Field CompactionCompaction EquipmentMost of the compaction in the field is done with rollers. The four most common types of rollers are 1. Smooth-wheel rollers (or smooth-drum rollers) 2. Pneumatic rubber-tired rollers 3. Sheepsfoot rollers 4. Vibratory rollers Smooth-wheel rollers (Figure 6.15) are suitable for proof rolling subgrades and for finishing operation of fills with sandy and clayey soils. These rollers provide 100% coverage under the wheels, with ground contact pressures as high as 310 to 380 kN/m2 (45 to 55 lb/in2). They are not suitable for producing high unit weights of compaction when used on thicker layers. Pneumatic rubber-tired rollers (Figure 6.16) are better in many respects than the smooth-wheel rollers. The former are heavily loaded with several rows of tires. These tires are closely spaced—four to six in a row. The contact pressure under the tires can range from 600 to 700 kN/m2 (85 to 100 lb/in2), and they produce about 70 to 80% coverage. Pneumatic rollers can be used for sandy and clayey soil compaction. Compaction is achieved by a combination of pressure and kneading action

Sheepsfoot rollers (Figure 6.17) are drums with a large number of projections. The area of each projection may range from 25 to 85 cm2 ( 4 to 13 in2). These rollers are most effective in compacting clayey soils. The contact pressure under the projections can range from 1400 to 7000 kN/m2 (200 to 1000 lb/in2). During compaction in the field, the initial passes compact the lower portion of a lift. Compaction at the top and middle of a lift is done at a later stage. Vibratory rollers are extremely efficient in compacting granular soils. Vibrators can be attached to smooth-wheel, pneumatic rubber-tired, or sheepsfoot rollers to provide vibratory effects to the soil. Figure 6.18 demonstrates the principles of vibratory rollers. The vibration is produced by rotating off-center weights. Handheld vibrating plates can be used for effective compaction of granular soils overa limited area. Vibrating plates are also gang-mounted on machines. These plates can beused in less restricted areas.

Factors Affecting Field CompactionIn addition to soil type and moisture content, other factors must be considered to achieve the desired unit weight of compaction in the field. These factors include the thickness of

lift, the intensity of pressure applied by the compacting equipment, and the area over which the pressure is applied. These factors are important because the

pressure applied at the surface decreases with depth, which results in a decrease in the degree of soil compaction. During compaction, the dry unit weight of soil also is affected by the number of roller passes. Figure 6.19 shows the growth curves for a silty clay soil. The dry unit weight of a soil at a given moisture content increases to a certain point with the number of roller passes. Beyond this point, it remains approximately constant. In most cases, about 10 to 15 roller passes yield the maximum dry unit weight economically attainable.

Figure 6.20a shows the variation in the unit weight of compaction with depth for a poorly graded dune sand for which compaction was achieved by a vibratory drum roller. Vibration was produced by mounting an eccentric weight on a single rotating shaft within the drum cylinder. The weight of the roller used for this compaction was 55.6 kN (12.5 kip), and the drum diameter was 1.19 m (47 in). The lifts were kept at 2.44 m (8 ft). Note that, at any given depth, the dry unit weight of compaction increases with the number of roller passes. However, the rate of increase in unit weight gradually decreases after about 15 passes. Another fact to note from Figure 6.20a is the variation of dry unit weight with depth for any given number of roller passes. The dry unit weight and hence the relative density, Dr, reach maximum values at a depth of about 0.5 m (1.5 ft) and gradually decrease at lesser depths. This decrease occurs because of the lack of confining pressure toward the surface. Once the relationship between depth and relative density (or dry unit weight) for a given soil with a given number of roller passes is determined, estimating the approximate thickness of each lift is easy. This procedure is shown in Figure 6.20b (D’Appolonia, Whitman, and D’Appolonia, 1969)

Specifications for Field CompactionIn most specifications for earthwork, the contractor is instructed to achieve a compactedfield dry unit weight of 90 to 95% of the maximum dry unit weight determined in the laboratory by either the standard or modified Proctor test. This is a specification for relativecompaction, which can be expressed as

where R relative compaction.For the compaction of granular soils, specifications sometimes are written in termsof the required relative density Dr or the required relative compaction. Relative densityshould not be confused with relative compaction. From Chapter 3, we can write

Comparing Eqs. (6.10) and (6.11), we see that

On the basis of observation of 47 soil samples, Lee and Singh (1971) devised a correlation between R and Dr for granular soils:

The specification for field compaction based on relative compaction or on relative density is an end product specification. The contractor is expected to achieve a minimum dry unit weight regardless of the field procedure adopted. The most economical compaction condition can be explained with the aid of Figure 6.21. The compaction curves A,B, and C are for the same soil with varying compactive effort. Let curve A represent the conditions of maximum compactive effort that can be obtained from the existing equipment. Let the contractor be required to achieve a minimum dry unit weight of gd(field) Rgd(max). To achieve this, the contractor must ensure that the moisture content w falls between w1 and w2. As can be seen from compaction curve C, the required gd(field) can be achieved with a lower compactive effort at a moisture content w w3. However, for most practical conditions, a compacted field unit weight of gd(field) Rgd(max) cannot be achieved by the minimum compactive effort. Hence, equipment with slightly more than the minimum compactive effort should be used. The compaction curve B represents this condition. Now we can see from Figure 6.21 that the most economical moisture content is between w 3 and w4. Note that w w4 is the optimum moisture content for curve A, which is for the maximum compactive effort.

The concept described in the preceding paragraph, along with Figure 6.21, is attributed historically to Seed (1964) and is elaborated on in more detail in Holtz and Kovacs (1981). Table 6.2 gives some of the requirements to achieve 95-to-100% relative compaction(based on standard Proctor maximum dry unit weight) by various field compaction equipment (U.S. Department of Navy, 1971).