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FIELD WING CUTTING EXPERIMENT

25/05/2021

The field shear test is intended to determine the undrained shear resistance of soils, used for soft cohesive soils, saturated with water. This test does not apply to soils with rapid drainage, swollen soils, soils with lots of rocks and shells. Here are some basic theories to better understand the nature of this experiment...

Applied standard

22TCN355-06: Field wing cutting test procedure

ASTM D2573: Standard Test Method for Filed Vane Shear Test in Cohensive soil

Torque and torsion/ Torque and torsion

Two equal and opposite forces acting in the same plane at a distance from each other.

The magnitude of each force is denoted by F , the distance between their lines of action is d and the generated Torque will be ( Fxd ). If F is in (N), d is in (mm), then Torque is expressed in (Nmm).

Two equal and opposite forces form a pair acting on one end of the shaft. The moment of forces is called “ torque - Torque ”, T. (1)

If the other end of the rod is resisted by a torque, the rod is said to be subjected to torsion under the torsion action of two opposing force pairs.

A torque that can be resisted by a uniformly distributed shear stress ( Shear stress ), s , per unit area (N/mm ² ) acting around the curved surface of a cylinder attached to a shaft rod. The area of the curved surface is πdh (mm ² ). Therefore, the total force will be equal to the circumference s .πdh (N) and provide a torque ( torque torque - Resisting torque ) T _r. (2)

Field wing cutting experiment

The above principle is used in the blade shear test to determine the shear strength of soft clays by measuring the torque applied until failure.

The device consists of a cross-shaped propeller of diameter D (mm) and height H (mm), which is attached to the lower end of a small diameter rod. The propeller rotates an earth cylinder, when a torsion force is applied to the rod. This is done via a torsion device at the upper end of the rod, which allows measurement of the applied torque.

As long as the applied torque is less than that required to shear the ground, it will be resisted by a torque equal to and vice versa provided by the shear resistance of the soil acting on the surface of the spindle. When the torque applied to the impeller is increased to a value just sufficient to rotate the earth cylinder, it is assumed that the maximum shear resistance, i.e. the shear strength of the soil, is achieved simultaneously. on all sliding surfaces.

The total torque, T _r , is made up of 2 components T ₁ and T ₂ , where the torque T ₁ is provided by the shear force on the surface around the cylinder and the torque T ₂ is provided by the shear force on each sides of the cylinder remaining.

T _r = T ₁ + 2T _{2 (3)}

Apply formula (2) to calculate the value of T ₁

T ₁ = D ² H s /2

The stresses acting on the other two faces of the cylinder. The stress is assumed to be uniformly distributed, so that the total shear force acting on each face is πD ²s /4 (N) (ignoring the diameter of the shaft). Swing arm length is not immediately obvious but usually through calculation, like the method below.

The circular base of the cylinder can be divided into many small arcs, each of which is approximately an isosceles triangle of height r, where r = D/2. If the stress acts uniformly over the whole area, the line of force acts through the center of gravity of the triangle, i.e. at a distance (2r/3) from the vertex. If all the triangles were put together, the total force would be at a distance of 2r/3 (i.e. D/3) from the center of the circle. Then T ₂ will be equal to:

Thus, the total torque:

If the shear strength of the soil ( vane shear strength ), τ _υ , is _expressed in units of kPa, s = τ _υ / 1000 (N/mm ² )

Let K = πD ² (H/2+D/6) be the blade coefficient - depending on the blade size , Torque will be equal to:

T _r = K.τ _υ

Application of soil shear parameters and field shear test

Many stability problems in soil are related to the sliding failure mechanism of a soil mass with the main soil mass. The sliding surface can be flat or curved, it is assumed that the soil along the entire sliding surface is in a state of failure, i.e. has reached the maximum shear strength of the soil.

In practice it is necessary to ensure that this situation will never happen. And in order to keep the deformation within allowable limits, a factor of safety is applied to ensure that the shear stress in the soil is not greater than a certain percentage of its maximum shear strength.

Below are some cases where it can be assumed that the water content of the soil remains unchanged and that the analysis of the undrained shear strength of the soil can be used to evaluate.

Foundation bearing capacity for structures on a saturated homogeneous clay base, immediately after construction.
Earth pressure on retaining wall, for conditions prevailing immediately after construction.
Soil pressure against temporary bracing in the excavation.
Soil pressure acting on the bottom of temporary excavation pits in clay.
Stabilize the slope immediately after excavation.

Stability of embankment or backfill during construction.

For long-term stability analysis such as retaining walls, backfills, earth dams, the shear resistance parameter will be used.

For long-term stability analysis of slopes in overconsolidated clays, it is based on residual shear strength parameters of the soil . (Skmpton, 1964; Skempton & La Rochelle, 1965; Symons, 1968).

The field shear test can determine the shear resistance of soft soils with very small values (below 20kPa) and with reasonable accuracy. In some cases it is necessary to know this value. For example, in the reinforcement of a soft ground by the backfill method, the shear strength will indicate the maximum safe pressure that the ground can maintain at its initial stage, and thus the embankment thickness can be designed. in the first stage. Consolidation then increases the shear strength and construction can proceed in stages based on shear strength criteria.

In addition, based on the shear resistance of the soil, it is possible to determine the negative friction force during the construction of piles, etc.

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