Site Investigation And Sub Soil Exploration –– : ––
(I) Site investigation. —
A knowledge of the ground which forms a site is essential to its safe and economic development .
(II) Purpose / Necessity of site investigation and sub soil exploration —
(a) The depths and lateral extent of different soil strata existing at a site, including their visual identification, and to determine the level of bed rock.
(b) To obtain disturbed and undisturbed samples of the different soil strata.
(c) To determine their identification and other properties such as unit weight, water content, relative density, unconfined compressive strength, angle of internal friction, cohesion etc. This will include the identification of existing weaker strata below the ground.
(d) To determine the position of water table and its fluctuation .
(III) Soil Exploration —
Whenever we plan an engineering structure , such as a dam, bridge , building , highway etc, it is necessary to know the details of soil strata existing at the proposed construction site. The process of exploring the site is called soil exploration .
(IV)Type of Exploration —
(a) General exploration : —
The details of soil exploration are generally required to be explored for the design of routine engineering structure and is called the general exploration .
(b) Detailed exploration : —
A more detailed exploration may sometimes be needed to determine the other soil properties like permeability , compressibility , density index , pore pressure etc or insitu values of properties like bearing capacity , etc. in case of heavier or complicated structure . That exploration is called the detailed exploration .
(V) Methods of soil exploration —
( 1) Open excavation
(2 ) Borings
( 3) Sub surface sounding
( 4) Geographical methods .
(VI) Open excavation —
A pit can be excavated for exploring shallower depths , say of the order of 2 to 5m , or so such a pit can be easily excavated at the proposed construction site , if the soil has a bit of cohesion, and the soil samples can be lifted from such different depths, besides making the easy visualization and examination of the different strata. Even undisturbed soil samples can be lifted from such a pit by a process called chunk sampling.
(VII) Boring —
Soil samples can be lifted from deeper depths by drilling bore holes by using mechanical devices called samplers.
(a) The process consist of
i) Drilling a hole and visually examining the cuttings coming out from different depths.
ii) Lifting the soil samples from different depths by using mechanical devices called samplers.
(b) Methods of boring —
i) Auger boring: –
This is simplest method of boring a hole by hand drilling. These can be used for shallower depths generally confined to depths of about 5m or so. In cohesive and other soft soils above water table, augers may be used.
ii) Auger and Shell boring: –
Augers are suitable for soft or stiff clays and very stiff and hard clays. Clays and sand pumps for sandy soils. Cylindrical augers and shells are used for making deep boring. Hand operated, mechanized ring are used for depths 25m, 50m respectively.
iii) Wash boring: –
This is a simple and fastest method, used for making holes in all types of soils except boulders and rocks.
iv) Percussion boring: –
This method is used to make hole in all types of soils including boulders and rocks.
v) Rotary boring (Mud rotary boring): –
This method is used to advance hole in rocks and soils. Rotating core barrels which are provided with commercial diamond bits or a steel bit with slots are used for rotary drilling. This method is used to obtain the rock cores, so this method is called as core boring or core drilling.
(VIII) Soil samples and sampling —
i) Disturbed sample: —
In disturbed sampling, the natural structures of soil gets partly or fully modified or destroyed, although with suitable precaution the natural water content may be preserved. Disturbed sample can be obtained by direct excavation by auger and thick wall samplers.
ii) Undisturbed sample: —
In undisturbed sample, the natural structure and properties remain preserved. These samples are used to tests for shear, consolidation and permeability
iii ) The field test adopted for determination of bearing capacity of soil are —
i) Static cone penetration test ii) Dynamic cone penetration test
iii) Standard penetration test
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