Soil sampling methods practised for developing the structural designs using the geotechnical investigation, to examine the safety concerns for the house construction.
Top 3 Soil sampling methods used in construction
The following are the 3 soil sampling methods utilised in house construction for subsurface geotechnical investigations.
- Direct soil sampling methods
- Semi-direct soil sampling method
- Indirect soil sampling methods
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Direct soil sampling methods:
a. Pits or trenches.
b. Drifts and shafts.
a.Pits or trenches
Pits or trenches are the practices of open excavations. Examination of this soil is done in natural condition in this method.
By practicing soil sampling techniques, the required soil samples can be collected and they utilized to achieve the engineering properties of soil.
b.Drifts and Shafts
Drifts are the tunnels made in a horizontal direction to find the nature of the soil strata.
Perpendicular holes of large size driven into the ground to find the nature of the soil strata are called Shafts.
Semi-Direct soil sampling method:
a. Borings
Borings
Boring is the vertical boreholes dug into the ground to get the report about the soil strata.
The following are the techniques used for doing boreholes:
a. Boring using Augur
An auger is a machine that is used for driving a borehole into the soil.
Mechanical augers, Hand Augers are the two auger varieties used in general.
b. Augur, shell boring type
For blocking the soils dropping from bore sides, casing or shells are utilised.
The casing is utilised for sandy or stiff clay’s.
c. Boring with wash
Except those which are mixed with rocks and stones, this method utilised for causing holes in all types of soil. This process is used for examination underneath the groundwater table.
d. Drilling with percussion
This technique is utilised for creating holes in stones, rocks and other hard strata. For this method, a heavy drill bit is pushed into the ground by a repeated number of blows.
e.Drilling by rotation
In this method, the borehole is driven by a rotating drill rod. It is operated by a rotary drive mechanism by the application of blow-ward pressure.
For getting rock core samples, this process is used.
Indirect soil sampling methods
i. Penetration tests
ii. Geo-physical methods
a.Penetration tests
For estimating the soil resistance to the penetration of a sampler or a cone or different fashioned tool these tests are penetration tests are done. In general two important types of tests are practised :
I. penetration by standard tests
II. penetration test by using cone
I. Penetration by Standard test
For this standard test, a split-spoon sampler having an inner diameter of 35 mm and an outer diameter of 50.8 mm is practised. A hammer of weight 6.3kgs with a free-fall of 75 mm this sampler is pushed.
The blows that are needed for penetration of 300 mm is termed as a standard penetration value or number denoted as N.
II. penetration test by using cone
Cone test is practised as a Dutch cone test. On the basis of loading, the following are the two types of penetration tests:
1.The static cone penetration test
2. Dynamic cone penetration test.
To receive a record of the resistance of soil these tests are executed by penetrating a dutch cone with a base if 10 sq-cm and an estimated angle of 60 degrees.
For receiving the resistance, the cone is forced downwards at a steady rate to a depth of 35 mm each time.
The estimated relationship among standard penetration test number N,cone resistance (QC) is:
- Gravels qc= between 8N to 10N
- Sands qc=between 5N to 6N
- Silty sands qc = between 3N to 4N
- Silts and clays qc=2N
b. Geo-physical methods
These geophysical methods involve the technique of obtaining the sub-surface details by means if measuring some physical properties of the subsurface and correlating them. These methods define the situations over extended areas in a quick limit.
The two geological methods used are:
- The Seismic refraction method
- The resistivity method by electricity
The resistivity method by electricity:
By measuring the electrical resistivity of subsurface strata are obtained by using electrical resistivity method. The electrical resistivity methods that practised are:
1.Proofing method
2.Sounding method
1. Proofing method:
In that proofing method, the depth of investigation is fixed by keeping the distance between the electrodes constant. At a selective depth for the examination over wide areas, this proofing method is preferred.
2.Sounding method:
The spacing among electrodes is not the equivalent in the sounding method. Until the investigation depth is reached, the spacing among electrodes is slowly increased. For studying strata changes with a rise in depth this technique is practised.
II. Seismic refraction method:
Seismic refraction methods work by sending shock waves with various velocities in distinctive soils. This method is extremely fast in determining the profiles of distinctive strata presented the deeper layers have more density and greater velocities.
Soil examination program
For estimating the durability of the land site geotechnical examinations needs to be done.
The conduction of examinations in the site or lab for gathering necessary data is called a Soil examination program.
The soil examination program is to get detailed information about:
- Engineering characteristics and nature of soil strata
- The water table at the underground level situation
- Finding the occurrence of soil strata.
Depth of examination:
The depth of examination required at a particular site depends majorly on the variety of the sub-surface strata in both parallel and perpendicular directions. The depth of exploration is administered by the depth of the influence zone, which will be created by an Isobar diagram.
What are the types of Soil samples?
1. Disturbed samples
2. Undisturbed samples.
1. Disturbed samples:
The physical structure of the soil in this sample gets disturbed either completely or partly. Nevertheless, those samples will be describing the composition of the soil. These can be utilised to define the index properties of the soil.
2. Undisturbed samples:
The water content of the soil, physical structure in this sample are retained.
For defining index properties, the engineering properties of soils those samples are utilised.
Sample disturbance:
The disturbance of the soil sample depends on the variety of samples and the process of sampling. The disturbance of the specimen is directed by the following factors:
1.Non-return valve.
2.Cutting edge
3.Inside wall friction
Conclusion :
The soil sampling methods which are discussed above are practised for subsurface soil investigations and those soil investigations records are necessary for structural design and interpretation of a building for home construction.
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