Subgrade, An Important Road Surface

Subgrade is one of the most crucial part of embankment fills or natural surface just below the sub-base or lower sub-base of road pavement and shoulder. The surface above the subgrade is known as the formation level or finishing level. Subgrade is the in situ material upon which the pavement structure is placed or constructed at selected location.

Formation level is defined as the final level of soil surface after completion of earthworks and when trough the process of compaction, stabilization and reinforced. The subgrade main function is to withstand the loading of road pavement (sub-base, base, etc.) above it.

Although there is a tendency by looking at the pavement performance in terms of pavement structure and mix design alone, the subgrade can often be the overriding factor in pavement performance.

Unsuitable Materials for Subgrade

The soil number one enemy is water which effect the quality

Not every soil is suitable of becoming filing or embankment materials in road construction. Some consideration should be made in terms of specification and observation in choosing the right soil.

Unsuitable soil materials for subgrade (or embankment fills) are as follows:

• Clay soil which contains the value of Liquid Limit more than 80% and/or Plasticity Index more than 55%,
• Having the value of Lost On Ignition (LOI) more than 2.5%,
• It is flammable materials (oily), and organically clay soil,
• Contain lots of rotten roots, grass and other vegetation,
• Considered as unstainable soil or toxic and categorized as peat soil,
• Soil which is soft and unstable because it is too wet or dry which makes it difficult to compact properly.

Testing for Subgrade

The Casagrande Apparatus

There are several testing method that were used

to test the subgrade layer and also embankment layers. The notable and most recommended test (among others) to be carryout are as follows:

• California Bearing Ratio (CBR), as accordance to: BS 1377: Part 4 1990, ASTM D1883-05 or AASHTO T-193
• Compaction Test, as accordance to: BS 1377: Part 4 1990, ASTM D-698 or AASHTO T-99
• Liquid Limit (LL) and Plastic Limit (PL)test, as accordance to: BS 1377: Part 2 1990, ASTM D-4318 or AASHTO T-89
• Lost On Ignition (LOI) test, as accordance to: BS 1377: Part 3 1990 or AASHTO T-267

Performance of Subgrade
The subgrade’s performance generally depends on two interrelated characteristics:
Load Bearing Capacity
The subgrade must be able to sustain loads transmitted from the pavement structure. The load bearing capacity is frequently affected by the types of soil, moisture content, and degree of compaction. A subgrade that can sustain a highly sum of loading without an excessive deformation was considered good quality.

The types of soil especially from gravel type considered the best and from peat type considered as the worst material. Moisture content of soil is also important and determine by conducting the soil compaction test at lab as to find out which type contains more water. The degree of compaction normally reflect to the method of compaction used at construction site, by means of machinery and the numbers of passes.

Changing in Volumes
In most cases, soils will undergo some amount of changes in volume when exposed to excessive moisture, rise in temperature or in freezing conditions. For instance, some clay soils would shrink and swell depending upon its moisture content, whereas soils with excessive fines may be susceptible to frost heave in freezing areas.

As a conclusion, the subgrade must be form properly to prevent any possible damage to the road pavement. Factors of choosing the right or suitable materials, affecting the strength, materials specification, materials classification, and method of testing is vital for the road construction especially in earthworks stage.