Presentation of Tests Results
Results from DCPs are typically presented in one of the following ways:
- 1. Incremental number of blows for a given driving distance;
- 2. Cumulative number of blows for the entire depth;
- 3. Penetration distance achieved with each drop of the hammer; or
- 4. Dynamic penetration resistance, rd or qd, calculated from different driving formulae.
Incremental Penetration Resistance
Results of DCPs are often simply presented as a plot of incremental penetration resistance versus depth, as previously shown in Figure 3.7. The penetration resistance is defined over a specific penetration interval, typically 10-20cm (4-12 in.). An example using results from a super heavy DCP profile conducted at a medium dense fine sand site is shown in Figure 3.9. These results clearly show changes in driving penetration resistance at several depths, indicating some changes in stratigraphy.
In this case, a 63.5 mm (2.5 in.) diameter 60° cone was used with AW rods in lengths of
1.5 m (5 ft.) and an automatic SPT hammer to perform the tests. Since the tests were performed above the water table, there is little possibility of the sand collapsing back around the drive rods, and therefore, almost no side resistance builds up on the rods. As a check in the field, the engineer checked the rod friction by simply rotating the rods by hand after each section of rod was driven. Additionally, the penetration record indicates lower blow counts after penetrating through a stiff layer where blow counts were high, again indicating little buildup of rod friction.
Figure 3.9 Incremental driving resistance from super heavy DCP (blows/6in.).
Figure 3.10 Cumulative penetration resistance.
Cumulative Penetration Resistance
DCP results can also be presented as cumulative penetration resistance or total driving record as a function of depth. This is done by numerically adding the results for each consecutive driving interval. The incremental results from Figure 3.9 are shown as cumulative driving resistance in Figure 3.10 as an example. These results are typically interpreted by identifying changes in the slope of the penetration record. Figure 3.10 indicates changes at depths of about 15, 23, and 41 ft. In this case, the changes may indicate differences in grain- size distribution, relative density, or a different stratigraphic unit.
Penetration Distance per Hammer Blow
It may be useful to record the actual penetration distance achieved with each drop of the hammer. This requires a measurement of the advance of the drive rods after each hammer blow. This is usually done by measuring the distance between a mark on the rods and some stationary reference point, usually the top of the ground or casing or using an electronic system. The record of penetration distance per hammer blow may give a more refined look at changes in penetration resistance that might be missed using the incremental penetration resistance, which tends to “average” out the resistance over the interval.
A slight modification to this procedure is to choose a specific number of hammer blows, say ten, and measure the resulting penetration produced. The average penetration per blow over the measured interval is obtained by dividing the distance by the number of blows. Data from the penetration record may be used to calculate the incremental and cumulative resistance by simply counting the number of hammer blows that were required to produce the set driving increment.
Dynamic Penetration Resistance
Bolomey (1974) suggested that it may be useful to express the penetration resistance in terms of the resistance values, rd or qd, using different pile driving formulae. The unit dynamic driving resistance is obtained as
or
where:
rd and qd=dynamic penetration resistance (Pa)
M=mass of the hammer (kg)
M'=total mass of the rods, anvil, etc. (kg)
H=hammer drop height (m)
A=cross sectional area of the cone point (m2) e=average penetration distance (m/drop) = D/N N=number of blows/lOcm D=drive interval (mm) g=acceleration of gravity (9.81 m/s2)
Dynamic penetration resistance is particularly useful when comparing DCP results obtained from different equipment, such as of different size cones, hammer mass, or height at the same site.
Interpretation of Test Results
The interpretation and use of DCPs may be highly varied depending on the application, the soil, the need, and the experience of the engineer. For example, Stefanoff et al. (1988) found that the predominant use of DCPs was qualitative, i.e., to distinguish between different soil layers. However, DCPs have also been used to interpret individual soil properties in both coarse-grained and fine-grained soils and for the design of deep and shallow foundations. Because of the wide variation in equipment and definition of Nc obtained from any particular set of equipment, caution should be used when applying any empirical correlations.
