In situ tests

Penetration tests

The standard penetration test that was described in Chapter 8 is normally used for cohesionless soils, although Terzaghi and Peck (1948) give an approximate relationship for clays:

For square footings, qa = 16N (kN/m2) For continuous footings, qa = 12N (kN/m2)

where qa = safe bearing capacity (F = 3) N = uncorrected test number of blows.

The Dutch cone test, also described in Chapter 8, can be used for both cohesive and cohesionless soils.

Plate loading test

Loading tests are more applicable to cohesionless soils than cohesive soils due to the time necessary for the latter to reach full consolidation. Generally two tests are carried out as a check on each other, diiferent sized plates of the same shape being used in granular soils so that the settlement of the proposed foundation can be evolved from the relationship between the two plates. The loading is applied in increments (usually one-fifth of the proposed bearing pressure) and is increased up to two or three times the proposed loading. Additional increments should only be added when there has been no detectable settlement in the preceding 24 hours. Measurements are usually taken to 0.01 mm, and where there is no definite failure point the ultimate bearing capacity is assumed to be the pressure causing a settlement equal to 20 per cent of the plate width.

Vane test

In soft sensitive clays it is difficult to obtain samples that have only a slight degree of disturbance and in situ shear tests are usually carried out by means of the vane test (Fig. 14.3). The apparatus consists of a 75 mm diameter vane, with four small blades 150 mm long. For stiff soils a smaller vane, 50 mm diameter and 100 mm high may be used. The vanes are pushed into the clay a distance of not less than three times the diameter of the borehole ahead of the boring to eliminate disturbance effects, and the undrained strength of the clay is obtained from the relationship with the torque necessary to turn the vane. The rate of turning the rods, throughout the test, is kept within the range 6-12° per minute. After the soil has sheared, its remoulded strength can be determined by noting the minimum torque when the vane is rotated rapidly.

Figure 14.3 indicates that the torque head is mounted at the top of the rods. This is standard practice for most site investigation work but, for deep bores, it is now possible to use apparatus in which the torque motor is mounted down near to the vane, in order to remove the whip in the rods.

Soil Test Mechanics

Because of this development the vane has largely superseded the standard penetration test, for deep testing. The latter test has the disadvantage that the load must always be applied at the top of the rods so that some of the energy from a blow must be dissipated in them. This energy loss becomes more significant the deeper the bore, so that the test results become more suspect.

The actual stress distribution generated by a cylinder of soil being rotated by the blades of a vane which has been either jacked or hammered into it is a matter for conjecture. BS 1377 has adopted the simplifying assumption that the soil's resistance to shear is equivalent to a uniform shear stress, equal to the undrained strength of the soil, cu, acting on both the perimeter and the ends of the cylinder (see Fig. 14.3).

For equilibrium the applied torque, T, = moment of resistance of vane blades. The torque due to the ends can be obtained by considering an elemental annulus and integrating over the whole area:

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Responses

  • anne
    WHAT IS THE RELATIONSHIP BETWEEN VANE SHEAR TEST RESULT AND BEARING CAPACITY?
    6 years ago
  • faruz
    What is a typical vane test apparatus?
    3 years ago

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