WO1996031660A1

WO1996031660A1 - Underground drain system

Info

Publication number: WO1996031660A1
Application number: PCT/GB1996/000681
Authority: WO
Inventors: Stephen Geoffrey Higson
Original assignee: British Nuclear Fuels Plc
Priority date: 1995-04-06
Filing date: 1996-03-22
Publication date: 1996-10-10
Also published as: AU5151196A; GB9507106D0

Abstract

An underground cut-off drain system (1) for controlling liquids flowing in the ground in the vicinity of hazardous material (2), for example, radioactive waste, comprises a vertical band drain (7) and at least one filter layer (8, 9). The band drain and the filter layer are formed from aggregate material, typically gravel. The average particle size of the aggregate material forming the filter layer is smaller than that forming the band drain. Preferably a filter layer is disposed along each side of the band drain. Filtrate passing through the band drain (7) may be conducted to a controlled drainage arrangement (11) or to an aquifer.

Description

Underground Drain System

The present invention relates to an underground drain system. In particular, the invention relates to a cut-off drain system for use in controlling underground liquids, such as leachate and groundwater, flowing in the vicinity of contaminated or hazardous material.

Leachate emanating from an underground contaminated landfill or other hazardous material can be prevented from leaking into the surrounding ground by a vertical barrier known as a cut-off wall. Cut-off walls may also be used to prevent the ingress of groundwater into the landfill where the natural water table is high. The cut-off walls are usually made from cement-bentonite or clay-bentonite materials and, where it is necessary to prevent migration of gas, may incorporate a geomembrane. However, there is considerable uncertainty concerning the long-term durability of cement-bentonite cut-off wall systems, particularly with respect to the formation of cracks. Since cement-bentonite cut-off walls represent a relatively recent technology, their effectiveness over a long period of time under field conditions is uncertain.

According to the present invention there is provided an underground cut-off drain system for controlling liquids flowing in the ground in the vicinity of a hazardous material, the cut-off drain system comprising a substantially vertical band drain formed from aggregate material, and a substantially vertical filter layer disposed along at least one side of the band drain, the filter layer being formed from aggregate material having an average particle size which is smaller than the average particle size of the aggregate material forming the band drain.

Preferably, the filter layer is disposed along a side of the band drain facing the hazardous material.

In a preferred embodiment a filter layer is provided along each side of the band drain. Preferably, the band drain extends to a greater depth than that of the filter layer or layers.

The aggregate material for the band drain and the filter material may comprise gravel.

Advantageously, means are provided for conducting a filtrate passing from the band drain to a controlled drainage arrangement or an aquifer.

The cut-off drain system may form part of a waste disposal site for the burial of waste hazardous material, the site comprising a depression formed in the ground for reception and storage of the waste hazardous material, the cut-off drain system being disposed so as to intercept leachate emanating from said material.

The hazardous material may comprise low level radioactive waste.

According to the present invention in a second aspect there is provided a method of burying hazardous waste material which includes the steps of forming a cut-off drain system having a construction as defined above, excavating a depression in the ground in the vicinity of said cut-off drain system and filling the depression with said hazardous waste material.

In order than the invention may be clearly understood one embodiment will now be described, by way of example, with reference to the accompanying single figure which is a diagrammatic half cross-section through a waste disposal site incorporating a barrier system according to a preferred aspect of the invention.

Referring to the figure, a waste disposal site 1 is shown in which waste material 2 is disposed below the ground level 3 in a depression in the form an excavated repository 4. The repository 4 is covered by a suitable capping 5. The waste material 2 may be of a hazardous or toxic nature, such as low level radioactive waste.

Surrounding the repository 4 is a drain system in h form of a cut-off drain 6 extending vertically downwards from the ground level 3 to below the lower level of the waste material 2. The cut-off drain 6 comprises a vertical band drain 7 having a filter layer 8 disposed along a side thereof facing the waste material 2 and a filter layer 9 disposed along a side remote from the waste material. The band drain 7 extends to a greater depth than the depth of the two filter layers 8, 9. Preferably the band drain 7 communicates by means" of at least one drain pipe 10 with a secondary, controlled drainage arrangement, generally indicated by reference numeral 11. In certain situations the band drain layer may be connected to an underlying regional aquifer by means of a continuous trench drain or boreholes disposed at suitable intervals.

The media forming the filter layers 8, 9 and the band drain layer 7 are preferably aggregates comprising a gravel, which may consist predominantly of pebbles. For the filter layers 8, 9 the particle size selected for the gravel will depend on the particle size of the natural soil surrounding the waste disposal site. Normally, the filter layers will serve to protect the band drain layer 7 from the ingress of the natural soil particles and therefore the particle size of the filter media will be larger than that of the surrounding natural soil, but smaller than that of the band drain. However, if it is also necessary to protect the drainage system from suspended solids in the leachate which are capable of being transmitted through the natural soil, then the particle size of the filter medium may need to be smaller than that of the soil. Typically, the average particle size for the filter media may be within the range of 2mm to 60mm, the average particle size for the band drain being larger than that selected for the filter layers. A suitable cut-off drain may comprise, for example, a band drain 7 formed from a gravel having a nominal particle size of 20 mm and the filter layers 8, 9 formed from a gravel having a nominal particle size of 10 mm.

It will be appreciated that the particle sizes given above are typical only and, in practice, will be selected to suit a particular environment or application in which the cut-off drain system is used.

In use, leachate emanating from the waste material 2 is intercepted by the cut-off drain 6"aήd thereby prevented from leaking into the surrounding soil. Suspended solids in the leachate are removed by the filter layer 8. Leachate filtrate passes down the band drain 7 and is then conducted to the controlled drainage arrangement 11 by means of the drain pipes 10. In addition to intercepting leachate, the cut-off drain 6 also prevents the ingress of groundwater into the waste material 2. Suspended solids larger than a certain size in the groundwater are removed by the filter layer 9, the filtrate passing down the band drain 7 and then conducted to the controlled drainage arrangement 11. Thus, the filter layers 8, 9 serve to prevent clogging of the band drain 7. After a period of time, the filter layers 8, 9 may become blocked. This would be acceptable, since the cut-off drain would then act as a low permeability barrier, similar to a conventional cut-off wall.

In certain locations where the prevailing groundwater level is low, the filter layer 9 on the side of the band drain layer 7 remote from the waste material 2 may be dispensed with.

The cut-off drain system can be used in situations other than waste landfill sites where it is desirable to prevent leakage of leachate from a hazardous material into the surrounding soil or to prevent the ingress of groundwater into the hazardous material.

Claims

1. An underground cut-off drain system for controlling liquids flowing in the ground in the vicinity of a hazardous material, the drain system comprising a substantially vertical band drain formed from an aggregate material, and a substantially vertical filter layer disposed along at least one side of the band drain, the filter layer being formed from an aggregate material having an average particle size which is smaller than the average particle size of the aggregate material forming the band drain.

2. An underground cut-off drain system according to Claim

1, wherein the filter layer is disposed along a side of the band drain f cing the hazardous material .

3. An underground cut-off drain system according to Claim

2, wherein a further filter layer is disposed along a side of the band drain remote from the hazardous material .

4. An underground cut-off drain system according to any one of the preceding Claims, wherein the band drain extends to a greater depth, than that of the filter layer or layers.

5. An underground cut-off drain system according to any one of the preceding Claims, wherein the aggregate material comprises gravel.

6. An underground cut-off drain system according to any one of the preceding Claims, wherein means are provided for conducting a filtrate passing from the band drain to a controlled drainage arrangement or an aquifer.

7. An underground cut-off drain system according to any one of the preceding Claims, wherein the drain system forms part of a waste disposal site for the burial of hazardous waste material, the site comprising a depression formed in the ground for the reception and storage of the waste material, the drain system being disposed so as to intercept leachate emanating from said waste material.

8. A method of burying hazardous waste material comprising the steps of forming an underground cut-off drain system having a construction as defined in any one of the Claims 1 to 7, excavating a depression in the ground in the vicinity of said cut-off drain system and filling the depression with said hazardous waste material