Impact Soil Tester  clegg


Michael Thomas Letter


NEWSLETTER 3                                 SEPTEMBER 1986


 It has become evident that the Clegg Impact Soil Tester is being used in a variety of ways for compaction control.

Initially its application was restricted to pavement materials such as fine crushed rocks, gravels and soft rocks but this has been extended  to control of earthworks in general.

The main reasons given for its preference over conventional control by density testing are usually related to speed of testing, cost of testing and testing locations, e.g. remote area situations such as mining construction; trench reinstatement; small job such as parking areas. The main concern and deterrent to its more general use is associated with interpretation in relation to current specifications based on relative compaction using the well known Proctor test or derivatives of this.

The commonest procedure is to establish a minimum strength requirement in terms of Clegg Impact Value (CIV) for the particular moisture conditions, e.g. optimum moisture content (OMC) for modified Proctor. It is essentially a single value acceptance/rejection criterion. This target value is generally established for a particular material on the basis that the resulting decision to accept or reject is the same as if commonly accepted procedures had been used for the particular situation.

A more sophisticated approach establishes regression equations for CIV in relation to density and moisture content and uses either judgement or test to determine the appropriate value for moisture content. This may be by field trials or from laboratory tests in a CBR mould.

There also appears to be interest in adding an in situ strength requirement, such as CIV, to the compaction objectives and specification requirements.

Because the Clegg Hammer is becoming widely applied to compaction control there is a need for discussion on the fundamentals of compaction as a lead up to answering the commonly asked question - "how does it (the CIV) relate to Proctor?"

The attached note, BASICS OF COMPACTION CONTROL, outlines some of the basic concepts relating to compaction control on a strength basis and forms a convenient starting point.


It should be recognised from the outset that the logical objective of compaction is not in fact an arbitrarily selected density. The primary objective is the attainment of a certain minimum strength or compressibility (and sometimes permeability).

Before the Proctor concept was introduced compaction was rather a haphazard process with no moisture control. In the early 1930s Proctor introduced the concept of an optimum moisture content (OMC) with its corresponding maximum dry density (MDD). These conditions could be determined for each soil type in relation to the compaction effort. The well known standard Proctor test was developed to correspond to what was known to be suitable for earthworks and later the modified AASHO test was developed to correspond to the use of heavier vehicles. By compacting at or near the OMC more efficient use of rollers was achieved. It also became possible to check the effectiveness of the compaction by relating field density to the MDD achieved in the laboratory test for the chosen compactive effort.

The Proctor tests recognised two basic levels of strength - one resulting from 'light' compaction and one from 'heavy' compaction but did not define these in terms of specific strength parameters. For what may be described as practical reasons the control of compaction proceeded along the lines of relative compaction, i.e. by the use of the ratio of field dry density to laboratory maximum dry density. One of the main practical advantages was that field dry density as a soil property was independent of moisture content (although moisture content was crucial to the level achieved by the compaction process). On the other hand strength was dependent for a given soil on both density (degree of packing) and moisture content (pore water pressures). As a consequence compaction technology and specifications have evolved largely around relative compaction and associated field density testing.

The use of direct strength or stiffness measurement for control introduces the complication that these properties are very much dependent on both moisture content and density. Further the former not only influences the final strength but also plays a considerable part in the actual compaction process. However it is not the gravimetric moisture content value as such that is of concern but rather the form that the water is in. It may have effect as either capillary water with consequent apparent cohesion or as free water with the possibility of positive pore air-water pressures. Some compaction theories consider it is the change from negative to positive pore pressure that results in the loss of density after optimum moisture content. Also while a dry soil may exhibit adequate strength this may be reduced to an unacceptable degree by an increase in moisture content if this is not compensated for by strength resulting from density.

Conversely a wet near saturated soil may be in a critical low stability state due to positive pore pressure, even though compacted to relatively high density.

During the Proctor and similar impact compaction tests it may be observed that the sound of the impacts is reflecting the changes in stiffness in the soil as it is being compacted. By fitting an accelerometer to the hammer as in the case of the Clegg Hammer the response can be translated into a strength or stiffness parameter via the peak deceleration. Using this as the strength parameter the onset of strength loss associated with approaching MDD may be located on the moisture scale and the peak value may be determined. At this point the pore pressure is about zero so that the impact value represents mainly the material's strength due to packing of the particles, i.e. due to its density.

Specifications for compaction control generally take the form of either method or end result, e.g. by describing the roller size, number of roller passes, thickness of layer or alternatively by simply requiring a certain minimum percent relative compaction. The use of method specifications requires also the control of moisture content - it needs to be at or near the optimum. The use of end result requires moisture control for efficient achievement of density but the moisture content can be any value for the actual density determination.

The use of a strength measurement for compaction control has in the past been by means of various types of penetrometers, bearing tests and falling weight devices. More recently wheel load deflection measurements by the Benkelman Beam have been added to proof rolling procedures. Also devices have been fitted to rollers to monitor the ground stiffness. With all of these methods the difficulty lies in the selection of appropriate values to be achieved be it in terms of compaction effort, relative compaction, penetration resistance, deflection, etc. However it is evident that an in situ strength measurement in some form for compaction control is desirable and is being sought after.

The Clegg Hammer offers a practical and direct link between laboratory and field compaction. In its simplest application the selection of the target strength in terms of CIV can be made for the selected compaction effort applied to the particular soil at the moisture condition of no pore pressure, positive or negative, i.e. any wetter and reduction in strength results. It is important that this be for the field compaction actually used. If the laboratory determined target strength cannot be achieved, the field effort is inadequate. If much higher, the effort is being wasted - again it must be emphasised that the moisture condition must be as wet as possible, i.e. for the no pressure condition otherwise high values may give a false impression of adequate density. If conditions are such that testing must be performed at moisture contents lower than the critical value then the actual moisture content needs to be determined by laboratory test or by judgement. This enables the density to be determined by inference from regression equations using CIV, density and moisture content. On the other hand if testing above the critical point the lower CIVs may cause satisfactory work (density wise) to be rejected.

The basic concepts outlined above should be seen as broad generalisations. The response indicated may be expected to vary in degree from soil to soil. However it may be seen that the Clegg Impact Test approach is a logical extension of the Proctor system of compaction control, adding the factor of strength to the design, construction and testing processes. For typical actual data see Figure 3 and 4.







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