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Because suitable sites are becoming scarce in urban areas, the construction of buildings on fill or made-up ground has assumed a greater importance. A wide variety of materials is used for fills, including domestic refuse, ashes, slag, clinker, building waste, chemical waste, quarry waste and all types of soils. The extent to which an existing fill will be suitable as a foundation depends largely on its composition and uniformity. In the past the control exercised in placing fill has frequently been insufficient to ensure an adequate and uniform support for structures immediately after placement. Consequently a time interval had to be allowed prior to building so that the material could consolidate under its own
weight. Although this may be suitable for small, lightly loaded buildings it is unsatisfactory for more heavily loaded structures that can give rise to substantial settlement. The time taken for a fill to reach a sufficient degree of natural consolidation so that it becomes suitable for a foundation depends on the nature and thickness of the fill, the method of placing and the nature of the underlying ground, especially the groundwater conditions. The best materials in this respect are obviously well graded, hard and granular. By contrast, fills containing a large proportion of fine material may take a long time to settle. Generally rock fills will settle 2.5% of their thickness, sandy fills about 5% and cohesive material around 10%. The rate of settlement decreases with time but in some cases it may take 10–20 years before movements are reduced within tolerable limits for building foundations. In coarse-grained soils the larger part of movement generally occurs within the first two years after the construction of the fill, and after five years settlements are usually very small. The minimum time that should elapse before development takes place on an opencast backfill should be 12 years after restoration is complete. Frequently, poorly compacted old fills continue to settle for years due to secondary consolidation. Waste disposal or sanitary land fills are usually very mixed in composition and suffer from continuing organic decomposition and physicochemical breakdown which may leave voids. Methane and hydrogen sulphide are often produced in the process, and accumulations of these gases in pockets in fills have led to explosions. The production of leachate is another problem. Some material such as ashes and industrial wastes may contain sulphates and other
products which are potentially damaging to concrete. The density of waste disposal fills varies from about 120 to 300 kg/m3 when tipped. After compaction the density may exceed 600 kg/m3. Moisture contents range from 10 to 50% and the average specific gravity of the solids from 1.7 to 2.5. Settlements are likely to be large and irregular. The initial settlement of waste disposal fills is rapid and is due to a reduction in the void ratio. It takes place with no build up of pore-water pressure. Settlement continues due to a combination of secondary compression (material disturbance) and physico-chemical and biochemical action.
How do you identify Fill materials in the field?
When you are on site look for evidence of unusual grading or built up areas, filled in areas (gullies, ravines, ponds, or former wetlands), grade changes, and other constructed features such as retaining walls. Ask locals, property owners, and drillers/contractors about their knowledge of area development and conditions. A pre-drilling geophysical survey can also identify fill, possible fill, and changes in soil type. During a subsurface investigation like drilling or test pit excavation look for and note soft soils, low Standard Penetration Test (SPT) or ‘N’-values or abrupt changes in ‘N’-values during split-spoon sampling. Don’t hesitate to ask the driller for their opinion (some drillers are better or more experienced than others in how it ‘feels’ while drilling, and can tell if it feels like drilling through fill). Look for any evidence of buried non-native materials – if you encounter buried topsoil, significant rootlets at depth (not always, some rooting does go deep into subsoil), debris, pavement, coal fragments, clinkers, construction materials (glass, wood, bricks, metals); then everything found above level that is most likely fill. Consider how native soils are originally deposited (via glacial deposition, as sediment from water, wind-borne, etc.), and think how that deposition would look in situ. In many soils, not all (e.g. some tills and conglomerate-type soils) there will be distinct layering, sometimes very uniform in consistency with noticeable changes from one homogenous stratum to another. Then consider how fill may get placed at a site – bulk dumping from a truck, graded by a dozer, mechanically transported and placed. In comparison fill may look ‘dirtier’ with several types of soil in a mix or blend (e.g. a heterogeneous mix of visibly different silt, sand, gravel, organics), sometimes oddly mixed with inclusions. Unless it has been compacted it will likely be looser and appear unconsolidated, and could even get softer and looser with depth (native soils often gain strength with depth). Some of the hardest material to distinguish in the field is fill that is comprised of re-worked native soil, such as a large cut and fill grading for a site. If necessary have a more experienced person review with you and directly examine the soil/fill in the field or at least review retained samples with you. The more soil you drill, sample, see, feel and touch; the better you will get at the task.
