Principles

The “direct reading shear vane”, was an instrument originally designed to measure the shear strength of clays in trial pits and excavations for foundations over the whole range of strengths likely to be found on a building site. Soil scientists have applied these instruments to find out properties in the top layers of the soil profile.

The shear strength of a soil is determined when an increasing external force is applied to a soil, and a point is reached where the soil moves in the direction of the force. At the point of movement, the shear strength of the soil has been reached. A term commonly used in describing a soil that has its shear strength exceeded is ‘failure’. When the shearing, or failure has occurred the two forces holding the soil particles together have been overcome. Those properties are;

1.) cohesion force between the particles (N/m2 = Pa)

2.) angle of shearing resistance.

Source, civcal.media.hku.hk/

Calibrating the shear strength of a soil ‘c’, where the length of the vane is ‘h’, the diameter is ‘d’, and the torsional resistance is ‘T’.

Note symbol use: some authors prefer t for shear strength whilst others use ‘cu’, which stands for the undrained shear strength, hence the subscript ‘u’.

If ‘cuv’, is the undrained shear strength in the vertical direction, and ‘cuh’, is the undrained shear strength in the horizontal direction then:

For the vane you are using the size of the vane directly influences the amount of force required to determine the shear strength of the soil. If the vane width 'd', or the the vane height (length) 'h' are increased or decreased, the force required to reach soil failure will similarly increase or decrease.

The Coulomb equation is the accepted method of description for shear failure. Coulomb worked out that the the force required to alter a soil has to overcome two forces. Firstly the bond that exists between the soil particles across the site of failure, and secondly the frictional force between the two surfaces as a result of the movement.

Coulombs equation: F = C + N * tan. theta

In reality the 'N' value is neglible as it really describes vertical force downwards. There would probably be some force being exerted downwards due to human error, but what we are interested in is the application of force in the horizontal. The important part of the equation is the 'C', ie the force associated with the cohesion between the particles.

Agricultural, Environmental implications of measurement

The implications of a soils strength are immense when looked at from an agricultural or environmental perspective. The main concern is that there is an upper and lower limit of soil strength which is most suitable, if it lies outside this problems can occur. For example the soil strength needs to be high enough to hold a plants roots in the ground to keep the plant upright. But it should not be so high where it stops a plants roots, or organisms such as earthworms from being able to move through the soil.

Other important implications are allowing machines to be able to move across the soil surface. If the topsoil shears when experiencing a small amount of torque, machines such as tractors would be unable to travel as there wheels will simply spin. Earth structures such as dam walls must be able to withstand the pressure exerted by the water inside from pushing outwards. The soil must be able to support the foundations of buildings placed on it to avoid any movement from occurring.

Brief description of apparatus / method

The shear vane consists of a vane spindle attached to the bottom of a casing enclosing a steel helical spring. The inner end of the helical spring is attached to a central spindle which houses one end of the vane spindle. The other end of the helical spring is attached to the casing. The vane spindle is pushed into the ground, to the desired depth, usually 150mm. When pushing into the soil always keep sideways movement to a minimum.

Firmly holding the shear vane in one hand, rotate it clockwise at a speed equal to one revolution per minute. When the force exerted by the shear vane exceeds the shear strength of the soil it is registered on the dial on top of the casing.

Readings of shear strength were taken at 5 locations in a cultivated area, and in a pasture area for the same soil type.

Analysis of data

Using a direct reading shear vane the shear strength of the soil was measured. The readings for the pasture and cultivated areas were compared using an analysis of variance.

Means for Oneway Anova

Analysis of Variance

Results

Oneway Analysis of Shear Vane (Kpa) By Land use

There was a significant difference between the soil strength of the pasture and the cultivated area. The size of the mean values first brings this to our attention, and this is statistically backed up by the probability value of .0001. Being less than 5%, the means of the two areas being looked at, are statistically significantly different. Visually, when soil strength is plotted against landuse its also easy to see the shear strength of the two areas are different.

This result is one you would expect to find between these two areas. The area covered in pasture has had time for the pasture to become established, therefore has no recent history of soil disturbance. So from the time when the pasture was initially sown the soil has remained untouched and has been given time for a structure in the soil profile to reestablish itself. Compared to the cultivated area which had no plants growing on it, because it had recently been cultivated to keep it in a bare fallow state.

Once the soil of an area has been disturbed and a pasture is allowed to develop the structure and fertility of a soil does improve. This is also seen when incorporating a pasture component in a cropping cycle. This has a direct impact on the strength of a soil. In both cases the soil is not disturbed by cropping and cultivation which means the amount of disaggregation occurring has been significantly reduced which allows the levels of organic matter to increase. This effects the soil in many ways, but looking more precisely at soil strength, the increasing amounts of organic matter means the soil can hold more moisture which directly influences soil strength. The soil we are dealing with is a cracking clay where soil strength increases with decreasing moisture content.(Friend & Chan, 2001)

So the result we would expect is that the strength of a soil would be higher in a pasture, than in a cultivated area for the same soil. For a cracking clay the soil strength in both situations would increase as moisture content decreases. Our results confirm this and the results from the dynamic cone penetrometer further back this up.

Problems with method

The direct reading shear vane is probably one of the quickest ways of getting a value of shear strength for a soil on site. But its not the most accurate method available. Another method available is the 'Shear Box'. The shear box would give a more accurate reading as a sample of soil is used where the structure is still intact, and its examined under laboratory conditions. When using the shear vane you physically push it into the soil. In doing this you are altering the soil by pushing the soil in immediate contact with the shaft and vane closer together. This would only change the soil in a minor way, but it still could slightly change the strength in some way which could cause the final reading to deviate from its true value. Also the friction along the shaft itself is not taken into account. The shaft would experience a resistance to movement as you are rotating the shear vane. However small this resistance is, a force is required to overcome this which would cause the final reading to be higher than it actually is.

References

Campbell K.O., Bowyer J.W. (1988) The Scientific Basis of Modern Agriculture. Sydney University Press. Ch 10, pg186-88

Friend J.J.,Chan K.Y. (2001) A simple method for quantifying effects of management practices on cracking clays under dryland conditions. Australian Society of Agronomy

McBratney A.B.,Minqsny B.(2004) Laboratory and Field Soil Physics. (Notes and Exercises)

Serota S. ,Jangle A. (1972) A Direct Reading Pocket Shear Vane.