FM 5-410 CHAPTER 8 Soil Compaction Soil compaction is one of the most critical such as embankments, subgrades, and bases components in the construction of roads, air- for road and airfield pavements. No other fields, embankments, and foundations. The construction process that is applied to natural durability and stability of a structure are re- soils produces so marked a change in their lated to the achievement of proper soil physical properties at so low a cost as compac- compaction. Structural failure of roads and tion (when it is properly controlled to produce airfields and the damage caused by founda- the desired results). Principal soil properties tion settlement can often be traced back to the affected by compaction include— failure to achieve proper soil compaction. Settlement. Shearing resistance. Compaction is the process of mechanically Movement of water. densifying a soil. Densification is ac- Volume change. complished by pressing the soil particles together into a close state of contact with air Compaction does not improve the desirable being expelled from the soil mass in the properties of all soils to the same degree. In process. Compaction, as used here, implies certain cases, the engineer must carefully dynamic compaction or densification by the consider the effect of compaction on these application of moving loads to the soil mass. properties. For example, with certain soils This is in contrast to the consolidation process the desire to hold volume change to a mini- for fine-grained soil in which the soil is mum may be more important than just an gradually made more dense as a result of the increase in shearing resistance. application of a static load. With relation to compaction, the density of a soil is normally SETTLEMENT expressed in terms of dry density or dry unit A principal advantage resulting from the weight. The common unit of measurement is compaction of soils used in embankments is pcf. Occasionally, the wet density or wet unit that it reduces settlement that might be weight is used. caused by consolidation of the soil within the body of the embankment. This is true be- Section I. Soil Properties cause compaction and consolidation both Affected by Compaction bring about a closer arrangement of soil par- ticles. ADVANTAGES OF SOIL COMPACTION Densification by compaction prevents later Certain advantages resulting from soil consolidation and settlement of an embank- compaction have made it a standard proce- ment. This does not necessarily mean that dure in the construction of earth structures, the embankment will be free of settlement; its Soil Compaction 8-1 FM 5-410 weight may cause consolidation of compres- corresponding to a minimum swell and mini- sible soil layers that form the embankment mum shrinkage may not be exactly the same, foundation. soils in which volume change is a factor generally may be compacted so that these ef- SHEARING RESISTANCE fects are minimized. The effect of swelling on Increasing density by compaction usually bearing capacity is important and is increases shearing resistance. This effect is evaluated by the standard method used by highly desirable in that it may allow the use of the US Army Corps of Engineers in preparing a thinner pavement structure over a com- samples for the CBR test. pacted subgrade or the use of steeper side slopes for an embankment than would other- Section II. Design wise be possible. For the same density, the Considerations highest strengths are frequently obtained by using greater compactive efforts with water contents somewhat below OMC. Large-scale MOISTURE-DENSITY RELATIONSHIPS experiments have indicated that the uncon- Nearly all soils exhibit a similar relation- fined compressive strength of a clayey sand ship between moisture content and dry could be doubled by compaction, within the density when subjected to a given compactive range of practical field compaction proce- effort (see Figure 8-1). For each soil, a maxi- dures. mum dry density develops at an OMC for the compactive effort used. The OMC at which maximum density is obtained is the moisture MOVEMENT OF WATER content at which the soil becomes sufficiently When soil particles are forced together by workable under a given compactive effort to compaction, both the number of voids con- cause the soil particles to become so closely tained in the soil mass and the size of the packed that most of the air is expelled. For individual void spaces are reduced. This most soils (except cohesionless sands), when change in voids has an obvious effect on the the moisture content is less than optimum, movement of water through the soil. One ef- the soil is more difficult to compact. Beyond fect is to reduce the permeability, thus optimum, most soils are not as dense under a reducing the seepage of water. Similarly, if given effort because the water interferes with the compaction is accomplished with proper the close packing of the soil particles. Beyond moisture control, the movement of capillary optimum and for the stated conditions, the air water is minimized. This reduces the ten- content of most soils remains essentially the dency for the soil to take up water and suffer same, even though the moisture content is in- later reductions in shearing resistance. creased. The moisture-density relationship shown VOLUME CHANGE in Figure 8-1 is indicative of the workability of Change in volume (shrinkage and swelling) the soil over a range of water contents for the is an important soil property, which is critical compactive effort used. The relationship is when soils are used as subgrades for roads valid for laboratory and field compaction. and airfield pavements. Volume change is The maximum dry density is frequently generally not a great concern in relation to visualized as corresponding to 100 percent compaction except for clay soils where com- compaction for the given soil under the given paction does have a marked influence. For compactive effort. these soils, the greater the density, the greater the potential volume change due to The curve on Figure 8-1 is valid only for one swelling, unless the soil is restrained. An ex- compactive effort, as established in the pansive clay soil should be compacted at a laboratory. The standardized laboratory moisture content at which swelling will not compactive effort is the compactive effort exceed 3 percent. Although the conditions (CE) 55 compaction procedure, which has Soil Compaction 8-2 FM 5-410 been adopted by the US Army Corp of En- air contained in the voids of the soil by com- gineers. Detailed procedures for performing paction alone is not possible. Typically, at the CE 55 compaction test are given in TM moisture contents beyond optimum for any 5-530. The maximum dry density (ydmax) at compactive effort, the actual compaction the 100 percent compaction mark is usually curve closely parallels the zero air-voids termed the CE 55 maximum dry density, and curve. Any values of the dry density curve the corresponding moisture content is the op- that plot to the right of the zero air-voids timum moisture content. Table 8-1, page 8-4, curve are in error. The specific calculation shows the relationship between the US Army necessary to plot the zero air-voids curve are Corps of Engineers compaction tests and in TM 5-530. their civilian counterparts. Many times the names of these tests are used interchange- Compaction Characteristics ably in publications. of Various Soils The nature of a soil itself has a great effect Figure 8-1 shows the zero air-voids curve on its response to a given compactive effort! for the soil involved. This curve is obtained by Soils that are extremely light in weight, such plotting the dry densities corresponding to as diatomaceous earths and some volcanic complete saturation at different moisture soils, may have maximum densities under a contents. The zero air-voids curve represents given compactive effort as low as 60 pcf. theoretical maximum densities for given Under the same compactive effort, the maxi- water contents. These densities are practi- mum density of a clay may be in the range of cally unattainable because removing all the 90 to 100 pcf, while that of a well-graded, Soil Compaction 8-3 FM 5-410 coarse granular soil may be as high as 135 pcf. moisture, producing sizable changes in dry Moisture-density relationships for seven density. different soils are shown in Figure 8-2. Compacted dry-unit weights of the soil There is no generally accepted and univer- groups of the Unified Soil Classification Sys- sally applicable relationship between the tem are given in Table 5-2, page 5-8. Dry- unit weights given in column 14 are based on OMC under a given compactive effort and the compaction at OMC for the CE 55 compactive Atterberg limit tests described in Chapter 4. effort. OMC varies from about 12 to 25 percent for fine-grained soils and from 7 to 12 percent for The curves of Figure 8-2 indicate that soils well-graded granular soils. For some clay with moisture contents somewhat less than soils, the OMC and the PL will be ap- optimum react differently to compaction. proximately the same. Moisture content is less critical for heavy clays (CH) than for the slightly plastic, clayey Other Factors That Influence Density sands (SM) and silty sands (SC). Heavy clays In addition to those factors previously dis- may be compacted through a relatively wide cussed, several others influence soil density, range of moisture contents below optimum to a smaller degree. For example, tempera- with comparatively small change in dry den- ture is a factor in the compaction of soils that sity. However, if heavy clays are compacted have a high clay content; both density and wetter than the OMC (plus 2 percent), the soil OMC may be altered by a great change in becomes similar in texture to peanut butter temperature.