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During the SPT, it was possible to take disturbed and undisturbed samples using split barrel-sampler which were used for further laboratory tests. The in situ penetration test provides a soil sample for identification purposes and for the laboratory tests that allow the use of disturbed samples. The method of sampling soil consists of driving a split-barrel sampler to obtain a representative, disturbed sample and to simultaneously obtain a measure of the resistance of the subsoil to the penetration of a standard sampler. The test is conducted inside a borehole. A ‘split spoon’ sampler is attached to the bottom of a core barrel and lowered into position at the bottom of the borehole. The sampler is driven into the ground by a drop hammer weighing 68 kg falling through a height of 76 cm. The number of hammer blows is counted. The number required to drive the sampler three successive 150mm increments is recorded. The first increment (0-150mm) is not included in the N value as it is assumed that the top of the test area has been disturbed by the drilling process. The SPT N is the number of blows required to achieve penetration from 150-450mm. The hammer weight, drop height, spoon diameter, rope diameter etc. are standard dimensions. After the test, the sample remaining inside the split spoon is preserved in an airtight container for inspection and description.
Advantages and Disadvantages of SPT Standard Penetration Test
- Relatively quick and simple to perform
- Able to penetrate dense layers, gravel, and fill
- Equipment and expertise for the test is widely available
- In addition to overburden pressure and relative density the SPT N-value is also a function of soil type, particle size, and age and stress history of the deposit
- Provides a representative soil sample
- Provides useful index of relative strength and compressibility of the soil
- The SPT does not typically provide continuous data, therefore important data such as weak seams may be missed
- Somewhat slower than other sample methods due to sample retrieval
- The basic problems to consider are change in effective stress at the bottom of the borehole, dynamic energy reaching the sampler, sampler design, interval of impact, penetration resistance count.
- Samples that are obtained from the SPT are disturbed.
Cone Penetration Test (CPT)
What is CPT?
Cone Penetration Tests are conducted to obtain the cone resistance, the side friction and, if there is a piezocone, the pore pressure. The soil type can be determined by analysing these result the values can also be used in the design of shallow foundations through the estimation of stiffness and shear strength of cohesive soils. The cone penetration test (CPT) is a method used to determine the geotechnical engineering properties of soils and delineating soil stratigraphy. It was initially developed in the 1950s at the Dutch Laboratory for Soil Mechanics in Delft to investigate soft soils. Based on this history it has also been called the “Dutch cone test”. Today, the CPT is one of the most used and accepted in soil methods for soil investigation worldwide.
CPT is static penetration test in which the device is pushed rather than then driven by blows into the soil. The CPT consist of cone which been attached to a rod that been protected by an outer sleeve. A 60o cone with face area 10cm2 and 150cm2 of friction sleeve is hydraulically pushed into the ground at a constant speed ranging from 1.5 to 2.5 cm/s about 80mm deep. The force required maintaining this penetration rate, and the shear force acting on the friction sleeve is recorded. The friction ratio gives an indication of the soil type. The CPT Soil Classification will be based on ratio of tip and sleeve resistance. The tip resistance will be high in sand and low in clays while the sleeve friction will be low in sands and high in clays.
Advantages and Disadvantages of Cone Penetration Test
- Continuous data
- Reliable, repeatable test results
- Inability to penetrate through gravels and cobbles
- Elimination of operator error
- Less populated database than SPT due to the new technology
- Lack of sampling
SHEAR VANE TEST
What is Shear Vane Test?
The vane shear test is an in-situ geotechnical testing methods used to estimate the undrained shear strength of fully saturated clays without disturbance. The test is relatively simple, quick, and provides a cost-effective way of estimating the soil shear strength; therefore, it is widely used in geotechnical investigations. Under special condition, the vane shear test can be also carried out in the laboratory on undisturbed soil specimens however the use of the vane shear test in in-situ testing is much more common.
The vane shear test is an in-situ geotechnical testing methods used to estimate the undrained shear strength of fully saturated clays without disturbance. The results of the test are not reliable if clay contains silt or sand. The vane shear test apparatus consists of a four-blade stainless steel vane attached to a steel rod that will be pushed into the ground. For a weak soils which has Cu less than 50 kPa, the size of the blade is 75mm wide x 150 mm long while for the slightly strong soils which has Cu between 50 kPa to 100 kPa, 50 x 100 mm blade should be use. The height of vane is usually twice its overall widths and is often equal to 10 cm or 15 cm.
The test can be conducted either from the ground surface or from the bottom of a borehole or a test pit. If conducted from the bottom of a borehole, the test area should be should be at the depth of least three times the borehole diameter lower that the borehole bottom in order to avoid the borehole disturbance effects. The test starts by pushing the vane and the rod vertically into the soft soil. The vane is then rotated at a slow rate of 6° to 12° per minute. The torque is measured at regular time intervals and the test continues until a maximum torque is reached and the vane rotates rapidly for several revolutions. At this time, the soil fails in shear on a cylindrical surface around the vane. The rotation is usually continued after shearing and the torque is measured to estimate the remoulded shear strength. The undrained shear strength of the saturated soil is proportional to the applied torque and the dimensions of the vane.
Advantages and Disadvantages of Shear Vane Test
- Rapid and economical test
- Reproducible results in homogeneous soils
- Inexpensive method for measuring clay sensitivity
- Limited to soft to stiff cohesive soils
- Field vane shear strength must be corrected
- Results can be affected by anisotropic soils, sand lenses, shells, and seams
What is Wash Boring?
A popular method to drill larger size holes in soft formations. Casing with a casing crown attached is rotated into the ground and water is used to flush out the drilled formation. It is one of a boring system by which material loosened by a bit is borne to the surface in the annular space between the bit and casing by water forced down through the pipe bearing the bit
For test boring over 3 meter in depth, this method can be conveniently used. In this method a hollow steel pipe known as casing pipe or drive pipe is driven into the ground for a certain depth. Then a pipe usually known as water jet pipe or wash pipe, which is shorter in diameter, is lowered into the casing pipe. At its upper end, the wash pipe is connected to water supply system while the lower end of the pipe is contracted so as to produce jet action.
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Water under considerable pressure is forced down the wash pipe. The hydraulic pressure displaces the material immediately below the pipe and the slurry thus formed is forced up through the annular space between the two pipes. The slurry is collected and samples of material encountered are obtained by settlement. In this process the particles of finer material like clay, loam etc. do not settle easily and the larger and heavy particles of the soil may not be brought up at all.
Moreover, the exact position of a material in the formation cannot be easily be located. However the change of stratification can be guessed from the rate of progress of driving the casing pipe as well as the color of slurry flowing out. Yet the results obtained by wash boring process give fairly good information about the nature of the sub-soil strata. This method can be adopted in soft to stiff cohesive soils and fine sand.
Advantages and Disadvantages of Wash Boring
- Can be used in difficult terrain
- Low equipment cost
- Used in uncased holes
- Depth limited to about 30m
- Difficulty in obtaining accurate location of groundwater level.
- Undisturbed soil samples cannot be obtained.
PERCUSSION RIG BORING
What is Percussion Rig Boring?
Percussion Rug Boring is widely used in UK. Percussion drilling method of advancing a boring is of common use in drilling water wells. This method consists of breaking up of the sub-strata by repeated blows from a bit or chisel. This type of boring can make a borehole up to 150 mm – 300 mm diameter and depth up to 50 – 60 m. However this method is not recommended for loose sand or clayey soils.
This method consists of breaking up of the sub-strata by repeated blows from a bit or chisel. Percussion methods start by raise up the tool assembly by the winch to 1 m above the bottom of the hole. It allows the tool assembly to fall under its own weight to drive the cutting tools into the soil. The material thus pulverized is converted into slurry by pouring water in the bore. At intervals the slurry is bailed out of the hole and dried for examination.
The slurry will be removed by bailers or pump. After that, tool will be raise to surface when the cutting tools are full of soil to obtain undisturbed sample. This method can be adopted in rocks and soils having boulders. In general, a machine used to drill holes is called a drill rig generally power driven, but may be hand driven. A winch is provided to raise and lower the drilling tools into the hole.
Advantages and Disadvantages of Percussion Rig Boring
- Simple to operate and maintain
- Suitable for a wide variety of rocks
- Operation is possible above and below the water-table
- It is possible to drill to considerable depths
- Slow, compared with other methods
- Equipment can be heavy
- Problems can occur with unstable rock formations
- Water is needed for dry holes to help remove cuttings
ROTARY AUGER BORING
What is Rotary Auger Boring?
A drill bit is pushed by weight of drilling equipment and rotated by a motor. This drilling method with mud pump for flushing can be direct circulation and reverse circulation, with tricone bit and drag bit Nevertheless, the torque is relatively critical for this drilling method. It is widely used in soft to medium hard ground conditions. This type of boring machine can be also use for piling works. It can bore the hole up to depth of 50 m and diameter of 300 mm to 2 m.
When rocks or hard pans are to be penetrated for examination, core drilling is done to get undisturbed samples of the formation. In this process a hole is made by rotating a hollow steel tube having a cutting bit at its base. The cutting bit makes an annular cut in the strata and leaves a cylindrical core of the material in the hollow tube. Two types of cutting bits are generally used, namely, diamond bit and shot bit. Diamond bit consists of industrial diamonds set in the face of the bit and in shot bit, chilled shot is used as an abrasive to cut the hard pan. When core samples of small diameter are needed, diamond bit is preferred. There are two main type of rotary auger boring which are:
- Consists of an open-top metal cylinder with cutter mounted on a baseplate as the soil is cut, it passes into the bucket, which is then raised and emptied at intervals.
- Can be used in frim soil.
- Not suitable for cohesionless soil below water table.
- Flight Auger
- Short flight
- Simple flight
- Continuous flight
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