The present invention relates to a method and apparatus for simulating the behaviour in the field of a capping for a land waste disposal site. In particular, the invention relates to simulating the subsidence of the waste material disposed in a trench beneath the capping.

It is now generally recognised that landfill waste disposal facilities must be properly engineered in order to minimise the release of hazardous substances into the environment. The two most likely pathways by which contamination might spread are the escape of leachate into the groundwater, and gas into the atmosphere. A land waste disposal site may use a combination of mineral and geome brane cut-off walls, engineered drainage and gas venting systems and a capping to control the movement of fluids into and out of the facility. An essential requirement is the provision of a capping having at least one water barrier layer formed from a low permeability material so as to limit the inflow of water, due to rainfall, into the facility.

While the appropriate combination of control and containment systems may depend on the conditions under which the waste disposal site is intended to operate, it is important that the extent to which the capping can maintain its efficiency throughout its design life is known. Subsidence due to biodegradation of the waste material beneath the capping, or a reduction in the water content of the low permeability layer, are potentially damaging occurrences. Thus, there is a need to be able to predict the likely effects of such events on the performance of the capping over a long period of time.

According to a first aspect of the invention there is provided a method for simulating the behaviour under subsidence conditions of a capping covering waste material in a land waste disposal site, the method comprising the steps of locating a scale model strip of the capping in a

casing having a fixed member and a movable member arranged therein, the strip of capping being positioned so that a first portion thereof extends over the fixed member and a second portion thereof extends over the movable member, placing the casing on a rotatable member and rotating said rotatable member so as to subject the strip of capping to a centrifugal effect, and moving said movable member so as to obtain relative movement between said first and second portions of the strip of capping whereby to simulate subsidence of the waste material beneath the capping.

The movable member may be moved between an initial position and a final position in a series of steps. Alternatively, the movable member may be moved continuously between the initial and final positions.

Preferably, the method further includes the step of measuring the displacement of the strip of capping caused by the relative movement between the first and second portions of the strip of capping.

Water may be sprayed over the strip of capping so as to simulate rainfall.

The method preferably includes the step of measuring the amount of water drained through the strip of capping. According to a further aspect of the invention there is provided apparatus for simulating the behaviour under subsidence conditions of a capping for covering waste material in a land waste disposal site, the apparatus comprising a rotatable member, a casing mounted on said rotatable member, a fixed member located in said casing for supporting a first portion of a scale model strip of the capping, a movable member located in said casing for supporting a second portion of the strip of capping, drive means for rotating said rotatable member so as to subject the strip of capping to a centrifugal effect, and means for moving said movable member so as to obtain relative movement between the first and second portions of the

strip of capping whereby to simulate the subsidence of the waste material beneath the capping.

Preferably, the movable member has a capping support surface which, in an initial position of said movable itu.mber, lies in a plane containing or substantially containing a capping support surface of the fixed member.

The capping support surface of the movable member may, after movement thereof to a final position, lie in a plane parallel to or substantially parallel to a plane containing the capping support surface of the fixed member.

Alternatively, the movable member may be pivotably mounted at an end thereof adjacent to the fixed member so that the capping support surface of the movable member, after movement thereof to a final position, lies in a plane which is inclined with respect to the capping support surface of the fixed member.

Preferably, the movable member is connected to an actuator, the actuator being operated so as to move the movable member from said initial position to the final position in a series of steps.

Alternatively, the actuator may be operated so as to move the movable member continuously from said initial position to said final position.

The actuator preferably comprises an hydraulically- operated cylinder and piston assembly.

The casing may by equipped with measuring means for measuring the displacement of the strip of capping caused by the relative movement between the first and second portions of the strip of capping.

Suitably, the measuring means comprise a plurality of displacement transducers arranged at spaced intervals along the strip of capping, the transducers being operable to emit an electrical signal having a value related to the corresponding displacement.

Nozzle means may be provided within the casing for spraying water on the strip of capping.

Preferably, the strip of capping comprises a low permeability water barrier layer.

The water barrier layer preferably comprises a material formed from a mixture of sand and bentonite.

The strip of capping may further comprise an upper layer of granular material arranged above the water barrier layer and a lower layer of granular material arranged below the water barrier layer.

Water collection means are preferably provided for collecting water drained from the strip of capping.

The water collection means may include a collection tank located outside the casing and in communication with a region located beneath the strip of capping so as to receive water therefrom.

In addition, the water collection means includes a aperture arranged within the casing to collect water moving through the upper layer of granular material, and a further collection tank located outside the casing and in communication with the aperture so as to receive water therefrom.

Preferably, means are provided for measuring the amount of water received by said collection tank.

The measuring means may comprise a float supported by the water in the collection tank, the float being αperatively connected to a displacement transducer which emits an electrical signal having a value related to the displacement of the float.

The effects of long-term environmental changes on a capping for covering waste material in a land disposal site is investigated in accordance with the present invention by accelerated testing using a physical modelling technique. This technique is carried out by subjecting an idealised capping to an enhanced gravity level in a centrifuge. For true similarity of the capping

characteristics a 1 : n scale model is tested in the centrifuge at a gravity level of n times the normal gravity.

Referring to Figure 1, a geotechnical centrifuge 1 of known type is provided with an enclosure 2 which incorporates a power supply cabinet 3 at one end and an instrumentation cabinet 4 at the other end. A conical base pedestal 5, housed within the enclosure 2, supports a pair of parallel arms 6 which form a centrifuge rotor arranged for rotation about a vertical axis. Rotational drive for the arms 6 is derived from an electric motor 7 through a reduction gearbox 8 and a vertical drive shaft 9. A cross piece 10 connects the drive shaft 9 to the arms 6.

Suspended from one end of the arms 6 is a swinging platform 11 which is pivotably connected to the arms 6 by means of suspension links 12. Supported on the platform 11 is a water-tight casing 13 in which is formed a scale model of a strip of capping for a land waste disposal site and which is to be subjected to an accelerated testing procedure, as hereinafter described. Two catch tanks 14, 15 are supported on either side of the casing 13 for collecting and measuring water drained from the casing. On the ends of the arms 6 remote from the platform 11 are two counterweights 16 which can be adjusted to a suitable location on the arms to counterbalance the masses of the platform 11 and casing 13 during operation of the centrifuge 1. Various electrical and hydraulic services to and from the casing 13 are routed through a slip ring assembly 17.

The casing 13, see Figure 2, is of parallelepiped form and houses a fixed spacer 18 which is supported on a base 20 of the casing. The spacer 18 has a horizontal upper surface 19 and extends from a side wall 21 to approximately halfway along the base and to a height just below half the height of the wall 21. Adjacent to the

spacer 18 and secured to the base 20 is a double-acting piston and cylinder assembly 22. Supported on a piston rod 23 extending upwardly from the cylinder assembly 22 is a movable member in the form of a platen 24 which extends from a side surface of the spacer 18 to a side wall 25 of the casing 13. The platen 24 has an upper surface 26 which, in an upper initial position of the platen, forms a horizontal surface which is co-planar with the upper surface 19 of the fixed spacer 18. Extending through the platen 24 are a plurality of holes 27, the purpose of which will be hereinafter described. Below the platen 24 is a reservoir chamber 28.

Supported on the upper surfaces 19, 26 of the spacer 18 and the platen 24, respectively, is a scale model strip of capping 29. The strip of capping 29 represents an edge portion of a capping for covering waste material in a land disposal trench and is to be subjected to a test procedure for simulating long-term environmental changes. It will be apparent that the strip of capping can be of various forms depending on the particular characteristics to be investigated. In this particular embodiment, the capping 29 comprises a granular overburden layer 30, a low permeability intermediate layer 31, representing a water barrier layer, and a granular fill layer 32. The fill layer 32, which may comprise 18/25 Leighton Buzzard sand, is placed in the casing 13 on the surfaces 19, 26 and finished to the required profile using a scraper and a former. The profile of the fill layer 32 comprises a horizontal portion overlying the spacer 18 and an inclined upper surface in the region above the platen 24. Suitably, the barrier layer 31 is formed from an engineered soil comprising a mixture of sand and bentonite. Preparation of the engineered soil preferably includes mixing fine and coarse sands with water and then adding to this mixture a separately prepared mixture of calcium mont orillonite and water. The engineered soil is

compacted in a mould, at a moisture content in the range of 26 to 28%, into a form as shown in Figure 2. Thus, the barrier layer 31 is of uniform thickness having a horizontal portion 33 extending from the casing side wall 21 to a location above the platen 24 just beyond the edge of the spacer 18. Above the platen 24, the barrier layer 31 has an inclined portion 34 which subtends an angle of about 14° with respect to the platen surface 26. The barrier layer 31 has a further inclined portion 35 adjacent to the casing side wall 25, this further portion 35 having an angle of inclination with respect to the platen surface 26 which is less than that of the inclined portion 34. After placing the barrier layer 31 in the casing 13, a small fillet of engineered soil is placed around the edge of the casing to prevent leakage at the soil/casing interface.

The overburden layer 30, which may comprise 18/25 Leighton Buzzard sand, is then placed over the barrier layer 33 and profiled using a scraper and former as for the fill layer 32. The overburden layer 30 is of uniform thickness and generally follows the form of the water barrier layer 33. Typically, the mass density of the overburden and fill layers may be approximately 1.5 Mg/m ³ .

The strip of capping under test has a thickness of 75 mm at the portion supported by the spacer 18, the water barrier layer having a uniform thickness of 15 mm. At a 1 : 40 scale, these dimensions represent field dimensions of 3m and 0.6m, respectively. The overburden layer 30 has a uniform thickness of 45mm, representing a field dimension of 1.8m.

Rainfall is simulated by spraying water from a spray nozzle 36 which extends into the casing 13 at a location above the strip of capping 29. Water moving through the overburden layer 30 drains into an aperture 37 which communicates with the catch tank 15 through a drain pipe 38. Measurement of water draining into the catch tank 15

is achieved by means of a float 39 whose movement is sensed by a displacement transducer 40. Similarly, water moving through all the layers of the capping strip 29 drains through the holes 27 in the platen 24 into the chamber 28. The chamber 28 communicates with the catch tank 14 by means of a drain pipe 41. Measurement of water draining into the catch tank 14 is achieved by means of a float 42 whose movement is sensed by a displacement transducer 43.

Movement of the capping strip 29 is detected by a series of displacement transducers 44a, 44b, 44c, 44d, 44e, 44f and 44g. The transducers 44a, 44b, 44c and 44d are associated with the sloping portion of the capping strip 29 and the transducers 44e, 44f and 44g are associated with the horizontal portion of the capping strip.

In carrying out a test for simulating the behaviour of the capping strip 29 while under changing environmental conditions, the casing 13 and the catch tanks 14, 15 are mounted on the platform 11. The counterweights 16 are moved to the desired position on the arms 6 to ensure that the masses are balanced when the arms are rotated. The motor 7 is operated so as to rotate the arms 6, by transmission through the gearbox 8 and the drive shaft 9, to obtain a centrifugal acceleration of 40g. Under these conditions, the platform 11 and the casing 13 pivot about the arms 6 through approximately 90° to the position shown in chain-dot lines in Figure 1. The rate of water supplied through the nozzle 36 is selected so as to simulate a rainfall which may be experienced in the field. Typically, the rate of water may be approximately 0.04 1/min, which corresponds to an actual rainfall of 0.55 mm/hr at a scale of 1 : 40.

Under field conditions, biodegradation of the waste material covered by a capping wi 1 r° ult in subsidence. In this testing procedure subsidence is simulated by

operating the cylinder and piston assembly 22 so that the platen 24 is lowered from its upper initial position in a st.ries of 5mm steps. The steps are continued until the platen 24 has been lowered to a final position at a maximum depth of 50mm over a period of about 10 minutes. To simulate slower degradation of the waste material, the platen 24 may be lowered to the maximum depth over a longer period of time. This represents a drop of 2m in the field. During each step the platen surface 26 lies in a plane which is parallel to, or substantially parallel to, a plane containing the surface 19 of the fixed spacer 18. The displacement of the platen 24 is measured by viewing the platen relative to a scale (not shown) engraved on the fixed member 18 by means of a closed circuit television arrangement and an associated computer measuring system. Changes on the capping strip 29 are monitored by the displacement transducers 44a 44g, each of which emits an electrical signal having a voltage corresponding in value to the amount of capping strip displacement. These signals are transmitted through the slip ring assembly 17 and recorded and analysed by instrumentation of known type.

Water passing through the overburden layer 30 drains into the catch tank 15 through the aperture 37 and the drain pipe 38. A measurement of the amount of water drained from above the barrier layer 31 is represented by the upward displacement of the float 39. This displacement is sensed by the displacement transducer 40 which emits an electrical signal having a voltage value corresponding to the displacement. The signal is transmitted through the slip ring assembly 17 to analytical instrumentation of known type. Water passing through the barrier layer 31 drains through the fill layer 32 and through the holes 27 in the platen 24 into the chamber 28. From this chamber 28 the water drains into the catch tank 14 through the drain pipe 41. Upward

movement of the float 42 upon accumulation of water within the catch tank 14 is sensed by the displacement transducer 43. A corresponding electrical signal is transmitted to the analytical instrumentation through the slip ring assembly 17. It will be apparent that water movement through the entire capping strip 29 is only of significant volume following the development of a crack or rupture in the barrier layer 31 during the simulated subsidence of the waste material.

The results obtained in the above test procedure indicated that a rupture occurred in the barrier layer 31 at a relative displacement at field scale of 0.88m.

The foregoing embodiment of the invention, in which a step change in level between the moving platen 24 and the fixed spacer 18 is imposed, represents a probable worst case, and would be applicable to the edge portion of a disposal trench having a vertical retaining wall around its perimeter.

In Figure 3 a further embodiment of the invention is shown in which a ramp subsidence of the waste material is simulated. Reference numbers used in the above described embodiment are used in the following description to represent similar components. The casing 13 houses a fixed spacer 18 having a horizontal upper surface 19 and supported on the base 20 of the casing. Adjacent to the spacer 18 is a double-acting cylinder and piston assembly

22 which is secured to the casing base 20. A piston rod

23 extends upwardly from the cylinder assembly 22 and has a horizontal platen 24 mounted on its upper end. A reservoir chamber 28 is provided below the platen 24.

Pivotably attached by a pivot pin 45 to an upper inner corner of the spacer 18 is a ramp 46. The ramp 46 has an upper planar surface 47 and is provided with a plurality of drain holes 48. An end of the ramp 46 remote from the pivot pin 45 rests on the upper surface 26 of the platen 24. Mounted on the platen 24 and extending between the

end of the ramp 46 and the casing side 25 is a capping support plate 49 which is provided with a plurality of drain holes 50. A portion of the capping strip 29 is supported on an upper horizontal surface 51 of the plate 49.

The capping strip 29 has a granular overburden layer 30, a low permeability barrier layer 31 and a granular fill layer 32. It is set up in a similar manner to that previously described with reference to Figure 2. At the commencement of the test, the platen 24 is in its upper initial position, so that the upper surfaces 19, 47 and 51 of the spacer 18, ramp 46 and plate 49, respectively, all lie in a common horizontal plane.

A spray nozzle 36 is provided within the casing 13 for simulating the rainfall. Water moving through the overburden layer 30 drains into the aperture 37 which communicates with the catch tank 15 through the drain pipe 38. Water passing through the barrier layer 31 and the fill layer 32 drains through the holes 48 and 50 of the ramp 46 and the support plate, respectively. This water drains through the holes 27 provided in the platen 24 into the chamber 28 formed below the platen. From the chamber 28 the water flows into the catch tank 14 through the drain pipe 41. Each of the catch tanks 14, 15 is provided with a float connected to a displacement transducer 43, 40, respectively, for measuring the amount of water moving through the capping strip 29. A series of displacement transducers 44a 44g are provided for monitoring the movement of the capping strip 29 during the test procedure.

The casing 13 and the catch tanks 14, 15 are mounted on the platform 11, as shown in Figure 1, and the test procedure is carried out as described previously. To simulate subsidence of the waste material beneath a capping, the cylinder and piston assembly 22 is operated so that the platen 24 is lowered in steps over a period of

approximately 10 minutes to a final position, at which the slope angle A of the ramp 46 is about 14° with respect to a plane containing the surface 19, as shown in Figure 3. Again, to simulate slower degradation of the waste material, a slower descent of the ramp 46 could be imposed. This corresponds to a settlement of 25% of the thickness of the waste material and a slope of 45° of the disposal trench side. The horizontal extent of the slope is approximately 200mm, which corresponds to a trench depth of 8m at field scale.

Changes in the level of the capping strip 29 are represented by the voltage value of the electrical signals emitted by the displacement transducers 44a 44g.

The amount of water which drains into the catch tanks 14, 15 is represented by the current values of the electrical signals emitted by the transducers 43, 40 associated with the catch tanks, respectively. The results obtained from the test indicated that the barrier layer 31 did not fail.

In either of the embodiments described above it may be desirable to operate the cylinder and piston assembly 22 so that the platen 24 moves from the upper initial to the final lower position smoothly and continuously, rather than stepwise. This enables certain potential environmental conditions to be simulated.