Well-prepared, compacted and shaped subgrade ready for placement of aggregateCompaction is an essential component of earthworks; it should be an integral part of both the subgrade preparation, as well as the placement of the improvement layer. All fill material should be compacted; correct compaction of subgrade and especially fill material will ensure that natural settlement is minimised increasing its stability and life. Uncompact fills are likely to be prone to vertical settlement of the ground surface which may lead to construction or environmental issues. Weak soils are greatly strengthened with compaction.
Compaction consists of reducing the apparent volume of the soil by expelling air through mechanical action. Compaction reduces the voids, thus increasing the soil density, and therefore increasing its bearing strength. This also leads to less water absorption.
Often there will be more than one soil type on a construction site. Consideration will need to be given to how best to manage the range of soil types that may be present. Fill design determines how soils will be mixed (and behave), depending on the site geology and geography, and the contractor’s construction methodology. It is important that the suitability of the in-situ soils be assessed against the fill requirements before earthworks begin. Some material is not acceptable as structural or load bearing fill. A good reference is Methods of testing soils for civil engineering purposes – Part 4 Soil compaction tests (NZS 4402.4.1:1986.)
6.5.1 Optimum moisture content
Optimum moisture content
Optimum moisture content for an East Coast low plasticity soilCBR testing
Laboratory results from CBR testing of an East Coast lean clay soilMoisture content has a significant impact on the efficiency of compaction. A soil that is too dry does not have sufficient inter-particle lubrication for the soil to pack into a dense configuration, whereas a soil that is too wet, will either be prevented from dense packing, due to the presence of water within the pore spaces of the soil matrix, or will cause the fines to be ‘pumped out’ during the compaction process. Before the compaction process begins, the layers should be checked to ensure that the moisture content is uniform.
Through a series of lab compaction tests, the effect of moisture content on density is readily shown, as is the concept of an optimum moisture content (OMC) at which a soil will achieve maximum dry density for a given compaction effort.
For example, in the adjacent chart the OMC occurs at 18% moisture content and results in a maximum dry density at 1.75t/m3. The OMC for each soil varies and needs to be determined by laboratory analysis. Note that the dry density achieved when the soil is dry (13%) is no better than that achieved when the soil is saturated (22%).
A simple in-field approximation of OMC is that the soil should be moist, but not wet, when compacted. A rough check for most materials is to squeeze a lump in the hand and, if it just holds together when pressure is taken off and the material does not stick to the fingers, the water content will be approximately at optimum. If a soil is too dry, add water with a water truck. Compaction is impossible if a soil becomes too wet. In this situation, stop compaction and wait until the soil has time to dry. Options are to provide good drainage and daylighting, or alternatively, apply lime and/or cement to assist drying of a wet subgrade.
Compacting fill material around a culvert is important to avoid failure in the long termThe benefit of compaction at the OMC is that the soil will be at its densest, which provides highest saturated strength and reduces water infiltration. For example, the subgrade soil in the adjacent figure exhibits high strength when built at a low moisture content (CBR=15 at 14% moisture content), but rapidly loses strength upon becoming saturated reducing to CBR=3 due to low dry density. This occurrence is common, as many forest roads are constructed during the dry summer, only to fail when they become saturated during the following wet winter. The same soil constructed at OMC will exhibit lower strength during construction (CBR=9), but will retain higher strength when saturated (CBR-6). Note that when compacted at OMC this soil will increase its strength as it dries, exceeding CBR=15 when dried to 14% moisture content.
Granular fills are normally suitable for fill construction, as strength is usually adequate over a range of moisture conditions. Granular soils are free draining and are less likely to be above their OMC for compaction. Significant volumes of water can be added to granular pavement layers to aid compaction. This is particularly important when compacting aggregate layers. Compaction of granular soils can be carried out with static compactors, that simply apply weight and tend to compact from the bottom of the layer up; vibratory compactors, that use a mechanical action to consolidate soil particles; and impact compactors, that use a high-amplitude thump to compact material.
Compaction is not only important for subgrades and roads, but also around culverts to prevent the fill material from becoming saturated and failing.