40 Flat dilatometer test
COMMENTARY ON CLAUSE 40
The flat dilatometer test, more commonly known as the Marchetti Dilatometer (or DMT), is a simple, robust device with the potential to give a range of soil properties as well as to be used directly in semi-empirical design procedures. It consists of a stainless steel blade, 250 mm long, 94 mm wide and 14 mm thick with a tip angle of 16° having a flat, circular steel membrane mounted flush on one side. It is inserted vertically on the end of a set of rods to pre-determined depths in the ground. A test is performed by inflating the membrane by gas pressure and the movement of the membrane is measured and the pressures required to start it moving and to expand it a given distance are recorded. Readings are repeated at each depth interval down the profile (typically 200 mm). The pressure readings are corrected for stiffness effects, etc. and converted to dilatometer parameters; these parameters are then correlated to various soil/geotechnical parameters related to ground behaviour.
DMT testing should be specified and carried out in accordance with ISO/TS 22476-11, BS EN 1997-2 and NA to BS EN 1997-2. Any correlations used should be justified and reported in the Ground Investigation report.
NOTE 1 The DMT is suitable for use in sands, silts and clays, where the grains are small compared to the membrane diameter (60 mm), with a very wide range of strengths, from very soft to very stiff clay or weak rock. It is not suitable for gravels, although the blade is robust enough to pass through gravel layers of no more than about 0,5 m thickness.
NOTE 2 Unless hindered by impenetrable layers, it is a very good profiling tool. When specified and used correctly, reliable and repeatable data can be obtained. A wide range of correlations to standard and advanced geotechnical parameters is now available and test data can be used directly in the design of foundations. However, the confidence level in the selection of parameters for correlation purposes is to some extent related to experience in the interpretation of the DMT data in the type of deposits being investigated.
41 Standard penetration test
COMMENTARY ON CLAUSE 41
The main reason for the widespread use of the standard penetration test (SPT) is that it is simple and inexpensive and can be carried out by the drilling crew. The test uses a thick-walled sample tube, the outside diameter of which is 50 mm. This is driven into the ground at the bottom of the borehole by blows from a standard weight falling through a standard distance. The blow count (N value, number of blows to drive the sampler 450 mm) gives an indication of soil strength, relative density and other parameters which can be inferred; these are, at best, approximate, but they might give a useful guide in ground conditions where it might not be possible to obtain borehole samples of adequate quality, e.g. 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 silty or very sandy clays, or very stiff 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 might be necessary to use a different methods. The small sample that is recovered from the SPT split sampler is likely to have suffered disturbance but can normally be used for identification purposes.
The standard penetration test is a dynamic penetration test and should be carried out in accordance with BS EN ISO 22476-3, BS EN 1997-2 and NA to BS EN 1997-2. The records should be presented on borehole logs as blow counts for all increments, with the N value reported in the standard way.
NOTE 1 Even minor variations from the specified procedure, including the preparation prior to carrying out the test, can seriously affect the results. Maintaining verticality of the SPT rods at the top of the borehole and the hammer assembly is particularly important to ensure consistent energy imparted to the anvil. Annual calibration of the hammer is required by BS EN ISO 22476-3. Typical Energy Ratio (Er) values for UK hammers vary widely. These variations are particularly important if the test result is to be used for quantitative purposes; if the Energy Ratio (Er) obtained from this calibration differs too much from 60%, steps to stop using that hammer might be appropriate.
When the test is carried out in coarse soils below groundwater level, the soil can become loosened, even when the test is carried out in strict accordance with BS EN ISO 22476-3, BS EN 1997-2 and NA to BS EN 1997-2 and the borehole has been properly prepared. (This is particularly problematic in sands and silty sands where groundwater flow into the borehole (piping) can lead to "blowing sand" conditions.) Boreholes should be kept topped up with water to a level at least as high as the relevant groundwater level; a suitably gentle shelling action should be used, and undersize boring tools used to reduce the suction pressure when raising the tool.
NOTE 2 In certain circumstances where very low blow counts are recorded, equivalent to N values of say 5 or less, it can be useful to continue driving the sampler beyond the 450 mm distance specified, for a further 300 mm, recording blows for 75 mm increments as with the main test drive. Although this is not a standard penetration test, and is not be regarded as such, it might at least give an indication as to whether the deposit is really as loose as the standard test indicates.
When the test is being performed in gravel or coarser soil or in rock, the cutting shoe of the split-barrel sampler should normally be replaced by a solid cone of the same outside diameter and an included angle of 60° and then recorded as SPT(C).
NOTE 3 The measured N value can be correlated with other soil parameters and with the performance of structures. The test is usually carried out in sands and gravels, but it has also been used to assess the strength of other soils (silts and clays) and of weaker rocks (e.g. Southern Province Chalk Subgroup).
When there is good reason to believe that unrealistically low values are being recorded, then an alternative testing method, which can be performed independently of a borehole, should be used, e.g. the dynamic or static probing described in 38.1 and 39. In the construction of bored piles, the test is sometimes carried out in boreholes considerably larger in diameter than those used for ground investigation work. The result of the standard penetration test is dependent upon the diameter of the borehole, and so these tests should not be regarded as standard penetration tests. They might, however, provide useful information to a piling contractor, particularly if the contractor has considerable experience in their use.
Correlation of the test results with other geotechnical parameters should be made with great care. BS EN 1997-2:2007, Annex F gives some examples of interpretation; any correlations and corrections (see BS EN ISO 22476-3 and BS EN 1997-2 for discussion on these) used should be reported in the Ground Investigation report. In particular, correction of N values to take account of the
Energy Ratio of individual hammers should take account of such a procedure not having been used in the original data reviews/correlations.
NOTE 4 SPT results and soil parameter correlations derived from data from other countries might not correlate with results from SPTs derived in accordance with previous UK standards. However, with the adoption of BS EN ISO 22476-3, this situation is less likely in future.