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7
Soil Investigation
Our Equipment : Percussion Rigs
Machines brand : Dando / Pilcon
Make: UK
Engine Type: Lister
 
Our Equipment : Rotary Rig
Mobile 31 mounted on Bedford Truck
Make: UK
Engine Type: Lister
 
Rotary Rig
Mobile M-40
 
 

Available at the laboratory's permanent facility
The Standard penetration test (SPT) is in-situ dynamic penetration test designed to provide information on the geotechnical engineering properties of soil. The test procedure is described in the British Standard BS 1377 -9:1990 and ASTM D 1586. The test uses a thick-walled sample tube, with an outside diameter of 35mm, and a length of around 650mm. This is driven into the ground at the bottom of a borehole by blows from a slide hammer with a weight of 63.5 kg. (140 lb)falling through a distance of 760mm (30in). The sample tube is driven 150mm into the ground and then the number of blows needed for the tube to penetrate each 150mm (6 in) up to depth of 450mm (18in) is recorded. The number of blows required to achieve the final 300mm (12in) penetration is the standard penetration resistance, N. In cases where 50 blows are insufficient to advance it through a 150mm (6in) interval the penetration after 50 blows is recorded. The blow count provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae. The main purpose of the test is to provide an indication of the relative density of granular deposits, such as sands and gravels from which it is virtually impossible to obtain undisturbed samples. The great merit of the test, and the main reason for its widespread use is that it is simple and inexpensive. The soil strength parameters which can be inferred are approximate, but may give a useful guide in ground conditions where it may not be possible to obtain borehole samples of adequate quality like gravels, sands, silts, clay containing sand or gravel and weak rock. In conditions where the quality of the undisturbed sample is suspect, e.g. very salty or very sand clays, or hard clays, it is often advantageous to alternate the sampling with standard penetration tests to check the strength. If the samples are found to be unacceptably disturbed, it may be necessary to use a different method for measuring strength like the plate test. When the test is carried out in granular soils below groundwater level, the soil may become loosened. In certain circumstances, it can be useful to continue driving the sampler beyond the distance specified, adding further drilling rods as necessary. Although this is not a standard penetration test, and should not be regarded as such, it may at least give an indication as to whether the deposit is really as loose as the standard test may indicate. The usefulness of SPT results depends on the soil type, with fine-grained sands giving the most useful results, with coarser sands and salty sands giving reasonably useful results, and clays and gravelly soils yielding results which may be very poorly representative of the true soil conditions. Soils in arid areas, such as the Western United States, may exhibit natural cementation. This condition will often increase the standard penetration value. The SPT is used to provide results for empirical determination of a sand layer's susceptibility to earthquake liquefaction, based on research performed by Harry Seed, T. Leslie You, and others.


 
 
 
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