GB2338006A

GB2338006A - Consolidation of sedimentary beds

Info

Publication number: GB2338006A
Application number: GB9811872A
Authority: GB
Inventors: Gilliane Christine Sills
Original assignee: Oxford University Innovation Ltd
Current assignee: Oxford University Innovation Ltd
Priority date: 1998-06-02
Filing date: 1998-06-02
Publication date: 1999-12-08
Also published as: GB9811872D0; WO1999063164A1

Abstract

A water-containing sedimentary bed 1 is consolidated by adding a layer 3 on the top of the bed 1 and by pumping or draining the water from below. Layer 3 can be a sheet of non-woven textile, woven textile, plastic or geotextile, having a permeability which is low by comparison with that of the sediment bed 1. A considerable part of the head loss that occurs between the bottom of overlying water 5 and the base of the sediment bed 1 will occur through the low permeability layer 3. Thus, at the surface of the sediment 1, the pore water pressure u w will be considerably less than hydrostatic, and the effective stress considerably greater than zero. The bed surface will therefore consolidate and the effective stress gradient through the bed will be reduced.

Description

2338006 -I- A METHOD AND APPARATUS FOR CONSOLIDATION OF SEDIMENTARY BEDS

The present invention relates to the consolidation of sedimentary beds, in other words to the production of a consolidated layer of sediment, e.g. soil, from a suspension of sediment in water.

Operations such as dredging, oil extraction from oil sands, and other mining operations often result in the production of large quantities of water containing sedimentary material. Typically this is pumped or channelled into ponds where the sedimentary material settles out in time. However it can take many years for is the sediment to settle: twenty years is not uncommon, and sometimes the sediment and the depth are such that the sediment will not settle even in that time. This not only causes environmental problems, with society less willina to accept large areas of land being disfigured this way for so long, but also for economic reasons it helpful if the sediment settles and can be consolidated quicker because more material can then be added, or the water can be removed, the sedimentary bed compacted, and the land developed.

in is Similar problems arise when trying to prepare waterlogged areas of land for development.

one way of trying to accelerate the consolidation process is to add sand and gravel to the top of the bed to put a downwards loading on the sediment. However it is sometimes found that the sand and gravel simply sinks through the sediment, and thus does not load it. A more successful way is to arrange for water to be removed from the bottom of the sediment bed. Such removal can be by pumping (which has been used in the treatment of tailings ponds from oilsands mining) or by drainage. The theory behind these techniques can be explained as follows.

Consolidation in a sediment bed is achieved by an increase in the effective stress a',, which is defined as the difference between the total stress cF, applied to the bed and the pore water pressure u. The effective stress can be increased by an increase in the total stress (e.g. by laying sand or gravel over the bed) or a decrease in the pore water pressure, or both. The effect of these can be understood by simple analogy if one imacJ.nes squeezing a bag of crisps between the hands. The total stress is the pressure applied by the hands. When the bag is sealed the stress is resisted by the increased air pressure in the bag rather than the crisps. so the crisps are under relatively little stress (they can still rattle around inside the bag). But if one puts a pi, nhole in the bag to let the air leak out, so that the air pressure gradually becomes less than the applied stress (equivalent to letting the water drain out of the bed), gradually the crisps will be stressed and squeezed ther.

-oge- Thus, in a sediment bed, removing water from the bottom to achieve a suitable change in the pore water pressure is used, particularly in the case of a comparatively deep sediment bed. Physically one reason that removal of the water from the base of the bed results in quicker/increased consolidation is that the downwards movement of water through the sedimentary material creates a frictional draa on the sediment particles pulling them downwards.

3 5, In the accompanying drawings Figure 1 shows the total stress cy and the pore water pressure u, at the end of a period of consolidation under three different conditions.

Figure 1 (a) illustrates the situation where there is no base drainage. The pore water pressure, which is just the hydrostatic pressure, increases as a straight line with depth. The stress also increases with depth because of the weight (actually the buoyant weight) of the sediment, but in a slight curve because the density of the sediment increases towards the bottom of the bed. it can be seen that the distance between the two curves, which is the effective stress on the sediment, increases with depth.

Figure 1 (b) illustrates the situation with base drainage. Because the base is open the water pressure there is reduced giving an increased difference between the two curves and thus an increased effective stress and increased consolidation.

The effective stress can be further increased by reducing still further the pore water pressure at the base as shown in Figure 1 (c). This can be achieved by applying a negative head P' to the bottom of the bed, e.g. by pumping out the water.

It can be seen that in all three cases the effective stress increases steeply with depth into the bed, reaching a maximum at the base. This maximum value is determined by the pressure from the sedimentary column at the base and the overlying water level or imposed back pressure, so that it is possible to achieve quite high effective stress levels.

However, a significant drawback with these methods of inducing consolidation is that the effective stress level at the surface of the bed is always zero (in the figures the curves of pressure and stress always run together at the top of the sedimentary bed). This region therefore remains very soft. The soft surface means that it is difficult to get heavy equipment onto the bed to compact it for further use, e.g. for development. In addition, the higher the effective stress achieved at the base of the bed, the larger the effective stress gradient through the bed. Thus the bed is not uniformly consolidated.

Thus it is an object of the invention to provide a way of accelerating the consolidation process and/or achieving an increase in uniformity and in the consolidation of the ton of the sedimentary bed.

The present invention achieves this by adding a permeable laver on the top of the consolidating sediment, with this permeable layer having a permeability which is low by comparison with that of the sediment bed. A considerable part of the head loss that occurs between the bottom of the overlying water and the base of the sediment bed will occur through the low permeability layer. Thus, at the surface of the sediment, the pore water pressure will be considerably less than hydrostatic, and the effective stress considerably greater than zero. The bed surface will therefore consolidate and the effective stress gradient through the bed will be reduced.

According to the present invention, therefore, there is provided a method of consolidating a water-containing sedimentary bed comprising the steps: applying over the surface of 'the bed to be consolidated a low permeability laver, and removing water frorn below the bed to be consolidated.

Preferably the method includes the step of maintaining a layer of water overlying the low permeability layer, though the depth of this water layer need not be constant.

The permeability of the low permeability layer is preferably significantly lower than that of the sediment, more preferably two to three orders of magnitude lower.

The permeability of the low permeability layer is selected to be such that a significant proportion of the pressure drop from the bottom of the overlying water to the base of the sedimentary bed occurs in the low permeability layer, typically 20 to 60%, more preferabl 20-40%.

y Preferably the low permeability layer is a sheet material, for instance of textile (woven or non-woven) or permeable plastics sheet.

The method may include the step of pumping water from the base of the sedimentary layer. Water may be pumped onto the top of the low permeability layer to increase the water pressure thereon.

The method may further include as a preliminary step the laying down of a drainage layer of sand and/or gravel and/or textile onto which the sedimentary layer is deposited. This can facilitate the removal of water from the bed.

After consolidation using the method above, the low permeability layer may be removed and the bed further compacted using compaction equipment.

The invention also provides a sheet material for use in the above method, and having a permeability selected according to the nature of the bed to be consolidated.

The invention will be further described by way of nonlimitative example with reference to the accompanying drawings, in which:- is Figure 1(a) to (c) illustrate the pressure and stress in a sedimentary bed after consolidation according to three prior art methods;

Fiaure 2 (a) is a schematic diagram of the present invention; and Figure 2(b) illustrates the pressure and stress in a sedimentary bed after consolidation according to the present invention.

Fiaure 2(a) illustrates an embodiment of the invention in which a sedimentary layer 1 has overlying it a low permeability layer 3, in this case of thick canvas, and a layer of water 5. It should be noted that Figure 2 illustrates the situation after considerable consolidation has taken place. It should also be noted that, although not illustrated, arrangements are made to remove water from the bottom of the bed, either by puriping or by free drainage.

Ficrure 2(b) illustrates the pore water pressure U, and L-he total stress cy in the bed with the low permeability layer of the invention. Because the permeability of the aver is less than that of the sediment 1 a sianif.'-can-L is pressure drop occurs as the water moves downwards through the low permeability layer. This can be seen in the sharp deviation of the pressure curve marked 7. Thus at the bottom of the low permeability layer (which is the top of the sediment) the pressure is considerably less than it would be without the layer 3 and thus much less than it is in the prior art of Figure 1 (c). This means that the effective stress, and thus the soil consolidation, is much greater than zero there is a large difference between the curves of u and a at point 9 in Figure 2(b). It can be seen that the pressure drop through the low permeability layer is about 30% of the total drop from the bottom of the water 5 to the base of the sediment bed 1.

Of course for the low permeability layer other materials than canvas can be used; the requirement is for a material with the desired permeability two to three orders of magnitude less than that of the sediment. The permeability of sediments has a large range - five to six orders of magnitude. For instance, in Darcy's flow equation v = -ki where v is the flow velocity, 1 is the hydraulic gradient and k is the water permeability of the sediment, the range of sediment permeability is from 1 x 1W' m/s for very soft river mud to 1 x 10 m/s for cilsands fine tailings. Thus the material for the layer 3 must be selected having regard to the application. it is useful, though, to provide a range of materials which can be chosen for typical applications.

As illustrated in Figure 2(b) a negative head P' is applied at the bottom of the bed. However, if the depth of water above the low permeability layer 3 is increased, the curves on Figure 2(b) move to the right, while not changing shape, so this reduces the amount of negative head P' required and thus reduces the pumping requirements. Thus maintaining a high depth of water!aver 5 can be advantageous.

The time taken to achieve a given degree of consolidation depends on the nature of the sediment, for instance some give off gases which slows consolidation. However for a sedimentary bed of, say, 5 metres in depth, consolidation takes months to years. Nevertheless this is quicker than with prior art methods and it should be remembered that with the invention consolidation is achieved at the top of the sediment, which it is not with the prior art. This allows earth moving/ compacti on equipment to be moved onto the site to prepare it for use. Also the stress gradient- through the sediment is much less than with the p-rior art giving a greater uniformity of consolidation.

The invention can be applied to sites for disposal of dredging waste or similar material. To prepare such a site to receive material a drainage layer of sand and/or jravel is la'Ld to facilitate water removal, before the,..jaste is deposited and the low permeability layer 3 is subsequently laid over the top when the bed of waste is partly or wholly consolidated.

Thus in a typical site the invention is applied by first preparing the site to receive the material to be consolidated. This may involve laying down a drainage layer as ment-Loned, and/or arranging for pumping of water ir. and out of the site. Arranging for controlled ingress or egress of water from a nearly river or sea can also be used in some situations. Then the waste material (soil slurry) is pumped into the side for as long as desired. When the desired amount has been pumped in, the low permeabillity layer 3 is laid over the site. The material for the layer is chosen, as described above, by reference to the permeability of the sediment. Further water can be pumped/allowed in over the top of the layer 3 if desired, and water is allowed to drain, or is pumped, from the base of the bed. After sufficient consolidation has been achieved, which could be on timescales of months or years, the low permeability layer 3 can be removed.

Claims

1. A method of consolidating a watercontaining sedimentary bed comprising the steps of: applying over the surface of the bed to be consolidated a low water permeability layer, and removing water from below the bed to be consolidated.

2. A method according to claim 1 further comprising the step of maintaining a layer of water overlying the low permeability layer.

3C

3 -r- 3. A method according to claim 1 or 2 wherein the permeability of the layer is significantly lower than that of the sediment.

4. P. method according to claim 3 wherein the permeabilit of the low permeability layer is two to three orders of maanitude lower than that of the sedimentary bed.

5. A method according to claim 1, 2, 3, or 4 wherein the permeability of the low permeability layer is selected to be such that a significant proportion of the pressure drop from the top of the low permeability layer to the base of the sedimentary bed occurs in the low permeability layer.

6. A method according to claim 5 wherein the proportion of the pressure drop which occurs in the low permeability layer is 20 to 605---' of the total pressure drop from the top of the low permeability layer to the base of the sedimentary bed.

7. A method according to any one of the preceding claims whe-rein the low permeability layer is a sheet material.

8. A method according to any one of the preceding claims wherein the low permeability layer is selected from a non-woven textile, a woven textile and a permeable plastics sheet.

9. A method according to any one of the preceding claims further including the step of pumping water from the base of the sedimentary layer.

10. A method according to any one of the preceding claims further including the step of pumping water onto the top of the low permeability layer to increase the pressure thereon.

11. A method according to any one of the preceding claims further including as a preliminary step the step of laying down of a drainage layer of sand and/or gravel onto which the sedimentary layer is deposited.

12. A method according to any one of the preceding claims further including the step, subsequent to consolidation of the sedimentary bed, of compacting the bed using compaction equipment with or w-Lthout removing the low permeability layer.

13. A sheet material for use in the method according to any one of the preceding claims.

14. A sheet material according to claim 13, comprising a geotextile of low water permeability.

15. A method of consolidating a water-containing sedimentary bed substantially as hereinbefore described with reference to Figures 2(a) and (b) of the accompanying drawings.