When the preliminary investigation program does not provide sufficient information for design and construction of foundation further investigation is required. The preliminary investigation data are used as a basis of locating additional boring and determining additional samples. Detailed site investigation consists of two types.
i) accessible investigation
ii) inaccessible investigation.
Accessible investigation method of soil exploration consists of digging of trial pits and test trenches. They permit a direct inspection of soil and its stratification in place and obtaining true representative disturbed as Well as undisturbed samples. Test pits and trenches are excavated to such a size that a man can Work in them easily. The usual size of test pit is about a diameter of 1.5 m. The depth is usually limited to 6 m.
In sandy soil more excavation is necessary to achieve a stable slope. In sands it will be generally necessary to use timber or steel sheeting to prevent the Wall of the trenches from caving in. Presence of Water at shallow depth may cause problems and some form of dewatering is necessary in the more permeable soils. In clayey soil it is possible to take block samples.
Trial pits are suitable for investigating in all types of soils including those containing cobbles and boulders and considered as the best method of exploration. The investigation by trial pits and test trenches finds Wide application in highways, airports, transmission towers, and for small and light structures.
In inaccessible investigation method there is not direct access to the subsoil and the operation involved consists of making a hole in the ground by hand or power augurs, wash boring, percussion drilling and or diamond drilling and then extracting samples for identification and laboratory testing. The Various operations involved comprise:
- Disturbed samples
- Undisturbed samples
- Laboratory Testing
- In-situ Testing
Detailed Site Investigation has greater advantage over either method. It is particularly helpful when a deep hole is to be made into ground and where trial pits and trenches are not possible.