Foundations: Compaction Requirements.

a. General.   Guidelines for selecting compaction equipment and for establishing compaction requirements for various soil types are given in table 15-1.  If fill materials have been thoroughly investigated and there is ample local experience in compacting them, it is preferable to specify details of compaction procedures,
such as placement water content, lift thickness, type of equipment, and number of passes.  When the source of the fill or the type of compaction equipment is not known beforehand, specifications should be based on the desired compaction result, with a specified minimum number of coverages of suitable equipment to assure uniformity of compacted densities.

A summary of Density Methods for Building Foundation
Table 15-1.  A summary of Density Methods for Building Foundation


b. Compaction specifications.   For most projects the placement water content of soils sensitive to compaction moisture should be within the range of -1 to + 2 percent of optimum water content for the field
compaction effort applied.  Each layer is compacted to not less than the percentage of maximum density
specified in table 15-2.  It is generally important to specify a high degree of compaction in fills under
structures to minimize settlement and to ensure stability of a structure.  In addition to criteria set forth in TM 5-818-4/AFM 88-5, the following factors should be considered in establishing specific requirements:

(1) The sensitivity of the structure to total and differential settlement as related to structural design is particularly characteristic of structures to be founded partly on fill and partly on natural ground.

(2) If the ability of normal compaction equipment to produce desired densities in existing or
locally available materials within a reasonable range of placement water content is considered essential, special equipment should be specified.

(3) The compaction requirements for clean, cohesionless, granular materials will be generally higher than those for cohesive materials, because cohesionless materials readily consolidate,  or liquify,
when subjected to vibration.  For structures with unusual stability requirements and settlement limitations,  the
minimum density requirements indicated in table 15-2 should be increased.  For coarse-grained, well-graded, cohesionless soils with less than 4 percent passing the No. 200 sieve, or for poorly graded cohesionless soils with less than 10 percent,  the material should be compacted at the highest practical water content, preferably saturated.  Compaction by vibratory rollers generally is the most effective procedure.  Experience indicates that pervious materials can be compacted to an average relative density of 85 + 5 percent with no practical difficulty.  For cohesionless materials, stipulate that the fill be compacted to either a minimum density of 85 percent relative density or 95 percent of CE 55 compaction effort,  whichever gives the greater density.

(4) If it is necessary to use fill material having a tendency to swell, the material should be compacted at water contents somewhat higher than optimum and to no greater density than required for stability under proposed loadings (table 15-2).  The bearing capacity and settlement characteristics of the fill under these conditions should be checked by laboratory tests and analysis.  Swelling clays can,  in some instances, be permanently transformed into soils of lower plasticity and swelling potential by adding a small percentage of hydrated lime.


Compaction Density as a Percent of CE 55 Labortory Test Density
Table 15-2.  Compaction Density as a Percent of CE 55 Labortory Test Density

c. Compacted rock.  Compacted crushed rock provides an excellent foundation fill.  Vibratory rollers are preferable for compacting rock.  Settlement of fill under the action of the roller provides the most useful information for determining the proper loose lift thickness,  number of passes,  roller type,  and material gradation.  Compaction with a 10-ton vibratory roller is generally preferable.  The rock should be kept watered at all times during compaction to obviate collapse settlement on loading and first wetting.  As general criteria for construction and control testing of rock fill are not available, test fills should be employed where previous experience is inadequate and for large important rock fills.

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