To perform standard penetration to obtain the penetration resistance (N-value) along the depth at a given site.
Equipment & Apparatus
Tripod (to give a clear height of about 4 m; one of the legs of the tripod should have ladder to facilitate a person to reach tripod head.)
Tripod head with hook
Guide pipe assembly
Standard split spoon sampler
A drill rod for extending the test to deeper depths
Heavy duty post hole auger (100 mm to 150 mm diameter)
Heavy duty helical auger
Heavy duty auger extension rods
Rope (about 15 m long & strong enough to lift 63.5 kg load repeatedly)
A light duty rope to operate sand bailer
Chain pulley block
A straight edge (50 cm)
Identify the location of testing in the field
Erect the tripod such that the top of the tripod head is centrally located over the testing spot. This can be reasonably ensured by passing a rope over the pulley connected to the tripod head and making the free end of the rope to come down and adjusting the tripod legs such that the rope end is at the testing spot. While erecting and adjusting the tripod legs, care should be taken to see that the load is uniformly distributed over the three legs. This can be achieved by ensuring the lines joining the tips of the tripod legs on the ground forms an equilateral triangle. Further, it should be ensured that the three legs of the tripod are firmly supported on the ground (i.e. the soil below the legs should not be loose and they should not be supported on a sloping rock surface or on a small boulder which may tilt during testing.)
Advance the bore hole, at the test location, using the auger. To start with advance the bore hole for a depth of 0.5 m and clear the loose soil from the bore hole.
Clean the split spoon sampler and apply a thin film of oil to the inside face of the sampler. Connect an A-drill extension rod to the split spoon sampler.
Slip the 63.6 kg weight on to the guide pipe assembly and connect the guide pipe assembly to the other end of the A-drill rod.
The chain connected to the driving weight is tied to the rope passing over the pulley at the tripod head. The other end of the rope is pulled down manually or with help of mechanical winch. By pulling the rope down, the drive weight, guide pipe assembly, A-drill rod and the split spoon sampler will get vertically erected.
A person should hold the guide pipe assembly split spoon sampler to be vertical with the falling weight lowered to the bottom of the guide assembly.
Now place a straight edge across the bore touching the A-drill rod. Mark the straight edge level all round the A-drill rod with the help of a chalk or any other marker. From this mark, measure up along the A-drill rod and mark 15 cm, 30 cm and 45 cm above the straight edge level. Lift the driving weight to reach the top of the guide pipe assembly travel and allow it to fall freely. The fall of driving weight will transfer the impact load to the split spoon sampler, which drive the split spoon sampler into the ground. Again lift the drive weight to the top of travel and allow it to fall freely under its own weight from a height of 75 cm. as the number of blows are applied, the split spoon sampler will penetrate into the ground and the first mark (15 cm mark) on the drill rod approaches the straight edge.
Count the number of blows required for the first 15 cm, second 15 cm and the third 15 cm mark to cross down the straight edge.
The penetration of the first 15 cm is considered as the seating drive and the number of blows required for this penetration is noted but not accounted in computing penetration resistance value. The total number of blows required for the penetration of the split spoon sampler by 2nd and 3rd 15 cm is recorded as the penetration resistance or N-value.
After the completion of the split spoon sampler by 45 cm, pull out the whole assembly. Detach the split sampler from A-drill rod and open it out. Collect the soil sample from the split spoon sampler into a sampling bag. Store the sampling bag safely with an identification tag for laboratory investigation.
Advance the bore hole by another 1 m or till a change of soil strata which ever is early.
The test is repeated with advancement of bore hole till the required depth of exploration is reached or till a refusal condition is encountered. Refusal condition is said to exist if the number of blows required for the last 30 cm of penetration is more than 100.
The test will be repeated in number of bore holes covering the site depending on the building area, importance of the structure and the variation of the soil properties across the site.
The SPT values are presented either in the form of a table or in the form of bore log data.