Lesson 1/Learning Event 2
Grade Line
The investigation and tests previously described will result in classification of the subgrade and subsoil
in accordance with the Unified Soil Classification System. Table 5 of EN 5453, Soils Engineering, lists
the various soils in descending order of their desirability as subgrade soils. You should consider this
information together with information on the depth to water table, depth to ledge rock, and the
compaction and strength characteristics in locating the grade line of the top of the subgrade. Generally,
this grade line should be established to obtain the best possible subgrade material consistent with the
proper utilization of available materials; however, you must also consider economics of plans with
construction.
Subgrade Compaction Normal Cases
In general, compaction increases the strength of subgrade soils. The normal procedure is to specify
compaction in accordance with requirements in Figure 4.
The problem is relatively simple to fill sections since all the layers will be subjected to construction
processes and can be compacted during construction.
The problem is more difficult in cut sections. You must obtain compaction during construction to a
depth where the natural density will resist densification under further traffic. It is recommended that in
cut sections only the top 6 inches of subsoil be recompacted if required. Cohesionless soils (except silts)
can often be compacted from the surface with heavy rollers or very heavy vibrating compactors.
Cohesive soils (including silts) cannot be compacted in thick layers; therefore, it may be necessary to
remove, process, and replace soil in cut areas in order to meet the compaction requirements as
previously discussed. In addition, you should compare the natural densities occurring in the subsoil with
the compaction requirements for the deeper depths to determine if compaction of the subsoil is
necessary, or if the flexible pavement structure must be established so that these layers are deep enough
that they will not be affected by loadings to be applied.
Compaction of cohesive materials, including those of relatively low plasticity showing little swell,
should be accomplished at the optimum moisture content determined in the modified AASHO (CE 55)
compaction test. This moisture content is generally slightly drier than the true optimum for field
compaction using normal equipment. This difference has been recognized and accepted as a slight
safety factor. Cohesionless, free-draining materials should be compacted at moisture contents
approaching saturation.
Subgrade Compaction Special Cases
Although compaction increases the strength of most soils, some soils decrease in stability when
scarified, worked, and rolled. There are also some
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