The Standard Penetration Test (SPT) is an in-situ penetration test carried out in a borehole. Like the Dynamic Penetration Test (DPT), SPT is a soil test conducted to test for soil qualities before using a site for any particular purposes like building construction, road construction, and mineral exploration among others. SPT is a modification of DPT (Read about DPT by clicking here). The results from the SPT test results can be used to deduce properties of soil like relative density, angle of shearing resistance, and load-bearing capacity.
SPT is necessary to assess the suitability of soil for any particular potential purpose. Load bearing capacity and shearing resistance, for instance, show how much load a site can handle which can be used to determine for instance how many stories for a potential flat on the site or how much traffic load a potential road can handle best on the strength of soils. Relative density can help determine the soil’s drainage qualities.
Tools used to conduct the test include:
A standard split spoon sampler; Split spoon samples are taken at every change of soil stratum or at specified intervals of depth. Such samples are usually regarded as disturbed samples because the grain structure of the particle arrangement of the soil is altered.
A drop hammer weighs roughly 64 kgs.
A guiding rod; This is a solid steel rod, of sufficient stiffness attached at the top to the drive head and the bottom is the sample being tested.
A drilling rig; Used in making the boreholes. It could be powered by gasoline, diesel, electricity, or compressed air.
Procedure;
A borehole of reasonable depth is drilled using a drilling rig.
The drill is removed as well as the residue and space left where the sample is inserted.
The drop hammer, anvil, and driving rod are set up and aligned with the soil sample. The hammer is set at 760mm from the sample below.
The hammer is dropped multiple times and reset to its original height with each blow.
Once the Hammer reaches the first 150mm mark of penetration, the operator starts to count the number of blows needed to reach the 450mm mark. The total number is recorded as N.
Corrections; Due to certain factors that bias the results of the experiment, specific corrections are used to correct for the errors caused by such factors. Among them include; Dilatory correction and Overburden pressure correction.
Dilatory correction is a correction that remedies the error caused when testing silty fine sand soils that are below the water table thus experiencing a higher water pressure which in turn increases the soil’s resistance to penetration thus increasing the value of N. The formula for the correction, as derived from empirical data is as follows;
N(c) = 15 + 0.5(NR – 15), where N(c) is the corrected value for N, and NR the recorded value.
Overburden pressure correction corrects for the increased resistance of a sample due to repeated compaction which increases the value of N. This makes the value of N underestimated at shallow depths and overestimated at higher depths. The formula for the correction is;
Nc = CNN, where Nc is the corrected value for N the recorded value, and CN the correlation factor for overburden pressure.
Pros; The benefits of a Standard Penetration Test; The method is simple, relatively inexpensive, and rugged. The method has the advantage of providing a representative disturbed soil sample in addition to the undisturbed samples obtained during the test.
Cons; However, the drawback of a Standard Penetration Test is that the reliability of the method and the accuracy of the result depend largely on the experience and care of the engineer on site. This can be remedied by hiring a professional to carry out the test. read about the benefits of hiring a professional by clicking here.
Conclusion
In conclusion, the Standard Penetration Test is a valuable tool for assessing the strength of the soil. It is a simple and quick test that can provide reliable data about the soil conditions in a given area. This information can then be used to make informed decisions about the design and construction of foundations and other structures.