Dynamic Cone Penetration Test (DCP)

Introduction

As discussed in Chapter 2, the Standard Penetration Test (SPT) is typically performed at depth intervals of about 1.5 m (5 ft). A rapid and inexpensive alternative for obtaining more continuous data is to use a Dynamic Cone Penetrometer Test (DCP). These tests are also often referred to as drive cones, dynamic probing, or dynamic sounding Tests. Reviews of the DCP are given by Melzer Sc Smoltczyk (1982) and Stefanoff et al. (1988). Broms Sc Flodin (1988) presented an historical review of the development of DCP tests and reported that there are records tracing this type of test to Germany as early as 1699. The use of DCPs in the U.S. was described at least as early as 1948 by Terzaghi Sc Peck (1948).

DCPs may be performed using a drill rig, using essentially the same equipment used to conduct the SPT; they may be performed using portable automatic rigs, or they may be performed by hand using simple light weight equipment. In addition to their use in routine site investigation work and for estimating individual soil properties, DCPs are often used for shallow site work to evaluate footing subgrade characteristics, as a quality control tool for backfill around buried pipes or compacted fill for shallow foundations or retaining walls, to locate depth to shallow bedrock, and for a variety of other applications. Test results have been presented in clays (e.g., Butcher et al 1995; Lawson et al. 2018), sands (e.g., Palmer 6c Stuart 1957; Mohan et al. 1970; Singh 6c Sharma 1973; Coyle 6c Bartoskewitz 1980; Muromachi 6c Kobayashi 1982), and residual soils (Chang 6c Wong 1986; Kelley 6c Lutenegger 1999) as well as in gravelly materials (e.g., Rao et al. 1982; Hanna et al. 1986; Talbot 2017).

The DCP may be used to supplement other in situ tests, or it may be used in situations where site accessibility is very limited or mobilization costs for a drill rig or other equipment may be very high. They may also be used for shallow investigations where the use of a drill rig is not practical.

Mechanics

DCPs are conducted by attaching a cone on the bottom of a set of drive rods and attaching an anvil on the top of the rods. The drive rods are marked off in the specified driving intervals and then driven with a simple drop hammer system. The cone may be sacrificial or may be threaded or pinned onto the end of the rods. Figure 3.1 illustrates the principle of the DCP and shows the different components of the test. All that is needed to perform the test

Figure 3.1 Principle of Drive Cone Penetration Tests.

are a drop hammer, an anvil, a ser of drive rods, and a conical tip. DCPs have some potential advantages over other types of penetration tests:

• 1. They are simple to perform;
• 2. Most drilling companies have the necessary equipment to perform the test (e.g., SPT hammer and drill rods) and can obtain a suitable cone tip;
• 3. They may be used in a wide range of soil types;
• 4. They provide more continuous test results than the SPT;
• 5. They are rapid and inexpensive;
• 6. They may be used to provide a good indication of relative differences in density or stiffness of the soil being penetrated;

• 7. The tests do not require a large static reaction that is needed for the CPT/CPTU and DMT;
• 8. They generally do not require a borehole; therefore, there are no cuttings to dispose of;
• 9. There are essentially no moving parts to the equipment; and
• 10. The test does not require power or any electronic components.

However, this type of test also has some limitations, which include the following:

• 1. All forms of the test are not currently standardized;
• 2. There can be problems with unknown and nonuniform energy input during the test;
• 3. No soil sample is obtained;
• 4. Rod friction may cause errors in the test results; and
• 5. Interpretation of results is almost entirely empirical.

Even with these limitations, DCPs may be used in many site investigations to supplement test borings and other in situ tests. They provide a large amount of data quickly at very low cost. Compared to a CPT/CPTU deployed off the back of a drill rig, DCPs are much faster and easier to perform since no reaction is needed. Three or four DCP profiles can easily be conducted to a depth of 6.1m (20 ft) in about an hour using a drill rig with an automatic SPT hammer.

An early attempt to standardize the DCP was presented by ASTM D18 Subcommittee 2 (ASTM 1970) in which the use of a 57.2 mm (2.25 in.) diameter 60° cone with flush joint rods having an outside diameter of 44.4 mm (1.75 in.) was recommended. The cone was to be driven with a 63.5 kg (140 lbs) SPT hammer having a drop height of 76.2cm (30 in.). The number of blows for each 30.5cm (12in.) were to be recorded. There is no record that this suggested method was ever adopted by ASTM as a standard; however, a light duty DCP procedure is described in ASTM D6951. The DCP is also covered by the International Organization for Standardization (ISO) under Dynamic Probing ISO 22476-2, which describes four classes of DCP.