A variety of mechanical equipment is used to compact soils in the fi eld. Compaction is accomplished by static and vibratory vertical forces. Static vertical forces are applied by deadweights that impart pressure and/or kneading action to the soil mass. Sheepsfoot rollers (Figure ), grid rollers, rubber-tired rollers, drum rollers (Figure loaders, and scrapers are examples of equipment that apply static vertical forces. Vibratory vertical forces are applied by engine-driven systems with rotating eccentric weights or spring/piston mechanisms that impart a rapid sequence of blows to the soil surface. The soil is compacted by pressure and rearranging of the soil structure by either impact or vibration. Common types of vibrating equipment are vibrating plate compactors, vibrating rollers, and vibrating sheepsfoot rollers. Vibrating sheepsfoot and impact rammers are impact compactors.
The soil mass is compacted in layers called lifts. The lift thickness rarely exceeds 300 mm. Coarsegrained soils are compacted in lifts between 250 mm and 300 mm while fi ne-grained soils are compacted in lifts ranging between 100 mm and 150 mm. The stresses imparted by compactors, especially static compactors, decrease with lift depth. Consequently, the top part of the lift is subjected to greater stresses than the bottom and attains a higher degree of compaction. Lower lift thickness is then preferable for uniform compaction.
A comparison of various types of field compactors and the type of soils they are suitable for is shown in Table Generally, it is preferable to specify the amount of compaction desired based on the relevant Proctor test and let the contractor select the appropriate equipment. You will have to ensure that the contractor has the necessary experience. What’s next . . . When you specify the amount of compaction desired for a project, you need to ensure that the specifi cations are met. In next section, three popular apparatuses for compaction quality control tests are discussed.
COMPACTION QUALITY CONTROL:
A geotechnical engineer needs to check that fi eld compaction meets specifi cations. A measure of the degree of compaction (DC) is the ratio of the measured dry unit weight achieved to the desired dry unit weight.
Various types of equipment are available to check the amount of compaction achieved in the fi eld. Three popular pieces of equipment are (1) the sand cone, (2) the balloon, and (3) nuclear density meters.
Sand Cone—ASTM D 1556:
A sand cone apparatus is shown in Figure 5.8. It consists of a glass or plastic jar with a funnel attached to the neck of the jar.
The procedure for a sand cone test is as follows:
1. Fill the jar with a standard sand—a sand with known density—and determine the weight of the sand cone apparatus with the jar fi lled with sand (W1). The American Society for Testing and Materials (ASTM) recommends Ottawa sand as the standard.
2. Determine the weight of sand to fi ll the cone (W2).
3. Excavate a small hole in the soil and determine the weight of the excavated soil (W3).
4. Determine the water content of the excavated soil (w).
5. Fill the hole with the standard sand by inverting the sand cone apparatus over the hole and opening the valve.
6. Determine the weight of the sand cone apparatus with the remaining sand in the jar (W4).
7. Calculate the unit weight of the soil as follows: