Jurassic Coast - Chiswell5 Drain

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Chiswell case study: The Scheme

The interceptor drain during construction, summer 1986. The section through to the Cove Inn (near the crane) has already been buried

The shingle lies on an impermeable layer. The interceptor drain was located at the boundary, thereby ensuring that the water was collected

Pile driving along the line of the drain in February 1996

Excavation along the line of the piling which supported the culvert during construction. The piling was later cut off at the drain level, but remained below it within the beach.

Precast concrete sections were lowered into the ditch

To the north, the shingle was simply dug out and the concrete sections laid and then buried. The 'windows' on the right were filled with gabion baskets, allowing the water to drain through the beach and into the culvert

The interceptor drain outfall under construction. The structure leading away from the concrete sections is composed of gabion baskets

Interceptor drain

37. The consultants proposal to intercept the seawater which percolates through Chesil Beach under certain storm conditions, the main cause of flooding in the village of Chiswell, was the most vital part of the Sea Defence Scheme. Separate studies and investigations were necessary before a design could be implemented. The investigations undertaken yielded information on the permeability of the Beach, and these data were used by Hydraulics Research to construct a model of the Beach, from which the required capacity of the drain could be determined. The discovery of an interface between a relatively impermeable material comprising single-sized round pebbles in a sand matrix and the overlying shingle at certain levels within the Beach was a major step towards understanding and solving the flooding problem.

38. The ensuing studies established design parameters for the drain, the size and invert level of the culvert, and the depth at which percolation water should be intercepted. Water that percolates through the beach shingle flows into the drain through large openings or 'windows' along one side and in the roof of the drain. To prevent beach material entering the drain the openings are covered with BRC gabions filled with beach material. The interface between the sand matrix and the shingle dictates the level of the sills of the side 'windows'. Where the interface occurred above the formation level of the drain, it was decided that the gabions lined with Terram fabric should be filled with material with the same permeability as the sand matrix in order to avoid both migration of the fines and the risk of creating unstable conditions in the Beach. [See World Heritage Nomination, Coastal Processes, for an explaination of the posible formation of the beach].

39. Boreholes driven before the permeability studies had been carried out were taken to the underlying Kimmeridge clay at several locations, and indicated that cut-off could be achieved with interlocking steel sheet piles about 15.5 m long. A borehole investigation to confirm clay depths at closer intervals along the line of the drain was carried out under the main contract.

40. The configuration of the Beach and access thereto dictated the type of construction and alignment of the drain. A length of some 500 m through the built-up area required culverting, while the remaining length could be an open channel discharging under gravity into Portland Harbour after passing through a culvert under the Weymouth Road.

41. The economics of restricting the number of different cross-sections of the drain to a minimum and the need to maintain a consistent invert level of the drain led to the adoption of a standard internal height of drain of 3 m, the width increasing in steps of 1 m, 2 m and 3 m. The drain was designed to be in precast reinforced concrete to ensure quality control and to offset the difficulties of remote batching plants and a restricted site area. In-situ reinforced concrete transitions were constructed at changes in width and direction of the drain. All structural concrete was grade 40 and minimum cover to reinforcement was 100 mm.

42. The alignment of the open channel necessitated a side discharge type outfall from the drain. In order to avoid the removal of fines from the Beach as a result of the high velocities which might accompany rapid drawdown of water in the Beach, the culvert was designed to flow about half-full by discharging over a weired outfall. Pipes below the weir at invert level allow the culvert to drain down.

43. The existing culvert under the Weymouth Road had a capacity of 35 cubic metres/second compared with the assumed flow of 50 cubic metres/second . Fortunately, the invert level of the existing three 3 m wide x 1 m high culvert sections permitted sufficient clearance for a second and higher level of similarly sized culverts to be installed above, a solution which was both practical and economic.

44. The open ditch is mainly unrevetted with slack side slopes of 1 to 4. However, two lengths of the ditch required revetments: firstly, between the outfall structure and open channel section, where excessive turbulence could develop; secondly at the approach to the culverts under the Weymouth Road where a transition incorporates a stepped structure to minimise the lengths of channel affected by normal tide levels in Portland Harbour. In both these locations the use of BRC galvanised wire gabion mesh has been favoured on environmental, aesthetic and cost grounds, the channel bed being protected by Maccaferri PVC coated wire mesh mattresses of greater flexibility.

45. The contract for the construction of the interceptor drain was let to Dean and Dyball as a variation of price (VOP) contract, with a value of £1.7 million in April 1985. The further borehole investigation consisting of ten borings revealed clay underlying a thin strata of weak limestone. A statistical analysis of the varying depth at which clay was found resulted-in the length of the cut-of steel sheet piles being revised to 18 m.

46. It was decided to drive a trial panel of piles to ensure that the pile section required for the cut off was the most economic in terms of driveability and cost of material. The pile driving was troublesome. A range of British Steel's newly introduced W section piles was specified, but driving them to the depth required with any combination of hydraulic vibrating hammer and diesel hammers proved impossible. After driving many different pile types in the trial, the Larssen No 3 pile was selected. The piling of the cut off proceeded in the light of the experience gained in the trial, part driving with a vibratory hammer and completing with a diesel hammer. Increasing damage to the piles caused by the diesel hammer led to an experiment with a BSP HH 357 hydraulic hammer which was subsequently used throughout.

47. The residents of Chiswell-who understood that, if successful, the works would protect them from frequent flooding-accepted stoically the inconvenience of the contracting operations. The assistance of the Borough Council Environmental Health Officer was invaluable to the progress of the works.

48. In order to maintain the Beach profile, a row of temporary sheet piles was installed seaward of the permanent piles to support the excavation for the culverts and the gabions. Generally, the temporary piles were 8 m long (section 16W) and driven individually, using the contractor's own hydraulic, vibrating hammer (MGF) which was mounted on a modified Poclain 160 tracked excavator. The depth of excavation was closely controlled to ensure no overdig of the beach material.

49. The precast concrete units were lowered initially on to a prepared sand cement bed and were eased into position by means of winches, alignment being controlled by theodolite and level. The contractor had attempted the use of a laser for alignment purposes, but frequent disturbance of the equipment, necessitating remedial measures, led to the abandonment of the hi-tech approach.

50 The gabion boxes were assembled in groups of three or four units and, with the assistance of an excavator were manoeuvred into position. The boxes were then filled and the lids secured before the gabions were placed and stitched to the next level. The gabions, except for the lowest level ones, were lined with Netlon CE121 mesh to prevent loss of smaller material from the box. The lowest level gabions were lined with Terram before being filled with graded fill material of specified permeability. The procurement of a suitable fill to match the specified permeability proved very difficult but, eventually a blend of as-dug gravels and single-sized material yielded an acceptable result.

51. The installation of the culverts under the Weymouth Road was treated as a separate section of work. Two-way traffic flow had to be maintained along the road, which is the only land link with the Isle of Portland. Certain culvert units had to be threaded through live 33 kV and 11 kV cables and British Telecom ducts, which were temporarily supported on trestles.

52. Although the culverts had nominally the same dimensions as the underlying original units, a creep of 100 mm over the 49.6 m length in culvert was observed; this was attributed to the use of different joints. Gaps between adjacent rows of culverts were filled with a 3 to 1 cement mortar, and all units were sealed by means of a polyurethane sealant (Berger Flextron B). This same material was used to seal the Joints between adjacent units in the interceptor culvert. However, difficulties in achieving satisfactory adhesion in the wet conditions at the invert of the drain, following periods of rain, meant that certain joints had to be made in a different sealant suitable for these conditions, with a consequential loss in flexibility as a result of this modification.

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		Chiswell case study: The Scheme
The interceptor drain during construction, summer 1986. The section through to the Cove Inn (near the crane) has already been buried The shingle lies on an impermeable layer. The interceptor drain was located at the boundary, thereby ensuring that the water was collected Pile driving along the line of the drain in February 1996 Excavation along the line of the piling which supported the culvert during construction. The piling was later cut off at the drain level, but remained below it within the beach. Precast concrete sections were lowered into the ditch To the north, the shingle was simply dug out and the concrete sections laid and then buried. The 'windows' on the right were filled with gabion baskets, allowing the water to drain through the beach and into the culvert The interceptor drain outfall under construction. The structure leading away from the concrete sections is composed of gabion baskets		Interceptor drain 37. The consultants proposal to intercept the seawater which percolates through Chesil Beach under certain storm conditions, the main cause of flooding in the village of Chiswell, was the most vital part of the Sea Defence Scheme. Separate studies and investigations were necessary before a design could be implemented. The investigations undertaken yielded information on the permeability of the Beach, and these data were used by Hydraulics Research to construct a model of the Beach, from which the required capacity of the drain could be determined. The discovery of an interface between a relatively impermeable material comprising single-sized round pebbles in a sand matrix and the overlying shingle at certain levels within the Beach was a major step towards understanding and solving the flooding problem. 38. The ensuing studies established design parameters for the drain, the size and invert level of the culvert, and the depth at which percolation water should be intercepted. Water that percolates through the beach shingle flows into the drain through large openings or 'windows' along one side and in the roof of the drain. To prevent beach material entering the drain the openings are covered with BRC gabions filled with beach material. The interface between the sand matrix and the shingle dictates the level of the sills of the side 'windows'. Where the interface occurred above the formation level of the drain, it was decided that the gabions lined with Terram fabric should be filled with material with the same permeability as the sand matrix in order to avoid both migration of the fines and the risk of creating unstable conditions in the Beach. [See World Heritage Nomination, Coastal Processes, for an explaination of the posible formation of the beach]. 39. Boreholes driven before the permeability studies had been carried out were taken to the underlying Kimmeridge clay at several locations, and indicated that cut-off could be achieved with interlocking steel sheet piles about 15.5 m long. A borehole investigation to confirm clay depths at closer intervals along the line of the drain was carried out under the main contract. 40. The configuration of the Beach and access thereto dictated the type of construction and alignment of the drain. A length of some 500 m through the built-up area required culverting, while the remaining length could be an open channel discharging under gravity into Portland Harbour after passing through a culvert under the Weymouth Road. 41. The economics of restricting the number of different cross-sections of the drain to a minimum and the need to maintain a consistent invert level of the drain led to the adoption of a standard internal height of drain of 3 m, the width increasing in steps of 1 m, 2 m and 3 m. The drain was designed to be in precast reinforced concrete to ensure quality control and to offset the difficulties of remote batching plants and a restricted site area. In-situ reinforced concrete transitions were constructed at changes in width and direction of the drain. All structural concrete was grade 40 and minimum cover to reinforcement was 100 mm. 42. The alignment of the open channel necessitated a side discharge type outfall from the drain. In order to avoid the removal of fines from the Beach as a result of the high velocities which might accompany rapid drawdown of water in the Beach, the culvert was designed to flow about half-full by discharging over a weired outfall. Pipes below the weir at invert level allow the culvert to drain down. 43. The existing culvert under the Weymouth Road had a capacity of 35 cubic metres/second compared with the assumed flow of 50 cubic metres/second . Fortunately, the invert level of the existing three 3 m wide x 1 m high culvert sections permitted sufficient clearance for a second and higher level of similarly sized culverts to be installed above, a solution which was both practical and economic. 44. The open ditch is mainly unrevetted with slack side slopes of 1 to 4. However, two lengths of the ditch required revetments: firstly, between the outfall structure and open channel section, where excessive turbulence could develop; secondly at the approach to the culverts under the Weymouth Road where a transition incorporates a stepped structure to minimise the lengths of channel affected by normal tide levels in Portland Harbour. In both these locations the use of BRC galvanised wire gabion mesh has been favoured on environmental, aesthetic and cost grounds, the channel bed being protected by Maccaferri PVC coated wire mesh mattresses of greater flexibility. 45. The contract for the construction of the interceptor drain was let to Dean and Dyball as a variation of price (VOP) contract, with a value of £1.7 million in April 1985. The further borehole investigation consisting of ten borings revealed clay underlying a thin strata of weak limestone. A statistical analysis of the varying depth at which clay was found resulted-in the length of the cut-of steel sheet piles being revised to 18 m. 46. It was decided to drive a trial panel of piles to ensure that the pile section required for the cut off was the most economic in terms of driveability and cost of material. The pile driving was troublesome. A range of British Steel's newly introduced W section piles was specified, but driving them to the depth required with any combination of hydraulic vibrating hammer and diesel hammers proved impossible. After driving many different pile types in the trial, the Larssen No 3 pile was selected. The piling of the cut off proceeded in the light of the experience gained in the trial, part driving with a vibratory hammer and completing with a diesel hammer. Increasing damage to the piles caused by the diesel hammer led to an experiment with a BSP HH 357 hydraulic hammer which was subsequently used throughout. 47. The residents of Chiswell-who understood that, if successful, the works would protect them from frequent flooding-accepted stoically the inconvenience of the contracting operations. The assistance of the Borough Council Environmental Health Officer was invaluable to the progress of the works. 48. In order to maintain the Beach profile, a row of temporary sheet piles was installed seaward of the permanent piles to support the excavation for the culverts and the gabions. Generally, the temporary piles were 8 m long (section 16W) and driven individually, using the contractor's own hydraulic, vibrating hammer (MGF) which was mounted on a modified Poclain 160 tracked excavator. The depth of excavation was closely controlled to ensure no overdig of the beach material. 49. The precast concrete units were lowered initially on to a prepared sand cement bed and were eased into position by means of winches, alignment being controlled by theodolite and level. The contractor had attempted the use of a laser for alignment purposes, but frequent disturbance of the equipment, necessitating remedial measures, led to the abandonment of the hi-tech approach. 50 The gabion boxes were assembled in groups of three or four units and, with the assistance of an excavator were manoeuvred into position. The boxes were then filled and the lids secured before the gabions were placed and stitched to the next level. The gabions, except for the lowest level ones, were lined with Netlon CE121 mesh to prevent loss of smaller material from the box. The lowest level gabions were lined with Terram before being filled with graded fill material of specified permeability. The procurement of a suitable fill to match the specified permeability proved very difficult but, eventually a blend of as-dug gravels and single-sized material yielded an acceptable result. 51. The installation of the culverts under the Weymouth Road was treated as a separate section of work. Two-way traffic flow had to be maintained along the road, which is the only land link with the Isle of Portland. Certain culvert units had to be threaded through live 33 kV and 11 kV cables and British Telecom ducts, which were temporarily supported on trestles. 52. Although the culverts had nominally the same dimensions as the underlying original units, a creep of 100 mm over the 49.6 m length in culvert was observed; this was attributed to the use of different joints. Gaps between adjacent rows of culverts were filled with a 3 to 1 cement mortar, and all units were sealed by means of a polyurethane sealant (Berger Flextron B). This same material was used to seal the Joints between adjacent units in the interceptor culvert. However, difficulties in achieving satisfactory adhesion in the wet conditions at the invert of the drain, following periods of rain, meant that certain joints had to be made in a different sealant suitable for these conditions, with a consequential loss in flexibility as a result of this modification.
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