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The simplest classification of earthfill is into cohesive (clay-rich) materials and granular, sandy or non-cohesive ones. The engineering properties of the first group are implied by their Atterberg consistency limits (liquid limit, LL; plastic limit, PL; and plasticity index, PI), while the granular soils are classified in terms of their particle size distribution. The most widely used engineering soil classification, originally developed by the US Army Corps of Engineers but now known as the unified soil classification (USC), is based on these criteria.

• Granular soils In general, the granular soils provide superior fill because they are easily compacted to high densities at low moisture contents, can sometimes be ‘dried back’ when too wet for compaction, and become more homogeneous with working due to the kneading action of the gravel clasts.

• Cohesive soils The cohesive soils are difficult to compact uniformly because water cannot penetrate their low-permeability (but high- microporosity) clods. Therefore a given compactive effort can only achieve relatively low densities. The abundance of platy clay minerals causes them to become slippery when wet and to develop shear planes (‘laminations’) when excessively compacted.

Having been devised in North America, the USC is more applicable to the transported (alluvial, glacial, fluvioglacial and aeolian) surface sediments that are prevalent there, rather than to the residual soils and saprolite (weathered rock) that dominate the surficial geology of warmer regions. In particular, the USC ignores the strengthening influences of particle aggregation in residual soil fabrics, and of self-cementation in duricrust (lateritic and calcrete) soils.

However, the bulk of earthfill used in Australia and other non-glaciated countries is derived from highly to extremely weathered rock, which has become disaggregated by excavation, spreading and rolling. In a typical cutting the top 1–3 m is residual soil, usually clay-rich, which is unsaturated and often lateritized. This is underlain by 10–20 m of saprolite down to the base of weathering (the ‘weathering front’ or fresh rockhead). The saprolite becomes less weathered and more rock-like in its properties with depth, and usually becomes unrippable within the ‘highly weathered’ grade.

As initially broken out, this material is typically a rock-soil mixture with 70–80% of angular or slabby weak rock fragments up to 4 m in maximum dimension. With further cross-ripping and comminution beneath bulldozer tracks, this might break down to a maximum particle dimension of 1 m, and a soil content around 50%. Additional size reduction occurs during scraper loading, spreading and compaction, to the

point where the +200 mm percentage becomes negligible. Any oversize blocks can be fractured by grid rollers, or simply pushed to the edge of the fill to act as slope armouring.

Because the engineering requirements for earthfill are so flexible, it is more sensible to think in terms of the few materials that are unsatisfactory rather than the many that are—more or less—suitable for use in embankment construction. These ‘problem’ soils include the following:

• Expansive (cracking) clays, which are subject to large volumetric changes on wetting and drying. These are difficult soils to compact uniformly and may wet up in fills by capillary suction over several years.

• Dispersive clays, which erode easily on exposure and which may also undergo internal ‘tunnelling’ erosion. These soils contain aggregated clays that lose their cohesion (i.e. they deflocculate or ‘disperse’) due to reduction in porewater salinity. This is primarily a problem of small dams and can be controlled by better compaction, or by lime infusion.

• Silt soils (loesses), which are difficult to compact, are highly susceptible to capillary rise (causing waterlogging and frost heave), and may liquefy during prolonged seismic shaking. These soils may be very sticky just below their optimum moisture content (OMC), yet turn into a slurry just above it. They are also prone to piping and surface erosion.

• Some highly micaceous soils, which are difficult to compact because of the springiness of the platelets. Where the mica is too fine to be visible, these soils behave like silts, and polished shear planes may develop where the soil is overcompacted.

• Some andosols (allophane-rich volcanic ash soils), which have an extremely porous microfabric. These soils may appear dry yet have a very high moisture content, often well above their LL. If overworked during compaction, the cellular fabric collapses, releasing large amounts of water and turning the fill surface into a morass. They may also shrink irreversibly on drying.

• Halloysitic soils also develop on volcanic ash, though they may also result from tropical weathering of other parent materials. Like the andosols, they have unusual properties for clay-rich soils, notably a tendency to granulate and become non-plastic on drying. Like the andosols, but unlike swelling clays, this granulation is irreversible.

• Peat and organic-rich soils, which are highly compressible, shrink on draining and may even catch fire on desiccation due to spontaneous combustion. Peat itself would never be used as fill, since it contains as little as 5–15% solids, but its presence in estuarine deposits is a major cause of foundation settlement beneath embankments.

• Some very weak and porous rocks break down excessively and hence require careful handling when used as fill. They have very low in situ

density (<2.2 t/m3_{) and low intact strength (UCS<10 MPa), and include}

chalk, overconslidated clays, diatomite, some pyroclastic rocks and lateritic pallid zone leached clays.

Nevertheless, the most common reason for rejecting soil as fill is simply that it is too wet. Acceptance criteria for wet fill include moisture contents less than 1.1–1.3 times PL, or undrained shear strengths above 35–50 kPa for clays. Saturated clean sand and gravel drain quickly, but sands with even a small proportion of silty fines can remain in a semi-liquid state after spreading and are useless as fill. Unsaturated soils can be carefully moistened up, but drying back is not often practicable. Good construction practice with clay fills requires that they be lightly rolled to a smooth finish with a surface crossfall at day’s end, to limit possible infiltration by overnight rain.

Moderately wet clay soils can sometimes be satisfactorily compacted by using lighter rollers, since a small compactive effort implies a higher OMC. Conversely, heavy compaction lowers the OMC; the reasons for this will be discussed later. Where wet fill has to be used, lime stabilization of upper layers can provide a working surface (at a cost). Alternatively, settlement of weak embankments may simply have to be accepted, and compensated by stronger pavements and flatter fill slopes.

10.4 EARTHWORKS DESIGN