reinforced concrete support piles are commonly used as the piles. They are driven down into the load-bearing soil layer or the rock surface.

In general, the foundation typically includes a load-bearing ground floor and a crawl space. The ventilation and moisture control of the crawl space must be carefully planned.

The material ordered from the city may include soil investigation and groundwater data as well as soil maps. It is also possible to obtain general information on foundation types and other soil-related investigation needs.

In addition to the material, the builder must always perform a soil investigation at the site. Already when buying the plot, soil factors should be considered as one of the factors affecting costs already, or at the latest, when buildings are placed on the site and you are investigating the proper foundation type.