Standard Test Methods for Determination of the Impact Value (IV) of a Soil

Importancia y uso:

5.1 Impact Value, as determined using the standard 4.5 kg (10 lbm) hammer, has direct application to design and construction of pavements and a general application to earthworks compaction control and evaluation of strength characteristics of a wide range of materials, such as soils, soil aggregates and stabilized soil. Impact Value is one of the properties used to evaluate the strength of a layer of soil up to about 150 mm (6 in.) in thickness using a 50 mm (2 in.) diameter hammer or up to 380 mm (15 in.) in thickness using a 130 mm (5 in.) diameter hammer, and by inference to indicate the compaction condition of this layer. Impact Value reflects and responds to changes in physical characteristics that influence strength. It is a dynamic force-penetration property and may be used to set a strength parameter.

5.2 This test method provides immediate results in terms of IV and may be used for the process control of pavement or earthfill activities where the avoidance of delays is important and where there is a need to determine variability when statistically based quality assurance procedures are being used.

5.3 This test method does not provide results directly as a percentage of compaction but rather as a strength index value from which compaction may be inferred for the particular moisture conditions. From observations, strength either remains constant along the dry side of the compaction curve or else reaches a peak and, for both cases, declines rapidly with increase in water content beyond a point slightly dry of optimum water content, at approximately 0.5 percent. This is generally between 95 and 98 % maximum dry density (see Fig. 1 and Fig. 2). An as-compacted target strength in terms of IV may be designated from laboratory testing or field trials as a strength to achieve in the field as the result of a compaction process for a desired density and water content. If testing is performed after compaction when conditions are such that the water content has changed from the critical value, determination of the actual water content by laboratory testing enables the field density to be inferred from regression equations using IV, density and water content.

FIG. 1 Illustration of Target IV for Material with No Peak

FIG. 2 Illustration of Target IV for Material with Pronounced Peak

Note 2: Impact Value may be used as a means to improve the compaction process by giving instant feedback on roller efficiency, uniformity, confirming the achievement of the target strength, and by inference the achieved density. When inferring density solely from IV, however, it is considered as only indicative of density.

5.4 This test method may be used to monitor strength changes during a compaction process or over time due to seasonal, environmental or traffic changes.

Note 3: For in-place soil strength evaluation where there may be a dry and hard surface layer (crust), testing both the crust and the underlying layer may be required.

5.5 The standard instrument is based on a 4.5 kg (10 lbm) compaction hammer using a 450 mm (18 in.) drop height. The hammer is equipped with an accelerometer and instrumented using a peak hold electronic circuit to read the peak deceleration on impact. The circuitry is filtered electronically to remove unwanted frequencies and the peak deceleration is displayed in units of ten gravities (g) with the output below units of ten gravities truncated.

5.6 The peak deceleration on which IV is derived represents the area under the deceleration versus time curve which for most soils may be assumed as half a sinusoid. Applying double integration provides first the time-velocity relationship and second the time-penetration relationship. As force is also directly related to deceleration, the IV therefore represents both stress and penetration and may be taken as a direct measurement of stiffness or strength (see Fig. 3).

FIG. 3 Development of Force-Penetration from Deceleration Versus Time

5.7 Impact Value may be correlated with an unsoaked CBR.

5.8 Impact Value may be expressed as a hammer modulus, analogous with elastic modulus or deformation modulus.

5.9 The lighter hammers use the same accelerometer and instrumentation as the standard hammer. Utilization of lighter masses at 0.5 kg (1.1 lbm) and 2.25 kg (5 lbm) results in more sensitivity for lower strength materials compared to the standard mass; that is, the scale is expanded with these lighter hammer masses and provides more definition on softer materials, along with there being less indentation into the material. To avoid confusion, the IV of the lighter hammers is notated as IV/L for the 0.5 kg (1.1 lbm) mass and as IV/M for the 2.25 kg (5 lbm) mass.

5.10 The medium hammer provides a sensitivity between that of the standard hammer and light hammer.

5.11 Light Impact Value and Medium Impact Value have application to testing of sand, peat and for natural and artificial recreation turf hardness evaluation, where it is that the hardness of recreation turf surfaces affects ball bounce characteristics and the performance or injury potential to participants. Medium Impact Value is preferable over Light Impact Value in relation to assessing natural turf where there is thicker thatch and longer grass whereas Light Impact Value is preferable for finely mown grass surfaces where less indentation than that of the medium hammer is desired, such as testing of grass tennis courts and golf putting greens.

5.12 The medium hammer has application to testing of earthworks materials.

5.13 The heavy medium heavy hammer uses the same accelerometer and instrumentation as the standard hammer and tests through a larger zone both horizontally and vertically than the lighter impact soil testers because of its larger diameter mass. The IV of the heavy medium heavy hammer is notated as IV/HMH.

5.14 The Heavy Medium Heavy Impact Value has application to testing the same materials as those tested by the standard, light, medium and heavy impact soil testers.

5.15 The heavy hammer uses the same accelerometer and instrumentation as the standard hammer and tests through a larger zone both horizontally and vertically as compared to the 50 mm (2 in.) diameter impact soil testers. The IV of the heavy hammer is notated as IV/H.

5.16 The Heavy Impact Value has application to testing the same materials as those tested by the standard, light, medium, and heavy medium heavy impact soil testers but the greater mass of this impact soil tester provides less sensitivity of the output so is applicable for harder materials at the top end the scales or beyond the ranges of the lighter impact soil testers.

Note 4: The quality of the results produced by this test method is dependent on the competence of the personnel performing it and the suitability of the equipment and facilities used. Agencies that meet the criteria of Practice D3740 are generally considered capable of competent and objective testing. Users of this test method are cautioned that compliance with Practice D3740 does not in itself ensure reliable results. Reliable results depend on many factors; Practice D3740 provides a means of evaluating some of those factors.

Subcomité:

D18.08

Referida por:

F1702-10R18, F0355-23

Volúmen:

04.08

Número ICS:

13.080.20 (Physical properties of soil)

Palabras clave:

acceptance test; clegg impact value; compaction control; heavy impact value; heavy medium heavy impact value; impact soil test; impact test; impact test hammer; impact value; light impact value; medium impact value; soil test; stabilization testing; strength test; trench testing; turf testing;