Before selecting the site for construction, as civil engineers, we have to collect all the information about the condition of the proposed site so we can easily design the structure and plan techniques. This information is collected by performing site exploration.
Site Exploration | Soil Exploration
So the first question is what is site exploration or soil exploration or subsurface exploration in civil engineering? Site exploration is nothing but a method of getting information about the subsurface condition of the proposed construction site.
The main aim or objective of site exploration is to provide complete information about the soil, its different strata, water table conditions, engineering properties of soils, types of soils, etc.
The site investigation and site exploration are performed by the Geotechnical Engineers for every big project in construction.
Purpose Of Site Exploration
Here are the purposes why we should perform the site exploration during the selection of a site for construction:
1- To know what is the bearing capacity of the soil site, so we can estimate the maximum load the soil can sustain.
2- To estimate the water content, porosity, relative compaction, void ratio, density index, etc of the site soil.
3- For the easy selection of depth and types of foundation.
4- To estimate the water table, settlements, and lateral earth pressure.
5- For the prediction of foundation failure in the future and to investigate the safety of existing structures.
Stages In Site Exploration | Stages In Subsurface Exploration
Here are three stages in the site exploration or sub-surface exploration:
- Preliminary Exploration
- Detailed Exploration
In sub-surface exploration, reconnaissance is the first step of exploration in which the geotechnical engineer visits the site and inspects the site, studies the available maps, photographs, soil maps, etc.
The main objective of the reconnaissance in sub-surface exploration is to gather information about the existing drainage pattern, water table height, topography of the site, cracks in the settlement structure, slopes, landslide possibility, shrinkage cracks, flood marks, etc.
The preliminary exploration is the second step of the sub-surface exploration in which we determine the depth thickness, strength, and compressibility of soil, locating the boundaries of soil strata.
The preliminary exploration is performed by the cone penetration test, borings, pits test, geophysical methods such as seismic method, and Electrical Resistivity Methods.
The information about the strength and compressibility of soil is obtained by the cone penetrometer test.
The location of the boundaries of different strata is determined by the seismic and electrical resistivity methods.
The last step of the sub-surface exploration is the detailed exploration in which we determine the engineering properties such as permeability, compressibility, elasticity, angle of internal friction, dynamic behavior, etc.
Detailed exploration involves a vane shear test, plate load test, permeability test, extensive boring, sampling, etc.
Detailed exploration is mainly required for big projects which have heavy structural loads such as skyscrapers, bridges, dams, and multi-story buildings.
For small projects which have a low structural load, detailed exploration may not be required, the construction and design of the building are done on the basis of the reconnaissance and preliminary exploration.
Depth Of Exploration
The depth of exploration in construction mainly depends on the types of load, characteristics of soils, the profile of soil, the load distribution, the design of the building, etc.
The minimum depth of exploration should be equal to the significant depth( significant depth is the depth up to which the increase in stress due to structural loading causes shear failure or excessive settlement of the foundation).
The significant depth is taken as the depth at which the vertical stress is 20% of the load intensity.
The depth of exploration of the Strip Footing is about three times the width.
The minimum depth of exploration of the isolated spread footing is 1.5 times the width.
In the case of the Raft or Mat foundation, the depth of exploration is 1.5 times the width of the entire loaded area.
The depth of exploration in the Pile Foundation below the tip of bearing piles is 1.5 times the width of pile groups.
The formula for calculating the depth of exploration for the multi-storeyed building is D= C(S)0.7. Here S = number of stories, D= depth of exploration, and C is constant which is 3 for light steel and narrow concrete building and 6 for heavy steel and wide concrete building.
The minimum depth of exploration for a gravity dam is 2 times the height of the dam.
The depth of exploration in the road fills is 2 meters below the ground surfaces.
Frequently Asked Questions (FAQ)
What is significant depth?
Significant depth is the depth up to which the increase in stress due to structural loading causes shear failure or excessive settlement of the foundation.
What are the 3 stages of site investigation?
The three stages of site investigation are Reconnaissance, Preliminary Exploration, and Detailed Exploration.
What is the meaning of site exploration?
The meaning of site exploration is to obtain information about the sub-surface condition of the purposed site.