ANALISIS DE SENSIBILIDAD

Sand and gravel deposits are usually found in distinctive landforms such as dunes or alluvial terraces, and consequently a knowledge of Quaternary geology and geomorphic processes is useful in prospecting for them. Some of these landforms can be interpreted from large-scale orthophotomaps with contour intervals of 1–2 m, or from airphotos; very large deposits may be visible on satellite imagery. On the other hand, many of the best sand and gravel deposits owe their formation to higher-discharge bedload streams that were active during Pleistocene time, but which are now obscured by several metres of younger silts of no engineering use (but considerable agricultural value).

Figure 3.5 Aerial view (about 2 km across) of sand mining south of Adelaide. Note

urban encroachment on left and rectangular tailings dams, which occupy half the area of some pits. The sand pit on the far left has since been rehabilitated.

Another source of information is surficial geology maps—where these exist. ‘Drift’ or Quaternary geology maps showing glacial, fluvioglacial, alluvial and other unconsolidated deposits cover much of the UK and parts of North America. Even where these surficial deposits are accurately mapped, however, they may not indicate important channel sands beneath younger floodplain silts. In poorly mapped areas, meaning most of the world, logs from water bores and bridge drillholes can be valuable sources of information, since the best alluvial aquifers are also sources of granular materials. Although these logs may not be geologically precise, most drillers can recognize and record coarse gravels, cobbles and clean ‘running’ sand. Furthermore, cable tool drilling is equally applicable to water well drilling and to gravel investigations.

Problems in investigations

The investigation of sand and gravel deposits presents a number of problems. In the first place, alluvial sediments are extremely variable, both vertically and horizontally. Representative samples need to be large, up to 100 kg or more where the particles are very coarse, and sampling locations need to be close together (25–50 m in many cases). Great care needs to be taken to ensure that both the oversize and fines fractions—the ‘top and tail’ of the grading curve in other words—are included. Secondly, such materials are difficult to penetrate when coarse, densely packed and even cemented. Excavation walls are prone to caving, requiring casing in boreholes and bracing in test pits. Finally, most deposits occur close to or below the water table. This magnifies the problem of excavation support and impedes sample recovery.

Trial excavations

Trial excavations by means of backhoe pits, hydraulic excavator trenches and bulldozer costeans can be very satisfactory for investigating shallow deposits, since they allow both for bulk sampling and for assessing the variability of the deposit. The depth limits for these excavators are approximately 3 m, 6 m and 10 m respectively (less in cobbles), with the cost increasing with depth. Even the coarsest gravels can be adequately sampled above the water table, and some recovery below it is also possible. Costeans can be deepened by hydraulic excavator pits in their floors, a ‘trench-in- trench’ layout. The main problem with such excavations, apart from their depth limitations, is the surface disturbance created. This is directly proportional to their size and a major cause of compensation claims from land-owners.

In particularly inaccessible sites, or where backhoes cannot penetrate, hand-dug shafts may still be useful. Mechanical winches, grabs and small- scale blasting will greatly assist progress. Very precise sampling of gravel

and removal of large bulk samples for on-site screening are possible by this method. The drawbacks are high labour costs, the need to erect timber shoring for safety and the near-impossibility of excavating below the water table.

Sampling procedures

Sampling procedures for both trial excavations and boreholes have to take account of the likelihood that oversize blocks will be pushed aside in small- diameter drilling, and that fines will be either washed away completely or will be greatly underestimated. In gravels, 30–50 kg samples at 1 m intervals may be required, less in sands. Depth accuracy is not a serious problem in most cases, since samples are composited and related to layers rather than to specific levels. Once recovered, samples have to be carefully mixed and quartered prior to laboratory testing, and allowance made in the test reports for rejected oversize and lost fines. Because of the large samples required in gravel deposits, some on-site preparation and oversize scalping is often needed. A small grizzly, scales and even a small front-end loader may be required.

Borehole sample collection also needs some thought. Wet bailer samples should be tipped out into steel troughs to avoid loss of fines; spiral augers should be scraped rather than simply reversed to fling off the soil; and disc augers should be emptied onto canvas or plastic sheets rather than straight onto the ground.

Deleterious materials

A number of deleterious materials are commonly found in sand and gravel deposits, and these can easily be missed during drillhole sampling. The most prevalent contaminant is clay, in the form of lenses, partings, matrix and coatings on gravel clasts. Many clasts themselves, particularly in torrent bedded deposits, are unsound due to rapid erosion and short transportation distance. Gravel subject to fluctuating water tables may be weathered, stained or encrusted with iron oxides, or may accumulate carbonates and salts (mainly gypsum) in low-lying arid areas where groundwater discharges. Feldspathic and calcareous sands tend to be much weaker, dustier and less durable than quartzose sand. Other deleterious matter includes charcoal, coal, peat, wood fragments, mica, shell grit and some duricrust gravels.