Screw Plate Tests

One potential problem with traditional plate tests is that in some soils, the depth of investigation may be limited, simply because an excavation may be difficult. A variation of the PLT, the SPLT, has been used to overcome this problem and evaluate engineering properties in a wide range of soils. The SPLT allows loading to be performed without an excavation or a borehole, although sometimes the test is performed below an open borehole. In this test, the plate is constructed of a single helical flight with a shallow pitch that is screwed into the ground. According to Strout &c Senneset (1998), the SPLT originated in Norway in 1953 where it is known as the “field compressometer test”.

Normally, the SPLT is controlled at the ground surface, and the load and displacement are measured just as in the conventional PLT. Schmertmann (1970b) presented a suggested method for the SPLT; however, it appears that this test was never adopted by ASTM as a standard. Table 9.2 gives a summary of different soils in which SPLTs have been reported.

SPLTs may be particularly suited to soils where undisturbed sampling is difficult. Tests in clays may require long testing times to obtain a drained behavior because of the slow rate of consolidation. Rapid loading tests may be performed in clays for evaluating undrained shear strength.

Equipment

Figure 9.7 shows a typical arrangement of the SPLT. The equipment usually consists of some types of surface reaction frame and a helical screw plate of one revolution of helix attached to a load rod. Usually, either a hydraulic jack or a mechanical screw jack is used to apply the load. The load is measured with a load cell or a proving ring and deformation is measured using an independent reference beam. The test setup is similar in many respects to the conventional PLT.

Table 9.2 Summary of some reported uses of screw plate tests

Figure 9.7 Schematic of typical screw plate load test.

Different diameter plates with different thickness and pitch have been used, generally depending on the anticipated stiffness of the material being tested. The thickness of the plate must be sufficient to provide for a rigid loading. Plate diameters range from 60 mm (2.36 in.) to 350mm (13.75in.), although screw plates on the order of 150mm (6in.) appear to be the most common. Table 9.3 summarizes some reported sizes of different screw plates used in different soils.

Kay & Avalle (1982) and Bergado et al. (1986) described test equipment that included a screw mechanism with the same pitch as the helical plate, as shown in Figure 9.8. Advance of the plate using this “pitch-matched” arrangement helps reduce disturbance of the soil.

The diameter of the plate must be large enough so that the test results accurately determine the soil behavior but small enough to be applicable over a wide range of soil conditions and still be installed with reasonable ease. Experience suggests that a plate with a projected horizontal area on the order of 0.093 m² (1 ft²) (diameter = 172 mm (0.56 ft)) will meet this criteria and provide a sufficient rigidity.

Test Procedures

SPLTs are usually conducted as incremental loading tests, where a constant load is applied and the deformation is recorded at various time intervals. The test can include one or more unload-reload cycles in order to more accurately estimate elastic modulus. The test may also be performed as a continuous loading test. Schmertmann (1970b) recommended that to avoid disturbance, the advance of the screw plate should be controlled such that the downward movement of the plate for each complete revolution should be equal to the pitch. It was also recommended that a vertical testing interval of at least three plate diameters be used.

Table 9.3 Different-sized screw plates used