STANDAR PENETRATION TEST

STANDARD PENETRATION TEST 

Advantages

1. Relatively quick and simple to perform

2. Standard procedure

3. Equipment and expertise for the test is widely available in most countries. 4. Provides a representative soil sample.

5. Provides useful index of relative strength and compressibility of the soil.

6. Able to penetrate dense layers, gravel, and fill

7. Numerous case histories of soil liquefaction during past earthquakes are available with SPT N-values. The method based on this history can reflect actual soil behavior during earthquakes, which cannot be simulated in the laboratory.

8. The SPT is an in situ test that reflects soil density, soil fabric, stress and strain history effects, and horizontal effective stress, all of which are known to influence the liquefaction resistance but are difficult to obtain with undisturbed samples.

Disadvanteges

1. The SPT does not typically provide continuous data (e.g. 5 ft. intervals), therefore important data such as weak seams may be missed.

3. Limited applicability to cohesive soils, gravels, cobbles boulders. Somewhat slower than other sample methods due to sample retrieval.

4. 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

5. Due to considerable differences in apparatus and procedure, significant variability of measured penetration resistance can occur. 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.

6. Samples that are obtained from the SPT are disturbed.

RECOMENDS

The wash boring method or rotary drilling with a tricone bit should be used to minimize soil disturbance. Water or drilling mud in the borehole should be used to minimize the reduction in vertical effective stress within the soil at the sampling location. Water and drilling mud must be maintained at or above the groundwater table. The bottom of the boring should be between 2.5 and 6 in. in diameter, although a maximum diameter of 4 in. is preferred. Casing should not extend below the bottom of the boring before the SPT is performed. The measured N-value should be taken from the penetration between 6 and 18 inches. The first 6 in. below the bottom of the boring is considered to be disturbed material.

•SPT N-values should be corrected for 60% of the maximum free-fall

•energy (140 lb weight dropped a distance of 30 in) to the drill stem. This corresponds to the energy delivered by the hammer.

•Drilling mud should be used to prevent liquefaction of loose sands

•(sometimes called “running sands”) below the water table.

• The split spoon sampler should have a uniform inside diameter of 1 3/8”.

•A drilling bit should be used that produces an upward deflection of the

•drilling mud.

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