Compaction of Embedment Materials
Compaction of the embedment often will increase the stiffness of the insitu soil resulting in increased pipe support. The required degree of compaction will be set by the designer in consideration of height of cover, extent of live loading, water table elevation and soil properties. In general, "moderate" compaction requirements are satisfactory, corresponding to minimum field measured densities of 90% Standard Proctor Density.
Typical equipment used for compaction are hand held tamping bars, gasoline driven impact tampers ("whackers"), vibratory plates, and air driven impact tampers ("pogo sticks"). Compaction of the haunch material can best be accomplished by hand with tampers or suitable power compactors. The use of impact tampers directly above the pipe should be avoided until sufficient backfill (e.g., 12 inches) has been placed to ensure no local deformation of the pipe. The primary initial backfill should be placed in lifts that are brought up evenly on both sides of the pipe, and thoroughly compacted prior to placement of the next layer. In general, maximum lifts of approximately 12 inches for Class I, 8 inches for Class II, and 6 inches for all others are adequate.
Compaction by vibration is most effective with granular (Class I and II) materials. In cases where the engineer specifies a minimum soil density of 90% of Standard Proctor, mechanical compaction of Class I materials will be required. Impact compaction of Class I material should be considered for any application where the pipe will be below the ground water table, beneath deep cover, or where the stability of the insitu soil is in question. An alternate method of achieving compaction with Class I materials is shovel slicing. Mechanical compaction of Class II materials can be aided by slight wetting, although care must be taken not to saturate the material or flood the trench, particularly when the native trench material does not drain freely. Compaction by impact is usually most effective with Class III and Class IVA materials.
It is prudent to routinely check density of the embedment material. Checks should be made initially to verify that the compaction procedure being used achieves the desired density. Also, checks should be made at random intervals for QA and anytime there is a change in insitu soil strata or the embedment material.