Section 5: Destructive Evaluation of Pavement Structural PropertiesAnchor: #i1007053
Trenching and Coring
Trenching and coring have been used in forensic and routine evaluation to determine the source of the problematic layer or layers. For example, there was severe rutting on US 281 and the district expressed a need to determine the source of the rutting. Although FWD and GPR tests were performed, evaluation of the data could not differentiate from which layer(s) the rutting came. Trenching provides a viable option. For example, Figure 4-17 illustrates the pavement section profiles on US 281. Chalk and stringlines were used to differentiate different pavement layers, as shown in Figure 4-17 and Figure 4-18. In view of Figure 4-17 and Figure 4-18, the rutting was found to be from the surface AC layer.
Figure 4-17. Trench sidewalls showing the pavement layer profile.
Figure 4-18. Trench sidewalls showing severe rutting.Anchor: #i1007090
A trenching procedure has been developed to run tests and collect samples while minimizing disturbance to the base material. The saw cut pattern is shown in Figure 4-19. The size of each trench is 3 ft. × 12 ft. Each saw cut is deep enough to penetrate all the way through the AC layer(s). A strip about 6 in. wide is cut and removed by hand from one end of the trench, as shown in Figure 4-20. This creates a slot for the backhoe to reach into and lift the AC block without major disturbance to the base layer as shown in Figure 4-21. A saw cut is also made across the center of the trench so the AC blocks can be removed without breaking.
Figure 4-19. Saw cuts for trenching.
Figure 4-20. Removing asphalt concrete (AC) block with backhoe.
Figure 4-21. Flipping asphalt concrete (AC) block out with backhoe.
After the asphalt layer is removed and samples are collected, tests can be run, and samples collected on the base layer.
The backhoe then digs out the base layer and a few inches of subgrade. One of the transverse trench walls (at base and subgrade levels) is then smoothed using shovels, chisels, and brooms. Once a clean trench wall is achieved, the asphalt layers are highlighted with chalk and the base/subgrade layers are highlighted with string lines held with small nails. The thickness of each layer is then measured at regular intervals to determine its contribution to rutting.Anchor: #i1007144
To extract a core sample, the coring rig is placed at the desired location and the barrel-cooling water is turned on. Note that dry ice can be used to cool the core barrel if samples at in situ moisture content are desired, as show in Figure 4-22.
Figure 4-22. Coring with dry ice.
The barrel is spun at about 500 rpm and gradually lowered through the asphalt layer. If the base is stabilized, the operator may want to cut through it as well to obtain an intact sample.
After the barrel has cut to the desired depth, it is retracted while still spinning. Then the core barrel is stopped and the location of the core is observed. If the core has twisted off and is now lodged in the barrel, the barrel is struck lightly with a mallet to loosen the core. If the core is still in its original position, thin pieces of metal (or screwdrivers) are used to rock the core back and forth until it breaks free from the base or subgrade. Then the core can be lifted out with bent welding rods or loops of thin wire.
Normally, the core diameter is either 4 in. or 6 in. Figure 4-23 shows the extracted core samples with a 4-in. diameter.
Figure 4-23. Extracted core samples.Anchor: #i1007193
Shelby tube samples have been used to determine the in situ density, moisture content, plasticity index (PI), potential vertical rise (PVR), sulfate content, optimum soil stabilizer, and modulus of the subgrade soil.
The Shelby tube is a sharpened pipe that is pushed into soil by a hydraulic ram on a truck-mounted boom. An auger can be used to remove the HMAC and base layers and collect Shelby tube samples without introducing water in the process. After the HMAC and base layers are removed, the Shelby tube is positioned on the subgrade and pushed roughly 2 ft. (0.6 m) into the soil, as shown in Figure 4-24. The pipe is then pulled out and placed on a rack. A hydraulic ram then pushes the soil out of the pipe in (hopefully) one cylindrical piece. Aluminum foil and cardboard tubes are used to protect the Shelby tube samples, as shown in Figure 4-25. The tube is labeled with information including the location, orientation and depth of the sample.
If deeper samples are desired, the Shelby tube can be extended and pushed into the bottom of the same hole. That sample is then labeled as coming from a lower depth at the same location as the first. In this way, several samples can be collected at any location, then pieced together later for a complete soil profile.
Figure 4-24. Extracting subgrade samples with Shelby tube.
Figure 4-25. Aluminum foil cardboard tube to protect Shelby tube samples.