Earth structures can be divided into two types:
§ Earth structures which impose a load on the subsoil, such as dams, road and rail beds, general land fill, banks and depots for soil and waste;
§ Earth structures which remove a load from the subsoil, such as water courses, river widenings, harbours, excavations for roads or railway lines, building excavations and trenches.
Below a number of comments on each of these structures is given.
General
When a load is applied to highly compressible layers of low bearing capacity by fill material, the water pressures increase because in the first instance the load is supported by the pore water. The effective stresses present in these layers do not undergo any significant changes during this period, so that the shear resistance also remains unaltered. As the excess water drains away, the effective stresses increase and the ground consolidates. However, it takers a long time for the drainage process to be completed because of the layer thickness which is often great, and the low permeability of the layers to be drained. The filling time, i.e. the time required to apply the load, is therefore much shorter in practice than the time needed for the water to drain away and for the soil to consolidate. During filling operations, therefore, shear stresses may occur at the edges of the fill which cannot be absorbed by the shear resistance at that point. As soon as the filling operations cease, however, the excess water pressures no longer increase; from that time on the soil consolidates. In practice filling operations are usually phased, i.e. the load is applied in stages and there is a waiting time between the stages.
Where a load is removed by excavation, the opposite effect occurs. The effective stresses in the subsoil are reduced, making the soil less firm. In the first instance, the only result is a reduction in the water pressure, while the effective stress and/or the shearing resistance remain unaltered at all levels. The result is that the slopes and excavation bases are initially stable. With time, however, the pore pressures rise with the result that the shear resistance in the layers in question diminishes. Contrary to filling, excavations undergo adverse effects if there are waiting times.
If the quality of the subsoil is such that filling would result in unacceptably high compression or unacceptably long waiting times between the phases, measures must be taken in the form of special foundation techniques.
These techniques are aimed at the following:
o Improvement of the subsoil, so that its composition is less sensitive to the effects of compression and failure.
This kind of improvement can be achieved by replacing the soil down to a certain level with soil of a better quality as regards its resistance to compression or failure. In practice this often means excavating a layer of peat and replacing it with sand. Installation of stone columns has a similar effect.
o Reducing the load on the subsoil so that the compression is less marked, and the resulting shear stresses are reduced.
The load can be reduced by introducing lightweight filling materials, such as for instance Flugsand, aerated concrete of PS rigid foam.
o Rapid drainage of the subsoil so that the waiting times can be shortened. Rapid drainage of the excess pore pressure water can be achieved by installing horizontal and/or vertical drainage systems. Vertical drainage is usually applied in such cases. In exceptional cases, suction is applied to the system, which results in acceleration of dissipation of excess water pressures.
Dams
Dykes, dams and quays are primarily water retaining structures. Earth structures of this kind have therefore been constructed from time immemorial using low permeability fill material such as clay. Nowadays, however, a low permeability facing layer of clay is usually applied to a structure consisting of a stable and incompressible material such as sand. The clay used must fulfil certain requirements as regard workability, shrinkability and permeability. Small quays and retaining dams are still however often constructed entirely of clay or clay-type material.
A distinction must be made between dams which have to cope with a constant difference in the water level, such as polder dykes and canal banks, and dams exposed to varying water levels, such as sea and river dykes. In the case of the latter, the loads and load combinations are of particular importance in the design.
The geometry and choice of materials for dyke construction are influenced to a great extent by the maximum outside water level, wave height, wave load and changes to the water table resulting from the load exerted by the dyke structure, etc. in the case of dykes of this kind, it is above all necessary to take measures over a wide area, e.g. covering slopes with a facing to prevent attack by waves on the outer slope. As a rule, materials such as stone, either tipped or placed in position, concrete elements or asphalt are used.
Extra attention must be paid to the level of the water table: situations where the water table intersects the inner slope, with the result that water could escape from the slope, must be avoided at all costs. This can be achieved by making use for example of a more gradual slope or by altering the water table with a ditch or by introducing a filter.
Most dykes, dams and quays are protected against further surface erosion by a layer of grass over a layer of topsoil; this layer too must meet special requirements.
In many cases, dams in the Netherlands are also required to carry traffic. In that case, even greater demands are placed on the structure.
Road and railway beds
Earth structures for road and railway beds railway beds are intended first and foremost to transmit the traffic loads to the subsoil without causing unacceptable deformations and in particular differential settlements; in any case, traveling comfort imposes stringent differential settlement requirements, both longitudinal and transversal.
The surface layers of structures of this kind, i.e. the road, pavement and the railway track must not only exhibit a minimal level of deformation but also possess a high bearing capacity. It is thanks to these qualities that traffic loads are absorbed at quite a shallow depth.
Connections between earth structures and bridge structures demand particular care. Bridge structures almost always exhibit different settlement behaviour from earth structures on account of the different type of foundations.
In recent years, widening of existing roads and railways has been the principal problem. Deformation in a transverse direction to the existing track must be kept to a minimum.
The slopes of a road or railway embankment are covered with topsoil, with grass or other plants sown on it; this facing is mainly intended to prevent erosion.
Banks
Banks are often raised to prevent the leakage of liquids and to reduce noise; their shape resembles that of dykes.
Banks are often erected around storage tanks for liquids; in the event of a disaster they prevent the liquid from escaping. Because of the nature of the liquids, banks of this kind are generally covered with an impermeable facing on the tank side, which must also cover the area between the toe of the bank and the tank. It is essential to take into account at the design stage the consequences of any contact between the liquid from the tank and the material from which the bank and the ground surface adjacent to the tank are constructed.
In horticultural areas, banks are sometimes used to create retention areas for rainwater.
Noise barriers are mostly positioned alongside roads and have no other purpose than to reduce the noise level in the adjacent residential area. In addition to the acoustic requirements, the design of the barrier is greatly influenced by the available space, the available material and aesthetic considerations. Dimensions, angle of slope and covering can therefore differ in each individual situation.
Dumps
Dumps for primary or secondary materials may be temporary or permanent. Primary material include naturally deposited materials such as for instance san and clay. Secondary materials are industrial residues, such as fly ash and blast furnace slag, as well as refuse, which may or may not be recycled, such as refuse incineration plant slag and material from crushing plants.
Both categories, primary and secondary, may be contaminated to a greater or lesser extent, so that sometimes an environmental impact report is necessary, and extra care has to be taken with the structure, in the form of a variety of protective measures. In such a case, extensive drainage arrangements are often necessary, as well as sealing systems above and below ground level in the form of impermeable sheeting, sand-bentonite layers or special screens.
Generally speaking, the slopes of these dumps have no other function than fulfil stability requirements, although measures to prevent surface erosion may also be necessary.
If the dump reaches a great height because of lack of space, for instance, and/or the stability of the subsoil is threatened, soil improvement is applied in the form of stabilization or soil replacement after partial excavation.
Excavations for waterways, ports, etc.
Excavations for ditches, drainage ditches, canals and ports are often carried out under water. The design must therefore not only take into account the nature of the subsoil but also, in particular, the available dredging equipment.
At water level, a bank revetment or berm is often built to counter the eroding effect of waves. Washing way or erosion of the lower levels is usually prevented by using gradual slopes and/or other design techniques.
Road and railways cuttings
Sections of roads and railways in built-up areas, and also in areas of particular natural or historic interest, are being laid more often in cuttings. So far as possible a cutting of this kind is carried out using the natural slope of the soil.
If the groundwater level in the areas in question is high, it will have to be lowered permanently, and a polder system is the result. Lowering the groundwater level can however have adverse environmental effects, such as subsidence of buildings, damage to crops and complications for surface water management. In order to restrict these effects, percolation screens or bentonite walls are installed.
Excavations
For buildings and engineering structures whose foundations have to be located relatively far below ground level, dry excavations of considerable size are required. The soil layers beneath the floor of such excavations will experience a certain reduction in ground pressure, which will however be pertly or completely reversed after the construction has been built and/or the excavation has been reviled. In compressible soils it is especially important not only to prevent adverse effects on the foundation of the building, resulting from deformation of the subsoil, but also damage to the surrounding area.
The stability of the slopes and the excavation must be guaranteed for a specified period of the construction time; a pumping system is usually installed to prevent water escaping from the slope and/or seeping up through the floor of the excavation due to local excess water pressures. The pumping system should lower the original water table to such an extent that it lies below the excavation depth. Sometimes pumping has to be carried out at several levels: open pumping or pumping from well points or any combination of the two.
Draining can however also lower the water level outside the excavation, resulting in settlements which may cause damage to the foundations of building and other structures.
Deformation of slopes and the excavation floor should also be kept to a minimum; in particular, deformations around the edge of the building pit may result in damage to the structure, such as for instance skewing of piles driven inside the excavated area.
Trenches
Trenches are usually dug for the laying of cables and/or pipe lines and are comparable to the excavations described above. Trenches are however much smaller in cross-section and as a rule remain open for a much shorter period. If personnel have to work in the trench, the stability of the trench wall must be taken into account. The longer the trench remains open and the greater the risk of reduction of the shear resistance, the greater the risk of loss of stability.
If drainage is carried out, just as in the case of excavations, it is necessary to take account of settlement in the vicinity of the trench.
Horizontal and vertical deformations near to the trench can also cause damage to adjacent constructions.
