COMMENTARY ON CLAUSE 38
Probing from the surface probably represents the oldest method of investigating the depth to a hard stratum where the overburden is of low strength and not unduly thick. The simplest probe is a sharpened steel rod, which is pushed or driven into the soil until it meets resistance. The method is still of use where it is likely that there are relatively thin layers of low strength soils overlying much higher strength soils, when the thickness of the soft stratum might be determined over a wide area very quickly and economically. The method has many limitations, and a variety of more sophisticated methods have been developed in an attempt to overcome these and to extend the use beyond that of detecting a bearing stratum within the soils present.
38.1 Dynamic probing
COMMENTARY ON 38.1
The apparatus for dynamic probing comprises a sectional rod with a cone fitted at the base of a slightly greater diameter than the rod. It is driven into the ground by a constant mass that is allowed to fall on the rod through a constant distance, and arranged such that the mass falls through the constant distance without the operator having to use their judgement in any way. This is usually achieved using a mechanical latch on machine-driven equipment and mechanical indication on hand-operated apparatus. The number of blows to drive the tip a given distance is recorded as the blow count. The process is repeated throughout a profile. Dynamic probing using small track mounted machines (generally used also for dynamic sampling) and hand-operated equipment, provides an indication of stratigraphy and ground strength.
Interpolation of data between boreholes using site specific correlations with known ground property data might be possible. Where a ground investigation has been carried out by exploratory holes, it might be possible to use dynamic probing to check rapidly and cheaply that conditions on neighbouring positions are similar. As with other types of penetrometer, probing can give unreliable results in soils containing cobbles or boulders, which can easily be mistaken for bedrock. The main limitation of dynamic probing is that the soil being tested cannot be identified, although sampling techniques using the machine operated equipment have been developed. Some examples of the use and interpretation of dynamic probing results are given in BS EN 1997-2:2007, particularly Annex G, and NA to BS EN 1997-2, but these correlations have not all been proven in UK conditions.
The selection of the type of dynamic probe apparatus to be used and the method of working should be carried out in accordance with BS EN ISO 22476-2, BS EN 1997-2 and NA to BS EN 1997-2. The basis for the selection of which type is to be used should be that the driving energy (hammer mass and height of fall) is appropriate for the ground conditions anticipated.
There have been changes to some of the configurations commonly used in the UK and care should be taken to ensure that the correct name is assigned to the configuration being used.
The results of dynamic probing may be correlated with the results of other types of field test and/or used to estimate various soil parameters. Some correlations are given in BS EN 1997-2; these are subject to the caveats in NA to BS EN 1997-2. Where these and other published correlations are used, care should be taken to ensure that they are applicable for the driving energy of the dynamic probing apparatus used.
NOTE The fact that the rod is slightly smaller in diameter than the base of the cone to some extent prevents shaft friction influencing the results. In some soils where the hole closes in on the rods this factor is to be taken into account. One method is to correlate the torque measurements, which have been taken as part of the test before each new extension rod is added, with the blow count and subtract this from the measured blow count (see Butcher, McElmeel and Powell, 1995 ). Shaft friction can be eliminated or substantially reduced by boring a hole to the depth required for each test, or by providing the exterior of the rods with either a sleeve or a lubricating mud injected behind the cone. The size of the cone, mass of hammer and the distance through which the constant mass is allowed to fall have only recently been standardized in the UK, so there is only limited practical experience behind the test, although some published data are available linking the results of certain tests with soil parameters determined by other methods (see Butcher, McElmeel and Powell, 1995 ). Cone size and hammer weight are available in different configurations so as to ensure various ground types can be penetrated and measureable results obtained (see BS EN ISO 22476-2).
Other "non-standardized" dynamic probing devices exist and may be used, where appropriate; the most commonly used in the UK is the Mackintosh Probe (see Clayton et al., 1995 ); with this device the hammer is lifted by hand and the driving energy sometimes declines due to operator fatigue. Care should be taken when these devices are used for anything other than generalized profiling.
38.2 Dynamic cone penetrometer
COMMENTARY ON 38.2
The dynamic cone penetrometer (DCP) is a type of dynamic probe intended, primarily, to obtain values of California Bearing Ratio by correlation. There is no formal standard for this test but it is described in Design manual for roads and bridges, Vol 7 . The DPC results are presented as penetration/blow, i.e. the inverse of general practice for dynamic probing in 38.1. A summary of the history of its development and a review of the published correlations is given in Booth, Keeton and Gosling, 2008 .
In the absence of a standard, the test should be carried out and the data processed following the guidance given by the Highways Agency or in the manufacturer's literature. The correlation adopted should be appropriate to the cone used for the test but even so the results should be treated with caution.