The soil classifications are grouped into soil-stiffness categories (SC) based on the typical soil stiffness when compacted. Soil stiffness is a property of soil, generally represented numerically by a modulus of deformation that indicates the relative amount of deformation that will occur under a given load. Category-1 indicates a soil that generally provides the highest soil stiffness at any given percentage of maximum proctor density, and a soil that provides a given soil stiffness with the least compactive effort. Each higher number soil stiffness category provides successively less soil stiffness at a given percentage of r density and requires greater compactive effort to provide a given level of soil stiffness. Maximum standard proctor density is the maximum dry unit weight of soil compacted at optimum moisture content (the moisture content at which maximum density is obtained).
Crushed rock and gravel with <15% sand and ≤ 5% fines.
SC1 materials provide maximum stability and pipe support for a given density due to the low content of sand and fines. With minimum effort these materials can be installed at relatively high soil stiffness over a wide range of moisture contents. In addition, the high permeability of SC1 materials may aid in the control of water, and these materials are often desirable for embedment in rock cuts where water is frequently encountered.
GW, GP, SW, SP and dual symbol soils beginning with one of these symbols such as GW-GC, SP-SM, etc. containing <12% fines.
SC2 materials when compacted, provide a relatively high level of pipe support; however, open graded groups may allow migration and the sizes should be checked for compatibility with adjacent material.
GM, GC, SM, SC with more than 12% fines; and ML, CL or borderline soils beginning with one of these designations, such as ML/CL, with 30% or more retained on the No. 200 sieve.
SC3 materials provide less support for a given density that SC1 and SC2 materials. Higher levels of compactive efforts are required and moisture content must be controlled. These materials provide reasonable levels of pipe support once proper density is achieved.
ML, CL, or borderline soil beginning with one of these designations, such as ML/MH, with less than 30% retained on the No. 200 sieve.
SC4 materials require geotechnical evaluation prior to use. Moisture content must be near optimum to minimize compactive effort and achieve the required density. Properly placed and compacted, this material can provide reasonable levels of pipe support; however, these materials may not be suitable under high fills, surface applied wheel loads or under high energy level vibratory compactors and tampers. Do not use where water conditions in the trench may prevent proper placement and compaction.
CH, MH, OL, OH, PT or borderline soils beginning with one of these designations, such as CH/MH, and any frozen materials.
SC5 materials should be excluded from pipe zone embedment.
| GW |
Well-graded gravel |
| GP |
Poorly graded gravel |
| GM |
Silty gravel |
| GC |
Clayey gravel |
| SW |
Well-graded sand |
| SP |
Poorly graded sand |
| SM |
Silty sand |
| SC |
Clayey sand |
| CL |
Lean clay |
| ML |
Silt |
| OL |
Organic clay / Organic silt |
| CH |
Fat clay |
| MH |
Elastic silt |
| OH |
Organic clay / Organic silt |
| PT |
Peat |