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The SPT is carried out at every 0.75 m vertical intervals in a borehole. This can be increased to 1.50 m if the depth of borehole is large. Due to the presence of boulders or rocks, it may not be possible to drive the sampler to a distance of 450 mm. In such a case, the N value can be recorded for the first 300 mm penetration. The boring log shows refusal and the test is halted if |
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50 blows are required for any 150mm penetration |
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100 blows are required for 300m penetration |
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10 successive blows produce no advance. |
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Precautions |
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The drill rods should be of standard specification and should not be in bent condition. |
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The split spoon sampler must be in good condition and the cutting shoe must be free from wear and tear. |
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The drop hammer must be of the right weight and the fall should be free, frictionless and vertical. |
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The height of fall must be exactly 750 mm. Any change from this will seriously affect the N value. |
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The bottom of the borehole must be properly cleaned before the test is carried out. If this is not done, the test gets carried out in the loose, disturbed soil and not in the undisturbed soil. |
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