Composite Drain To Be Used In Soil Types Having Low Water Premeability

Abstract

Embankments for roads, railways etc are made by using material having low water permeability, such as clay. The material is laid up in different layers, which are separated from each other by means of horizontally arranged strips of drains. The drains comprise an inner core having wave-shaped surfaces and outer sheets of water filtering material, so that water from the clay may go through the outer sheets into the space between the inner core and the outer sheets and flow out.

Description

METHOD FOR MAKING EMBANKMENTS FOR ROADS, RAILWAYS, HOUSES ETC

The present invention relates to a method for making embankments for roads, railways, houses etc using material having low water permeability, such as clay. The invention comprises also a drain suitable for carrying out the method.

Clay and similar soil types are usually regarded as bad basic materials due to their low ability to let water penetrate through them. For this reason, it has hitherto been necessary to carry out extensive piling, if construction works shall be done on a clay base, or it has been necessary to take away large quantities of clay and replace them with better basic materials. This is a serious disadvantage giving rise to high construction costs.

If, however, these materials having a low water permeability could be kept dry, they would be extremely hard and make a good base for construction works. To lower the construction costs, many attempts have been made to arrange a good drainage. Such an attempt is to put clay in layers alternating with layers of gravel, so that horizontal drainage of the clay is obtained. Also this method is, however, expensive partly due to high costs of transportation and partly due to the difficulty in spreading the gravel in uniform layers. This latter difficulty is especially apparent when the gravel shall be spread over relatively large areas, as heavy machines cannot be moved on the clay.

A special drainage problem is present for instance with road constructions, where it is desired to use fine-grained, water-saturated soil types like clay from open cuts as embankment filling material. As the clay taken away usually cannot be used as embankment filling material without special measures, it is mostly transported to certain dumping-grounds. This causes large extra costs and is also unsuitable from a nature point of view. At present there is no practical and economical method for draining of clay in embankments. Generally, the filling must consist of gravel, which often has to be taken from open side cuts. In many areas, the supply of gravel is unsufficient, which causes high prices. Such open side cuts do also often come into conflict with nature interests. It is accordingly desired that the clay could be used for filling of road embankments. One condition for doing so is that the clay can be quickly dewatered by suitable measures, e g by means of drains.

Another example where drains advantageously can be used in connection with e g road construction is when the work is carried out winter-tide with frozen material in the embankment filling. The pore water in the frozen soil (the water in the cavities between the minerial grains) is frozen. When the spring comes, the soil thaws, and on account of the excess of water therein a pore water pressure arises, which substantially decreases the shearing strength of the soil. In fine-grained soils, it can take a very long time for the water to disappear, which often results in floating out and slides of the filling materials. By arranging drains in the frozen filling masses during the filling work, the drainage of these is facilitated when thawing. No appreciable pore water pressure arises and the risk for floating out and slides is eliminated.

According to the present invention, the above-mentioned disadvantages are eliminated. The invention comprises a method for making embankments for roads, railways, houses etc using materials having low water permeability, such as clay. The method is mainly characterized in that the material is laid up in several, mainly horizontal layers, which are separated from each other by means of sheets of a material having fine pores, so that predominantly horizontal drainage of the embankments is obtained.

A drain according to the invention, which is suitable for carrying out the above-mentioned method, comprises an inner core having wave-shaped, channeled surfaces. The core shall at least on one surface be equipped with a water permeable, filtrating sheet-shaped material, such as paper or the like, so that channels are formed between the core and the sheet-shaped material. It is advantageous that the drain comprises a disk-shaped, elongated material having wave-shaped surfaces on both sides and being included in paper.

Paper, which should preferably be unsized and have a surface weight of approx. 100 g/m.sup.2, may also be used as draining material. The papers is then arranged with the direction of the fibres in the same direction as the draining shall occur, i.e. across the road when embankments for roads are made.

When for instance roads are made in accordance with the invention, the embankment is laid up in such a way that a clay layer having a thickness of about 20 cm is spread out, which is thereafter covered by a layer of drains, a further layer of clay, another layer of drains thereon etc.

When road embankments are made according to conventional methods, it is usual that the embankment as it is built serves as a road for the transportation of filling material. By the method according to the present invention, however, the embankment is laid up from the direction toward which it is built.

The invention has made it possible to build road embankments at substantially reduced costs compared to conventional methods, primarily due to the highly reduced costs of transportation. The arrangement of the drains may be carried out by very simple means.

The invention shall be described more in detail in connection with the enclosed drawings, wherein FIG. 1 shows an embankment when being built and FIG. 2 shows a cross section of an embankment. The FIGS. 3 and 4 show cross sections of two different drains according to the invention.

In FIG. 1 a clay layer 1 is spread out on a suitably prepared surface 2, whereupon a layer of drains 3 is arranged on the clay layer 1. If the drain consists of paper, the direction of the fibres in the paper should be oriented across the embankment. The first clay layer is given a relatively limited length, but it extends over the whole width of the embankment. Thereafter a new clay layer is spread out upon the drain and then comes one more layer of drains etc. When the embankment has reached the intended level, the process is repeated with a new clay layer direct on the earth surface immediately in front of the just finished embankment, so that the embankment grows in short pieces.

From the cross section in FIG. 2 it appears how a finished embankment according to the invention consists of a plurality of clay layers separated from each other by means of drains.

In FIG. 3 the core material 11 of the drain consists of a thin, wave-shaped strip of plastics. On both sides of the strip, paper sheets 12 are anchored. Due to the wave-shape of the strip, longitudinal channels 13 are formed on each side of the core. When the drain is put into the soil, water will penetrate through the paper, into the channels and along these to the open earth surface. It is important that the paper acts as a filter.

If not, the channels will be filled with soil and thus become impermeable. It is of course possible to use other materials than plastics and paper in the present combination, but as these materials not only give the drain the desired stiffness and elasticity, but also are cheap, the combination is especially suitable. In some cases, however, it can be advantageous to have a core of metal having the same profile instead of a core of plastics.

FIG. 4 shows another embodiment of the invention, which differs from the embodiment according to FIG. 3 only in the way that the core 11 has been made even more robust. The waves on the both sides are mirror images of each other. This embodiment is preferred for production reasons when the core material consists of plastics, but otherwise it has the same effect as the embodiment according to FIG. 3.

When embankment fillings are to be horizontally drained, the drain according to the invention can also by reason of its strength take up horizontal shearing tensions in the filling, which has a stabilizing effect thereon. (The stabilizing effect of the drain is in this case -- besides occurring through dewatering of the clay -- the same as for the stabilizing pile construction for embankment fillings as is described in Vag- och Vattenbyggaren, No. 8, August 1968, Oleg Wager, Stability improving pile construction for embankment fillings.) By this means a heavier load may often be put on the embankment. An important difference between the paper drain of FIG. 2 and the drain according to FIGS. 3 and 4 is that the paper in the former acts as a drain having high resistance to water flow, while in the latter case the paper only acts as a filter. As the paper is stretched by means of the channeled core, it may also be thinner than the paper in the drain consisting only (as in FIG. 2) of cardboard. The capacity and hence the efficiency of the drain is increased hereby. The new drain of FIGS. 3 and 4 has two channel systems separated from each other, which makes it more reliable.

The drains according to the present invention may be manufactured in convenient widths adapted for the actual draining problem. The channel depth in the core may also be adapted to the dewatering demand. The distance between the drains is dependent on i.a. the character of the soil. For horizontal drainage the c/c distance is generally between 0.5 and 1.5 m.

Besides draining embankments for roads and the like, the drains are also suitable for draining around buildings and for draining of fields. On account of the high capacity and shape of the drain the advantage is obtained that the ditch, in which the drain is placed, may be made appreciably narrower than when using conventional drains. The filter layer makes it also unnecessary to have the same high requirements on the grain distribution of the material surrounding the drain as is the fact in connection with conventional drain pipes. The drain is practically not at all influenced by possible uneven settlements in the earth.

The invention is not limited to the embodiments shown but can be modified in different ways within the scope of the invention.

Claims

I claim:

1. A composite drain to be used in soil types having low water permeability, characterized in that it consists essentially of an inner core of water-impermeable material having wave-shaped surfaces and that the core on at least one surface is combined with a filtrating, water-permeable, sheetlike material, which maintains drainage channels formed between the core and the sheetlike material, said sheetlike material permitting the passage of water but preventing the passage of soil particles therethrough.

2. A composite drain according to claim 1, characterized in that the core comprises a strip-shaped, elongated material having wave-shaped surfaces on both sides.

3. A composite drain according to claim 1, characterized in that the core comprises a strip-shaped, elongated plastics material having wave-shaped surfaces on both sides, which both sides are combined with paper.