Site investigation or Sub-Soil explorations are done for obtaining the information about subsurface conditions at the site proposed for construction. Soil exploration consists of determining the profile of the natural soil deposits at the site, taking the soil samples and determining the engineering properties of soils using laboratory tests as well as in-situ testing methods.
Objectives of Site Investigation
Following are the objectives of site investigation or subsurface exploration.
- To know about the order of occurrence of soil and rock strata.
- To know about the location of the groundwater table level and its variations.
- To determine engineering properties of soil.
- To select a suitable type of foundation.
- To estimate the probable and maximum differential settlements.
- To find the bearing capacity of the soil.
- To predict the lateral earth pressure against retaining walls and abutments.
- To select suitable soil improvement techniques.
- To select suitable construction equipment.
- To forecast problems occurring in foundations and their solutions.
Stages in Site Investigation
Site investigation or sub-soil exploration is carried out stage-wise as given below.
- Site Reconnaissance
- Preliminary site exploration
- Detailed exploration
- Preparation of soil investigation report
1. Site Reconnaissance
Site reconnaissance is the first stage of site investigation. In this stage, visual inspection of the site is done and information about topographical and geological features of the site are collected. The general observations made in site reconnaissance are as follows :
- Presence of drainage ditches and dumping yards etc.
- Location of groundwater table by observing well in that site.
- Presence of springs, swamps, etc.
- High flood level marks on the bridges, high rise buildings, etc. are observed.
- Presence of vegetation and nature of the soil.
- Past records of landslides, floods, shrinkage cracks, etc. of that region.
- Study of aerial photographs of the site, blueprints of present buildings, geological maps, etc.
- Observation of deep cuts to know about the stratification of soils.
- Observation of Settlement cracks of existing structures.
2. Preliminary Site Exploration
Preliminary site exploration is carried out for small projects, light structures, highways, airfields, etc. The main objective of preliminary exploration is to obtain an approximate picture of sub-soil conditions at low cost. It is also called general site exploration.
The soil sample is collected from experimental borings and shallow test pits and simple laboratory tests such as moisture content test, density, unconfined compressive strength test, etc. are conducted. Simple field tests such as penetration methods, sounding methods, geophysical methods are performed to get the relative density of soils, strength properties, etc.
The data collected about subsoil should be sufficient enough to design and build light structures. Following are some of the general information obtained through primary site exploration.
- Approximates values of soil's compressive strength.
- Position of the groundwater table.
- Depth and extent of soil strata.
- Soil composition.
- Depth of hard stratum from ground level.
- Engineering properties of soil ( disturbed sample)
3. Detailed Site Exploration
Detailed exploration is preferred for complex projects, major engineering works, heavy structures like dams, bridges, high rise buildings, etc. A huge amount of capital is required for a detailed site exploration hence, it is not recommended for minor engineering works where the budget is limited. For such type of works, data collected through preliminary site exploration is enough.
In this stage, numerous field tests such as in-situ vane shear test, plate load test, etc. and laboratory tests such as permeability tests, compressive strength test on undistracted soil samples are conducted to get exact values of soil properties.
Different methods of site exploration which are used in both preliminary and detailed site exploration are explained in the link given below.
4. Preparation of Report of Sub-Soil Exploration
After performing preliminary or detailed site exploration methods a report should be prepared. A sub-soil investigation or exploration report generally has the following sections :
- Scope of site investigation
- Description of the proposed structure, purpose of site investigation
- Site reconnaissance details
- Site exploration details such as number, location and depth of boreholes, sampling details etc.
- Methods performed in site exploration and their results.
- Laboratory tests performed and their results.
- Details of Groundwater table level and position.
- Recommended improvement methods if needed.
- Recommended types of foundations, structural details, etc.