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30 Carl Bro LWD Device It is expected that these disadvantages of the equipment can be easily resolved with future modifications. These devices The Carl Bro system looks exactly like the Dynatest system, will likely make the technology and device more expensive. It except that it has additional sensors that are not attached to does, however, provide the agency with elastic modulus val- the frame. These extended geophones do not change the theory ues that can be used to confirm design assumptions with and applications. Although, the algorithms are slightly differ- proper calibration. In summary, the LWDs are believed to be ent to include input from the additional sensors, the theory less practical and effective for the uses that are the focus of and application appear to be the same. this study. The geophones are arranged linearly at set distances from the plate. Since the sensors are connected to each other by a bar, but separate from the loading plate, connecting and placing 1.4 Dynamic Cone Penetrometer them at a specific distance from the plate for each test becomes The DCP is used to estimate the strength and modulus of problematical. It is expected, however, that these perceived unbound materials and soils. The DCP is much like the LWD disadvantages can be resolved in future modifications to the in appearance (see Figure 7); however, it uses a 15-lb (6.8-kg) equipment. steel mass falling 20 in. (50.8 cm) that strikes the anvil to cause The process, from the beginning through the last of the five penetration of a 1.5-in. (3.8-cm) diameter cone (45 vertex drops, takes an average of about 5.5 minutes. The procedure angle) that has been seated at the surface or in the bottom of followed for using the system is listed. a hand augered hole (see Figure 8). The blows required to drive the embedded cone a depth of 13/4 in. have been correlated by 1. Locate test point (surface must be even (flat) and must be others to N values derived from the Standard Penetration cleared of anything that could cause part of the plate to Test (SPT). Experience has shown that the DCP can be used lose contact with the surface). effectively in augered holes to depths of 15 to 20 ft (4.6 to 6.1 m). 2. Set the loading plate on the surface to be tested (plate must The system has been used in the past for the testing of soils be flat on the surface). more than anything else. 3. Measure for geophone location. The technical skills and training requirements for the DCP 4. Set the geophone arm and line up the sensors. operator are no different than for a nuclear density gauge. 5. Set data acquisition key for collecting the deflection data. Advantages of the DCP include its simplicity, low maintenance 6. Drop hammer (first drop "seats" the plate and is not read). (using disposable tips, making sure that the allen screws 7. Repeat last two steps for five drops at each location are kept tight, etc.), mobility, and low cost. It can also be used (including the one to seat the plate). to test thick embankment layers, unlike some of the other NDT technologies and devices. This system had a wired connection to a laptop computer Conversely, the manual apparatus is slow (tests took 5 to and was more cumbersome to set up because of the additional 10 minutes at each location), its use is physically demanding, geophones. In addition, the seating drop of the plate some- and the test is actually destructive to bases and pavements, times moved the plate. This increased the variability in the data that is, the test creates a hole in the material. Use of the gathered from the geophones and increased the number of device can also be dangerous, if the operator's hand gets caught anomalies. The system is comparable in cost to the Prima 100. Summary This technology was tested on crushed aggregate base material, embankments, and prepared subgrades. However, there should be no difference between the procedures and the device's reaction to a hard base material and those of HMA mixtures. A key advantage of this technology is that it gives the operator a reading of the elastic modulus in about the same time required to obtain a nuclear density gauge reading. The disadvantages are that the devices have limited reliability because of the range and reliability of the wireless remote and its software logic. In addition, the resulting values for the upper layer are dependent on the stiffness and variability of Figure 7. DCP before assembly for use in measuring the supporting layer. the in-place strength of unbound materials and layers.