The Dynamic Cone Penetration Test For Soil Resistance
The Dynamic Cone Penetration Test (DCPT) is a widely-used and very simple test for soil compactness and load-bearing capacity. In this post, we’ll describe the test and the instruments involved, and give basic instructions in how to perform it.
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The Need To Measure Soil Strength
In any situation where soil must bear a significant load it is important to know the strength of the soil before the load is applied. This is true whether you’re constructing a home, an office building, or a highway, and it is true of areas that must bear occasional loads, including road shoulders and embankments. A means of accurately measuring soil load-bearing capacity is essential to any construction or civil engineering project.
Cone Penetration Testing
Cone penetration testing is a means of testing soil resistance by forcing a rod with a cone-shaped tip into the soil at a measured rate. The extent of penetration at each stage indicates the resistance to shear and the overall ability of the soil to bear a load.
The terms Cone Penetration Testing (CPT) and Dynamic Cone Penetration Testing (DCPT) actually refer to two very different methods of carrying out a cone penetration test. In CPT, the cone-shape tip is pressed into the soil at a measured rate, typically by a large (and expensive) machine that can deliver a considerable degree of steady pressure at a controlled rate.
Dynamic Cone Penetration Testing
DCPT, on the other hand, uses mechanical impact to force the cone-tip into the soil, and requires only a simple and inexpensive hand-held device. It can be performed by two people, making it ideal for use in highway engineering and other types of construction or engineering that require frequent or rapid soil inspection in widely separated areas.
Most DCPT devices (known as dynamic cone penetrometers) are strictly mechanical in operation, requiring the operator to lift a moderately heavy ring- or cylinder-shaped weight (the hammer) and release it repeatedly; the operator or an assistant must also record the penetration data based on visual measurements. There are, however, several automated DCPT devices on the market, with computer-controlled hammer action and data recording.
The Dynamic Cone Penetrometer
A typical Dynamic Cone Penetrometer (DCP) consists of two steel shafts; in operation, they are fitted together to form a single shaft. The upper shaft has a handle at the top; the hammer is fitted to this shaft, and able to slide freely on it. The lower shaft has a fixed metal cylinder, called the anvil, at the top; the shaft is marked in increments, typically 5 mm. The base of the upper shaft fits into the center of the anvil, and when the operator raises and drops the hammer, it lands on the anvil.
The cone is fitted into the lower end of the lower shaft; the impact of the hammer on the anvil drives the cone into the soil.
For highway and civil engineering use in the United States, DCP specifications are typically as follows:
- Hammer Weight: 8 kg (17.6 lb.)
- Drop Height: 575 mm (22.6 in.) (base of hammer to top of anvil)
- Lower Shaft: 1 m (40 inches) (variable)
- Cone Diameter: 20 mm (0.79 in.) (at base)
- Cone Angle: 60° (30° can be used for more resistant soils)
Note that the actual DCP specifications may vary considerably (hammers, for example, can weigh from 7 to 9 kg or beyond) depending on both the purpose and local standards.
Using The DCP: Assembly
Generally, the DCP will be disassembled when stored and transported, so you must assemble it before use. To do this, you will usually insert the bottom of the upper shaft into the anvil end of the lower shaft, line up the connecting-pin (or bolt) hole, and insert the pin/bolt, washer, and retaining clip. You should inspect the cone before attaching it to the lower tip of the shaft, both before and after use, to see if it is worn or damaged.
Some DCPs may have an attachable measuring device, which can be fixed to the shaft in such a way that it does not interfere with the motion of the hammer or the cone; if this is the case, it will probably come with special instructions for assembly and attachment.
Using The DCP: Preparing The Test Spot
You should, of course, have utility clearance before conducting any soil test; the DCP can penetrate as much as 36 inches into the soil, so it is not merely a surface test.
A DCP is designed to penetrate soil, rather than asphalt, concrete, or other hard surfacing materials. In order to test the soil beneath such a material, a hole (typically about 2 inches in diameter) should first be cut or drilled down to the actual soil. (Note that the cutting/drilling process should do as little as possible to disturb the underlying soil. If water-cooling is required, excess water should be drained away as soon as possible, and the test itself should be done before the remaining water has a chance to soak into the soil.)
Using The DCP: Setup
To conduct the actual test, first place the DCP in a vertical position, with the tip of the cone resting on the soil. You should establish a reference level before proceeding with the test. if you do not have a measuring device with a built-in reference, you can do this by placing a ruler, straightedge, or other flat, rigid object on the soil close to the cone; it should remain in that position throughout the test.
Drive the cone into the soil (by raising the hammer to about half height or less and dropping it) until the base (the wide part) is just below the reference. This is the point at which you begin measuring.
Using The DCP: Running The Test
To run the test, raise the hammer to the top of the shaft, being careful not to hit it against the handle. Let go of the hammer, allowing it to fall freely and hit the anvil. After each drop, record the position of the shaft in relation to the reference (usually to the nearest millimeter or 0.1 inch). Continue to do this either until the shaft reaches the required depth or until you have dropped the hammer the required number of times (based on your agency’s standards).
After the test, remove the shaft from the ground (by pulling it out or using a jack; not by forcing it up with the hammer), inspect it, disassemble it, and put it back in its transport case.
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