STANDARD PENETRATION TEST (SPT) - PROCEDURE & ADVANTAGES

Standard Penetration Test

The standard penetration test is an in-situ test that is carried out to provide information on geotechnical engineering properties of soil. The test is carried out in borehole.The test will measure the resistance of the soil strata to the penetration undergone.The main purpose of the test is to provide an indication of the relative density of granular deposits such as sand and gravels. It can also be used to determine the confined compressive strength of cohesive soil. This test is widely used because it is a simple and inexpensive. 

Apparatus For Standard Penetration Test

1. Standard Split Spoon Sampler
2. Drop Hammer of weight 63.5 kg
3. Drilling Rig
4. Driving Head
5. Guiding Rod

Procedure For Standard Penetration Test

The test is conducted in bore hole by means of a standard split spoon sampler. Once the drilling is done to the desired depth, the drilling tool is removed and the sampler is placed inside the bore hole. The standard split spoon sampler 50.8 mm outer diameter and 35 mm inner diameter, is driven into the undisturbed soil at bottom of the bore hole by blows from a slide hammer with a weight of 63.5 kg falling through a height of 750 mm at the rate of 30 blows per minute, the sample is driven into the soil.

The number of blows of hammer needed for the tube to penetrate each 150 mm is counted. Further it is driven by 150 mm and the blows are counted. Similarly, the sample is once again further driven by 150 mm and the number of blows recorded. The number of blows recorded for the first 150 mm is not taken into consideration. The number of blows recorded for last two 150 mm intervals are added to give the standard penetration number (N). 

In case if the number of blows for 150 mm drive exceeds 50, it is taken as refusal and the test is discontinued. The standard penetration number is corrected for extensive correction and overburden correction.

Advantages Of Standard Penetration Test

• Test is simple and inexpensive.
• Able to penetrate dense layers, gravel etc.
• Test can be applicable for variety of soil conditions.

Disadvantages Of Standard Penetration Test

• Test is time consuming.
• The sample retrieved for testing is disturbed.
• The results are not very precise and highly reliable.

Vane Shear Test

Vane shear test is used to determine the undrained shear strength of cohesive soils especially soft clay. This test is done in laboratory or in field directly on the ground. This test is carried out with equipment consisting of a rod with vanes mounted to it that is inserted into the ground and rotated. This test was introduce by L Carlson and AW Skempton in 1948. This test give accurate results for soils of low shear strength i.e, less than 0.3 kg/cm2.

Apparatus Required

1. Vane Shear Apparatus
2. Vernier Callipers
3. Soil Specimen Container

Vane Shear Apparatus

Procedure Of Vane Shear Test

1. Clean the vane shear apparatus and apply grease to the lead screw for better movement off handles.

2. Take the soil specimen in container which is generally 75 mm in height and 37.5 mm in diameter.

3. Level the soil surface on the top and mount the container on the base of vane shear test apparatus using screws provided.

4. Lower the vane gradually into the soil specimen until the top of vane is at a depth of 10 -20 mm below the top of soil specimen.

5. Note down the reading of the pointer on circular graduated scale which is initial reading.

6. Rotate the vane inside the soil specimen using torque applying handle at a rate of 0.1°/sec.

7. When the specimen fails, the strain indicator pointer will move backwards on the circular graduated scale and at the point stop the test and note down the final reading of pointer.

8. The difference between initial and final reading is nothing but the angle of torque.

9. Repeat the procedure on two more soil specimens and calculate the average shear strength value.

10. Measure the diameter and height of vane using vernier caliper.

11. Sensitivity of given sooil sample is determined by repeating the above test procedure on remoulded soil which is nothing but soil obtained after rapid stirring of vane in the above test.

Sensitivity (St) = Undisturbed shear strength/Remolded shear strength.

Advantages Of Vane Shear Test

• This test is flexible.
• Vane shear test is quick and easy.

Disadvantages Of Vane Shear Test

• It cannot be conducted on the fissured clay.
• This test is not suitable for clays which contain sand or silt laminations in it.