The RAP System
Testing soil grain sizes? That’s a RAP!
In 2005, 15 years after the first WILDCAT DCP was manufactured and sold, the RAP (Reusable Adhesive Point) system for the WILDCAT was introduced in order to add the ability to test for soil grain sizes. Based on John Schmertmann’s development of friction measurements in static cone penetration, the RAP system allows the user to measure torque resistance of the 4” long surface of the RAP bracket and point, and to correlate resistance, thus determining soil grain size and strength at 10 cm. increments to 26 ft.
The RAP system consists of a reusable RAP bracket, a small RAP point which is “lost” in testing (tips are sold in boxes of 32), a lead test rod that has been modified to allow for slurry flow (1/8” holes or ports drilled in them near the male end), a torque wrench with hex key, and RAP data logging and calculating software.
Testing begins by driving the RAP bracket and tip, attached to the lead testing rod, 30 cm. into the soil so that the RAP assembly stands freely without overturning. Then, the torque wrench, fitted with a 5/8” hex key, is inserted through the hole in the hammer top, locking into the 5/8” cap screw socket which attaches the anvil to the testing rods. A turn of that beam type torque wrench then measures the foot pounds of sustainable adhesion on the RAP bracket and cone assembly.
Blows per 10 cm are recorded as usual, but every 20 or 30 cm, driving is halted and additional torque measurements are taken. These measurements are recorded as well.
The measured adhesion, along with the blows per 10 cm, allow determination of the soils’ grain size, because for any relative density or consistency, granular souls have less adhesion that silts, silts have less adhesion than clays, and clays have less adhesion than organics.
Logging of grain size, consistency, and SPT N-values from the RAP system is complicated by, as well as made possible by, the longer cone mantle. Calculated dynamic cone resistances presume that the entire dynamic force reaction is provided by the bearing resistance of the soil. With the RAP, part of the resistance to penetration comes from the adhesion of the longer mantle, which is greater for clays than sands. Our logging software recognizes the adhesion’s effect on calculated dynamic cone resistances and adjusts the logged dynamic cone resistances appropriately.
Scores of RAP tests, done near known soil grain size soils, have proven the validity of RAP testing for grain sizes as well as N-values.
So, does that mean that all WILDCAT testing should be done using the RAP system? We believe not. The longer sleeve of the RAP bracket and cone combination causes the increased adhesion of clayey soils to need considerably more hammer blows per 10 cm. when testing with RAP. Despite our logging software correcting for this, RAP testing for N’ values may give less accurate equivalent SPT data.
Ultimately, we believe that the WILDCAT DCP testing should be used to determine the most accurate SPT N-values, and RAP testing should be used to determine soil grain size and to ESTIMATE SPT N-values.
WILDCAT RAP System consists of:
RAP bracket (P310/09RAP)
32 RAP lost points (P310/10RAP)
Torque wrench with hex key (P102/04)
RAP Wildcat Adhesion Cone logging software (P102/02RAP)