DESCRIPTION AND APPLICATION OF DUAL-
MASS DYNAMIC CONE PENETROMETER
The purpose of this appendix is to describe the DCP, its use, and the application of data obtained by its use. Procedures are presented for using the DCP to measure soil strength and correlating DCP index with CBR strength values required for operation of aircraft and military vehicles on unsurfaced soils. Procedures are also presented for using the DCP to evaluate aggregate surfaced roads and airfields for military operations based on the existing soil strength conditions.
DESCRIPTION, USE, AND MAINTENANCE OF DCP
DESCRIPTION OF DUAL MASS DCP
Dual-Mass DCP Device
The dual-mass DCP consists of a 5/8-inchdiameter steel rod with a steel cone attached to one end which is driven into the pavement or subgrade by means of a sliding dual-mass hammer (Figure J-1). The angle of the cone is 60 degrees, and the diameter of the base of the cone is 0.79 inches. The cone is hardened to increase service life. The diameter of the cone is 0.16 inch larger than that of the rod to ensure that the resistance to penetration is exerted on the cone. Figure J-2, shows an assembled DCP with vertical scale for measuring the cone penetration depth. The DCP is driven into the soil by dropping either a 17.6-pound or 10.1-pound sliding hammer from a height of 22.6 inches. The 17.6-pound hammer is converted to 10.1 pounds by removing the hexagonal set screw and removing the outer steel sleeve, as shown in Figure J-3. This procedure can be accomplished during a test since the outer steel sleeve is designed to slide over the DCP handle. The cone penetration caused by one blow of the 17.6-pound hammer is essentially twice that caused by one blow of the 10. l-pound hammer. The 10.1-pound hammer is more suitable for use and yields better test results in weaker soils having a CBR value of 10 or less. The 17.6-pound hammer penetrates high-strength soils quicker and may be preferred when these type soils are encountered. However, the 10.1-pound hammer can be used on soils up to CBR 80. The depth of cone penetration is measured at selected penetration or hammer-drop intervals, and the soil shear strength is reported in terms of DCP index. The DCP index is based on the average penetration depth resulting from one blow of the 17.6-pound hammer. The average penetration per hammer blow of the 10. l-pound hammer must be multiplied by two in order to obtain the DCP index value. The DCP is designed to penetrate soils to depths of 36 inches. Individual DCP index values are reported for each test depth resulting in a soil-strength-with-depth profile for each test location.
Dual-Mass DCP Kit
Figure J-4, shows a dual-mass DCP kit designed for Army engineer use. The kit includes the following items:
- Case assembly.
- Top rod threaded and welded to the handle.
- Bottom rod threaded and welded to the anvil.
- Dual-mass hammer.
- Vertical scale in centimeters and inches.
- Go/no go gauge.
- Six hardened, 60-degree, fixed cones.
- Three hardened cone adapters and 200 disposable cones.
- Two channel lock pliers.
- One can of light, lubricating oil.
- Thread-locking compound.
- Hexagonal wrench set (5/64 to 1/4 inch).
The DCP test kit shown in Figure J-4, is not a current government stock item, and it is not available on the commercial market. Test kits and component parts are currently manufactured at WES and are available to other government agencies for cost reimbursement. A US patent on the DCP test kit is pending. Until a patent license with a commercial manufacturer can be obtained, the test kit can be made by the user or a contractor. A complete set of plans can be obtained by contacting WES at (601) 634-2282.
The disposable cone is for use in soils where a standard cone is difficult to remove. The disposable cone mounts on an adapter and is shown in Figure J-5. At the conclusion of the test, the disposable cone easily slides off the cone adapter, allowing the operator to easily remove the DCP device from the soil. The disposable cone remains in the soil. Use of the disposable cone approximately doubles the number of tests per day that can be run by two operators.
Go / No Go Gauge
The go / no go gauge is used to ensure the cone's base diameter is within proper tolerance. Each new cone should be checked before use and at selected usage intervals to ensure the cone's base diameter is within a proper tolerance of between 0.78 and 0.8 inch. The cone must be replaced if its base diameter fits into both ends or neither end of the go / no go gauge. The cone is within proper tolerance when it fits into only one end of the gauge.
The DCP test causes wear to the metal parts of the DCP device. Parts of the DCP device will eventually suffer fatigue failure and will have to be repaired or replaced. In order to ensure maximum service life, the DCP should be inspected before it is used to ensure all joints are tight. Thread-locking compound should be used on loose joints. Also, the cone's base diameter should be checked to ensure it is within tolerance. If the cone point becomes bent or too blunt to penetrate around aggregate, it must be replaced.
Two people are required to operate the DCP. One person holds the device by its handle in a vertical position and taps the device using the hammer until the base of the cone is flush with the surface of the soil. The second person then checks the device for a zero reading by holding the vertical scale between the soil surface and bottom of the hammer. The bottom of the 4-inch-diameter portion of the hammer should read 0 millimeters on the vertical scale. In weak soils, the weight of the DCP device will sink the cone past its zero reading. In this case, a zero-blow penetration reading is recorded, in millimeters, at the actual measured pretest depth. The hammer is then raised to the bottom of the handle and dropped. Care should be exercised when raising the hammer to ensure the hammer is touching the bottom of the handle but not lifting the cone before it is allowed to drop. The hammer must be allowed to fall freely with its downward movement not influenced by any hand movement. The operator should also be careful not to exert any downward force on the handle after dropping the hammer. Both the operator and the recorder should keep track of the number of hammer drops (blows) between measurements. The recorder is responsible for recording the number of hammer blows between measurements. He is also responsible for measuring and recording the penetration after each set of hammer blows. Penetration measurements are recorded to the nearest 5 millimeters. As an example of how to read the penetration depth, Figure J-6 shows a penetration depth of 150 millimeters.
The cone must penetrate a minimum of 25 millimeters between recorded measurements. Data taken at less than 25-millimeter penetration increments are unnecessary and sometimes result in inaccurate strength determinations. The number of hammer blows between measurement recordings will generally be 20, 10, 5, 3, 2, or 1, depending on the soil strength and thus cone penetration rate. Both the operator and recorder should be alert to sudden increases in the cone-penetration rates during the test. Any noticeable increase in the penetration rate indicates a weaker soil layer. The operator should stop and allow the recorder to record the blow count and penetration depth whenever a weaker soil layer is encountered.
After the cone has been driven to the desired test depth (maximum 39 inches), it is extracted from the soil by driving the hammer against the top handle. Caution must be exercised during this operation to prevent damage to the DCP device. The hammer must be raised in a vertical direction (rather than in an arcing motion), or the rod may be bent or broken where it connects to the anvil. In soils where great difficulty is encountered in extracting the DCP device, disposable cones should be used. Using disposable cones will save wear and tear on both the device and the operator. In soils with large aggregate, the DCP may try to penetrate the soil at a slant rather than from a vertical direction. The operator should not apply force to the handle of the DCP in an attempt to force it to penetrate the soil vertically. Lateral force on the handle in an attempt to make the DCP penetrate the soil vertically will cause the upper handle rod to fatigue and break at the point where it screws into the anvil. Instead, the test should be stopped when the handle deviates laterally 6 inches or more from the vertical position, and a new test should be attempted at another location.
The DCP should be kept clean, and all soil should be removed from the penetration rod and cone before each test. A light application of spray lubricant or oil should be applied to the hammer slide rod before each day's use. All joints should be constantly monitored and kept tight. Loose joints will lead to equipment failure Any problem joints should be treated with a joint-locking compound. The lower penetration rod should be kept clean and lubricated with oil when clay soils are tested.
SOIL STRENGTH EVALUATIONS WITH DCP
Number of Measurements
The number of measurements to be made, location of the measurements, depth of measurements required, and frequency of recording data with depth vary with type of road or airfield pavement operation and with time available for conducting the tests. For this reason, hard-and-fast rules for the number of tests required in evaluating roads and airfields are impracticable. Soil conditions are extremely variable. The strength range and uniformity of the soils or existing pavement materials will generally control the number of measurements necessary. In all cases, it is advisable to first test those spots that appear to be weakest. since the weakest condition controls the pavement evaluation, Penetrations in areas that appear to be firm and uniform may be few and widely spaced. In areas of doubtful strength, penetration tests should be more closely spaced. No less than three penetration tests should be made in each area having similar type soil conditions.
Reading Depths in Soil
Soil strength usually increases with depth, but in some cases a thin, hard crust will overlay a soft layer, or the soil will contain thin layers of hard and soft material. For this reason and the fact that many aircraft and some military vehicles will effect the soil to depths of 36 inches or mom, it is recommended that each penetration be made 10 a depth of 36 inches unless prevented by a very hard condition at a lesser depth. Soil test depths may be reduced when required traffic operations are known and the thickness requirements indicate that a reduced thickness above the subgrade controls the evaluation.
Correlation of DCP Index with CBR
Correlation of DCP index with CBR is necessary since the CBR is the soil strength value used for designing and evaluating unsurfaced. aggregate-surfaced, and flexible pavements for military roads and airfields. A data base of field CBR versus DCP index values was collected by WES technicians from many sites and different soil types (Table J-1). In addition, correlation test results by Harison (1987), Kleyn (1975), Livneh and Ishai (1987), and Van Vuuren (1969) were compared with the data-base test values (Figure J-7). General agreement was found between the various sources of information. The equation log CBR = 2.46 - 1.12 (log DCP) was selected as the best correlation. In this equation, DCP is the penetration ratio in millimeters per blow for the 17.6-pound hammer. Figure J-8, shows a plot of the correlation of CBR versus DCP index. Table J-2, shows a tabulated correlation of DCP index with CBR.
The data can be tabulated in spread-sheet format with the only data input values required being that of the number of hammer blows, hammer weight, and cone penetration recorded to the nearest 5 millimeters after each set of hammer blows. Figure J-9, shows a sample format for a DCP data sheet.
The user should group test data for locations having similar type soil conditions. For each location group, an individual should make a combined data plot showing CBR, interpreted from Figure J-8 versus depth in inches as shown in Figure J-10. From this data, an average data plot of CBR versus depth in inches should be developed. Average data plots for each location having similar type soil conditions are used in the following pavement evaluations.
APPLICATION OF DCP DATA
TM 5-822-12 can be used for evaluating the potential of military operations on unsurfaced soils and aggregate-surfaced roads and airfields based on existing soil conditions. The evaluation procedure is the reverse of the design procedure. CBR and thickness evaluation data from the DCP tests are used to enter the appropriate set of design curves in this manual or in FM 5-430-00-1/AFPAM 32-8013, Vol 1, to determine the allowable design index for roads or allowable gross weight and aircraft pass configuration for airfields. The design index for roads is then used to determine the allowable road class and number of vehicle passes per day for various traffic categories.
For unsurfaced soils in which the soil strength increases with depth, the average strength of the top layer is first used in order to make sure that compaction to a higher strength or the addition of a surfacing aggregate layer is not required. If the top layer of soil is adequate to support the desired design index or aircraft passes, then the strength of weaker soil layers beneath the top layer is used in order to check for adequate thickness requirements of the surfacing layers of soil.
For aggregate-surfaced roads and airfields, both the subgrade soil strength and aggregate layer strength should be used to ensure that the aggregate thickness and strength requirements are adequate for a given design index or aircraft pass level.
Because soil conditions are immediately and significantly affected by weather, an evaluation is valid only for the period immediately after measurements are made for unsurfaced pavements. However, it can usually be assumed that the evaluation will remain constant as long as no rain occurs. Gravel-surfaced pavements will be affected to a much lesser extent by rain.
DCP tests in highly plastic clays are generally accurate for depths to approximately 12 inches. At deeper depths, clay sticking to the lower rod may indicate higher CBR values than the actual values. Oiling the penetration rod will help prevent the clay from sticking to the penetration rod; however, it will not significantly improve the test results. A 2-inch-diameter (or larger) auger can be used to open the test hole up after each 12-inch DCP test penetration. This will eliminate clay lower-rod friction problems and allow the test to accurately measure the clay soil strength for an additional 12 inches.
Many sands occur in a loose state. When relatively dry, such sands show no DCP index values for the top few inches and may show increasing DCP index values with depth. The confining action of aircraft tires will increase the strength of the sand. Generally, any dry sand or gravel will be adequate for aircraft in the C-130 class, regard less of the DCP index values. All sands and gravels in a quick condition (water percolating through them) must be avoided. Evaluation of moist sands should be based on the DCP tests as described earlier.
Soil remolding is the changing or working of a soil by traffic. The effects of traffic remolding may have a beneficial, neutral, or detrimental effect and result in a change of soil strength. Additional DCP tests should be run after some traffic has been applied to determine any changes that may have occurred in soil strengths.
If the cone does not penetrate 25 millimeters after 10 blows with the 17.6-pound hammer (20 blows with the 10.1-pound hammer), the test should be stopped. If this firm material is a stabilized soil or high-strength aggregate base layer, it should be cored or drilled with an auger to allow access of the DCP cone to underlying layers. The DCP test can then proceed through the access hole after the depth of the material layer has been recorded. The material layer is assigned a CBR value of 100+. However, if a core or auger drill is not available, the 17.6-pound DCP hammer can normally be used to drive the lower rod and cone through the firm material. If the cone penetration was stopped by a large rock or other object, the DCP should be extracted and another attempt made within a few feet of the initial test. The DCP is generally not suitable for soils having significant amounts of aggregate greater than a 2-inch-sieve size.
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