Plate Load Test (PLT) and Screw Plate Load Test (SPLT)

Introduction

Plate Load Tests (PLTs) are one of the simplest of all in situ tests to perform and to interpret, and yet they are perhaps one of the most underutilized tests by the profession. Historically, a rigid circular plate has been used near the surface to apply a vertical load to the soil, as shown in Figure 9.1. Prior to the development of “modern” techniques for investigating soils and determining soil properties, PLTs were used to make a direct assessment of the load-bearing characteristics of soils for shallow foundations. Reports of load tests are available beginning at the turn of the century and continuing up to the present day (e.g., ENR 1922, 1932).

Concept of plate load test (PLT)

Figure 9.1 Concept of plate load test (PLT).

PLTs are considered both a specific property measurement test and a prototype test in which the load vs. deformation performance of a rigid plate is determined for use in the design of shallow foundations or end bearing of deep foundations. Screw plate load tests (SPLTs) have generally been used for specific property measurement because of their smaller size.

One advantage of PLTs is the ability to evaluate the mass soil behavior since a large volume of soil is involved in the plate response. Although PLTs may be used in virtually any soil type, the test is particularly useful for evaluating the behavior of structurally dependent soils and soils that may not allow undisturbed sampling or where scale effects may significantly influence the interpretation of strength and stiffness.

Plate Load Test

The PLT is highly versatile; the plates may be either round or square; they may vary in dimension to suit the soil and the project; they may be constructed of steel or concrete. Tests may be conducted at the ground surface or below grade. Typical sizes of plates for most routine testing range from 0.15 m (6 in.) to 0.91m (36 in.). The equipment and procedure for conducting the PLT is described by ASTM Method Dll94 Standard Test Method for Bearing Capacity of Soil for Static Load and Spread Footings. Table 9.1 gives a summary of some reported uses of PLTs in different soil materials.

Equipment

The equipment used to conduct a PLT is simple, and consists of a rigid loading plate, a reaction system, a loading jack (usually hydraulic), a load cell, and a reference beam and settlement measuring system. Steel plates may be used provided that they are sufficiently stiff to represent a rigid loading. ASTM D1194 requires that steel plates must be at least 25.4mm thick. Rigid loading may be accomplished by using a series of stacked plates or by using plates with stiffening ribs, as shown in Figure 9.2.

Reaction can be provided by a number of different systems, as shown in Figure 9.3. In most cases, the reaction is provided by dead load (Figure 9.3a); anchor piles or grouted anchors (Figure 9.3b); and helical anchors (Figure 9.3c). In the case of large plate tests, in which a prototype-scale or full-scale concrete footing is used, the reaction may be provided by a central internal anchor, as shown in Figure 9.3d.

The applied load is measured using an independent calibrated load cell. The pressure being applied to the hydraulic jack may also be monitored using a pressure gauge; however, this should be considered a backup measurement of load and not the primary measurement of load. Settlement of the plate is usually measured by analog or digital dial gauges or by LVDTs. A reference beam or some other form of independent reference is needed to measure the plate deformation during the test. The use of electronic load and settlement equipment also allows for an automated data acquisition; however, the test can be kept very simple.

Test Procedures

The procedure used to conduct the PLTs is relatively simple; prepare the soil surface so that the plate rests uniformly on the soil; arrange a loading system to provide a sufficient reaction; and provide a means for measuring plate deformation. Normally, the test is conducted

Table 9.1 Summary of some reported uses of plate load tests in different materials

Soil type

References

Residual soils

Chin (1983)

Filho & Celso (1983) Ferreir & Teixeira (1989) Consoli et al. (1998)

Gravels

Wrench (1984)

Wrench & Nowatzki (1986)

Sand

Ismael (1987)

Ismael (1996) Kesharwani et al. (2015)

Dense glacial till

Klohn (1965)

Soderman et al. (1968) Radhakrishna & Klym (1974) McKinlay et al. (1974)

Fisher (1983)

Soft clay

Housel (1929)

Bergado & Chang (1986)

Stiff clay

Burland et al. (1966)

Ertel (1967)

Lo et al. (1969)

Marsland 1971)

Marsland (1977)

Bauer et al. (1973)

Marsland & Randolph (1977) Tand et al. (1986)

Lefebvre et al. (1987)

Bauer & Tanaka (1988) Marsland & Powell (1991)

Peat

Landva (1986) Nichols et al. (1989)

Waste fill

Eliassen (1942)

Landva & Clark (1990) Watts & Charles (1990) Van Impe (1998)

Bedrock

Ward et al. (1968)

Seychuk (1970)

Rozsypal (1983)

Marsland & Butcher (1983) Lo& Cooke (1989)

in a load-controlled manner with loads being applied incrementally and the deformation monitored over time under each load increment. ASTM D1194 suggests that equal load increments of no more than 95kPa (1.0 tsf) should be used or alternatively, loads of not more than 10% of the estimated ultimate bearing capacity may be used. The load is to be maintained for a minimum of 15 min.

ASTM recommends that the test is typically continued until “a peak load is reached or until the ratio of load increment to settlement increment reaches a minimum, steady magnitude”. Unless a distinct, well-defined failure occurs, the test should be conducted to a point where a minimum settlement of 10% of the plate diameter or width has occurred.

Typical geometry of steel plates for PLT

Figure 9.2 Typical geometry of steel plates for PLT.

Different arrangements for load reaction for PLT

Figure 9.3 Different arrangements for load reaction for PLT.

An alternative deformation-controlled loading procedure is also allowed by ASTM as follows: apply the load increments corresponding to approximately 0.5% of the plate diameter; after applying each settlement increment, measure the load at some fixed time intervals, e.g., 30s, 1, 2, 4, 8, 15min, until the variation of the load stops or until the rate of the variation of the load on a load vs. log-time scale becomes linear.

Tests on the Ground Surface

Tests performed on the ground surface are the easiest to arrange since all of the equipment is readily accessible. Most surface tests are used to evaluate the deformation characteristics of fills and for the support of structural slabs. In some cases, the dead weight reaction may be supplied by heavy equipment, such as a fully loaded dump truck, provided that the vehicle does not interfere with the plate and influence results.

Tests in an Excavation/Test Pit

Occasionally, PLTs may be performed in excavations, as shown in Figure 9.4b. ASTM D 1194 states that “the distance between test locations shall not be less than five times the diameter of the largest plate used in the tests”. In general, the test equipment and procedure is the same as used for surface tests. The primary purpose of performing tests in an excavation is to be able to evaluate the soil behavior in specific strata or at specific elevations that may correspond to the anticipated foundation locations.

Tests in Lined Borings

A special type of PLT may be performed in lined borings, and used to determine soil properties at various depths, as shown in Figure 9.4c. Marsland (1975) described the deep large-diameter plate tests performed using special test equipment, shown in Figure 9.5. A large-diameter steel casing would typically be used as the liner. Using this equipment, tests may be performed at any depth as the drilling proceeds to evaluate specific soil behavior in different strata.

PLTs performed (a) on ground surface; (b) in excavations, or (c) in lined borings

Figure 9.4 PLTs performed (a) on ground surface; (b) in excavations, or (c) in lined borings.

Equipment for PLTs in deep boreholes. (After Marsland 1971.)

Figure 9.5 Equipment for PLTs in deep boreholes. (After Marsland 1971.)

Horizontal Plate Load Tests

In some cases, it may be desirable to conduct PLTs in the horizontal direction, either in an open trench, along the walls of a bored hole, or in tunnels to obtain a measure of soil deformation characteristics in the horizontal direction, as shown in Figure 9.6. In this case, the test is performed by jacking against opposite sides of the excavation walls. The use of lateral PLTs to evaluate lateral soil stiffness has been reported by a number of investigators

(e.g., Slack & Walker 1970; Sherif & Strazer 1973; Radhakrishna & Klym 1974; Reddy et al. 1979; Wrench & Nowatzki 1986).

 
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