The SPT is the most commonly used in situ test in a bore hole in the USA. The test is made by
making use of a split spoon sampler shown in Fig. 9.7. The method has been standardized as ASTM
D-1586 (1997) with periodic revision since 1958. The method of carrying out this test is as follows:
- The split spoon sampler is connected to a string of drill rods and is lowered into the
bottom of the bore hole which was drilled and cleaned in advance.
- The sampler is driven into the soil strata to a maximum depth of 18 in by making use of a
140 Ib weight falling freely from a height of 30 in on to an anvil fixed on the top of drill rod.
The weight is guided to fall along a guide rod. The weight is raised and allowed to fall by
means of a manila rope, one end tied to the weight and the other end passing over a pulley
on to a hand operated winch or a motor driven cathead.
- The number of blows required to penetrate each of the successive 6 in depths is counted to
produce a total penetration of 18 in.
- To avoid seating errors, the blows required for the first 6 in of penetration are not taken into
account; those required to increase the penetration from 6 in to 18 in constitute the N-value.
As per some codes of practice if the N-value exceeds 100, it is termed as refusal, and the test
is stopped even if the total penetration falls short of the last 300 mm depth of penetration.
Standardization of refusal at 100 blows allows all the drilling organizations to standardize costs so
that higher blows if required may be eliminated to prevent the excessive wear and tear of the
equipment. The SPT is conducted normally at 2.5 to 5 ft intervals. The intervals may be increased
at greater depths if necessary.
Standardization of SPT
The validity of the SPT has been the subject of study and research by many authors for the last
many years. The basic conclusion is that the best results are difficult to reproduce. Some of the
important factors that affect reproducibility are
- Variation in the height of fall of the drop weight (hammer) during the test
- The number of turns of rope around the cathead, and the condition of the manila rope
- Length and diameter of drill rod
- Diameter of bore hole
- Overburden pressure