CHAPTER 3: TEST PROGRAM
3.3. Instrumentation
An SPT hammer system (as shown Figure 3.5) is comprised of the hammer itself, the mechanism that lifts and drops the hammer, (the anvil, stem and anvil or drive-head) and the operator. Two shapes of hammers are in common use; the safety hammer and the donut hammer. The safety hammer, which is relatively long and therefore, has a corresponding small diameter. The safety hammer has an internal striking ram that greatly reduces the risk of injuries. The donut hammer is short in length and therefore larger in diameter than the safety hammer. The longer safety hammers are more efficient in transferring energy into the rods than the squatter donut hammers. In an energy calibration study by Kovacs et al. (1983), the mean energy ratio delivered by a safety hammer was found to be about 60%, whereas the mean energy ratio for a donut hammer was about 45%. The common practice in performing the SPT is to raise the hammer 762 mm by means of a rope wrapped around a rotating pulley and then throw the rope smartly to dissociate it from the pulley, in this way letting the hammer fall onto the anvil fastened to the top of the drill stem. Since the rope is rarely completely dissociated from the pulley, the actual energy delivered using this technique depends on the skill of the operator, smoothness of cathead (amount of rust) and very much on the number of times the rope is originally wrapped around the pulley. Kovacs et al.
(1982) recommended that two turns of the rope around the pulley should be used to minimize the importance of the number of turns and operators characteristics as variables of the delivered energy. To eliminate the variability of the energy delivered to the hammer that rises using the rope and pulley technique, an automatic trip
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hammer has been introduced. A mechanical system raises the hammer and a tripping device releases it from a 762mm height. It has been found that these systems also do not deliver the theoretical free-fall energy to the drilling rods, probably because of the energy losses associated with the anvil system at the top of the drill stem. In the United States, the two most common SPT hammer systems are the safety hammer with cathead and rope mechanism and the automatic trip hammer system.
3.3.1. Auto Trip Hammer
The autotrip hammer (Figure 3.1) was designed for driving the SPT spoon and conforms to the requirements of BS 1377: Part 9: 1990 for carrying out the Standard Penetration Test (SPT) or Super Heavy Dynamic Probing (DPSH). The hammer consists of a weight of 63.5 kg complete with pick-up and self-tripping mechanism that ensures that the weight has a free-fall of exactly 762 mm. The inner shaft acts as a guide that permits the weight to drop with minimal resistance and ensures that the weight strikes the anvil squarely. The hammer is supplied with a safety cross bolt which secures the sliding outer sleeve to the inner guide rod when the hammer is not in use or during transport. The overall length of the hammer is 1.8 m and 2.6 m when fully extended. The total weight, including assembly, of the hammer is 105 kg.
Autotrip hammers have advantages for standard penetration test (SPT) of consistent drop height and low friction loss during hammer fall. These advantages, however, generate high energy transfer ratios “ER”, typically about 90%. This efficiency causes lower sensitivity and higher energy correction coefficients. For SPT at WLA and GVDA, the energy produced by the automatic trip hammer was mechanically reduced to increase sensitivity by inserting a 127 mm long sleeve into the hammer mechanism.
This insertion reduced the drop height from 762 mm to 635 mm.
3.3.2. Specification of SPT Spoon
SPT spoon is usually does not comply with ASTM. Most of the cases, driving shoe of SPT spoon is sharper than that specified in ASTM. So the smaller SPT-N value is
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obtained than actual value. In Figure 3.2 the driving shoe of SPT spoon is 16º to 24º angle and the cutting edge is not too sharp which gives erroneous value of SPT.
Figure 3.2 representing the standard shoe of SPT spoon and conventional shoe of SPT spoon. In this research the standard shoe of SPT spoon is used. The drawback of conventional shoe of SPT Spoon is that the cutting edge is too sharp and it entered into the deep soil and we get the lower value of SPT which is not the correct value.
The cost of foundation will be expensive. So for controlling the quality of a sample Standard Shoe of SPT spoon is essential.
3.3.3. Fabrication of Standard Shelby Tube
The specification of Shelby tube sampler is described in “Standard Practice for Thin- Walled Tube Sampling of Soils for Geotechnical Purposes” of ASTM D 1587. This sampler consists of a thin-walled tube with a cutting edge at the toe. A sampler head attaches the tube to the drill rod, and contains a check valve and pressure vents.
Generally used in cohesive soils, this sampler is advanced into the soil layer, generally 152.4 mm (6") less than the length of the tube. The vacuum created by the check valve and cohesion of the sample in the tube cause the sample to be retained when the tube is withdrawn. Standard ASTM dimensions are shown in Table 2.1. It should be noted that ASTM allows other diameters as long as they are proportional to the standardized tube designs, and tube length is to be suited for field conditions. Soil sampled in this manner is considered reasonably undisturbed.
Most of the Shelby tube samplers used in Bangladesh have high area ratio, very rough inner surface, irregular cross sections and no specification for cutting edge. Sampler is pushed into soil by impact loading not by static thrust. It has long been recognized that if side friction becomes too great the sample will jam in the tube. Apart from the inconvenience of low percentages of recovery, this is associated with very high levels of disturbance (Clayton and Siddique, 1999). This type of undisturbed sample may lead to very conservative design of foundations causing more foundation cost.
International Society for Soil Mechanics and Foundation Engineering (ISSMFE, 1965) has made specific recommendations about leading edge, area ratio and length to
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diameter ratio for a good tube sampler (ISSMFE, 1965). Siddique (2000) reported the current practices of soil sampling in Bangladesh. As a first attempt to implement good quality Shelby tube sampling in the field in Bangladesh, this study compared the unconfined compressive strength and consolidation properties of undisturbed soil samples collected by using locally available currently practiced Shelby tube samplers and Modified Shelby tube samplers . Modified Shelby tubes was fabricated as per recommendations of ISSMFE (1965) having inside diameter 72 mm, wall thickness 1.9 mm, area ratio 10%, inside clearance ratio 0.0%, leading edge taper angle 600 up to thickness of 0.3 mm, cutting shoe taper angle 120, B/t ratio 38 and smooth inner/outer surface. Sampler quality parameters of both modified Shelby tube samplers are given in Table 3.1.