Design and construction of a reinforced soil embankment on soft soil

Transcription

1 Design and construction of a reinforced soil embankment on soft soil Russell Jones, Golder Associates Gareth Swift, University of Salford

2 Overview Background Site details Design philosophy Design issues Construction issues Conclusions

3 Background Introduction Site is located south of Basildon in Essex Bounded by: Wat Tyler country park on western boundary Pitsea Marshes (SSSI) along the northern boundary East Haven Creek along southern boundary (Thames beyond)

4 Background Introduction Pitsea landfill site located south of Basildon in Essex, and covers an area of approx. 284ha (>50ha currently operational)

5 Background Introduction Paper and presentation deals with design and construction issues relating to a large leachate lagoon to be constructed on soft, compressible soils Design carried out in June 2002 Construction between July 2002 and January 2003 Lagoon filled to capacity March 2008 Final Certificate issued by the Panel Engineer July 2008

6 Background Client brief: Minimum leachate capacity of 150,000m 3 Maximum cost of build 1.3mill Maximum bund height 8mAOD (planning constraint) needs to be buildable!

7 Background Additional design constraints: Ensure stability Minimise soil imports Maximise lagoon area (hence, minimise bund height) Minimise excavation in to existing soils

8 Background Additionally Satisfy requirements of Reservoir Act 1975 the Act applies to Large Raised Reservoirs, defined as: being designed to hold or capable of holding more than 25,000m 3 of water as such above the natural level of any part of the land adjoining the reservoir (including the bed of any stream)

9 Overview Background Site details Design philosophy Design issues Construction issues Conclusions

10 Site Details Ground conditions The area is relatively even but groundwater levels close to, or at, the ground surface Waste dating from the 1950 s underlies the site in a layer between 1m and 7m thick Waste comprises ash, clinker, glass, cans in hydraulic continuity with the surrounding landfill This overlies a generally soft stratum of alluvial clays and sands (m v m 2 /kn).

11 Site Details Ground conditions A similar, but earlier, lagoon encountered significant difficulties during construction associated with the high groundwater levels and the trafficability of the waste and the soft alluvial material.

12 Ground conditions Site Details

13 Overview Background Site details Design philosophy Design issues Construction issues Conclusions

14 Original design philosophy Design Philosophy Reducing the existing ground level of approximately 3 m AOD to a minimum formation level of 1.0 m AOD Pumping and dewatering required (estimated quantity 20,000 to 75,000m 3 ) Forming embankment slopes of 1v:4h utilising the excavated waste Using a single ethylene inter-polymer alloy geomembrane liner on both the base and embankment slopes Liner manufactured to a specific prefabricated size and shape to suit the design

15 Original design philosophy Design Philosophy

16 Original design philosophy Design Philosophy

17 Original design philosophy Design Philosophy Ground conditions are poor at best: Groundwater/leachate levels at or near to the ground surface; and Material to be excavated to produce a formation level comprised 1950 s waste of questionable engineering integrity Dewatering logistically difficult due to up-gradient landfill (with leachate) Single geomembrane liner not the most effective barrier Tenderers comments

18 Design Philosophy Proposed design No excavation Reinforced basal platform 20 m wide by 700 mm thick constructed at the existing ground level Steeper face angles Reinforced soil perimeter embankment, 5.5 m high External side slopes 1v:2h Internal side slopes 1v:1v

19 Original design philosophy Design Philosophy

20 Proposed design Design Philosophy

21 Proposed design Design Philosophy

22 Design Philosophy Proposed design Base area of approximately 32,000 m 2 lined with a composite lining system 2 mm thick Linear Low Density Polyethylene (LLDPE) sheet Geosynthetic clay liner (GCL) Underdrainage geocomposite drainage layer to limit hydraulic pressures from the leachate and gas Lining of the perimeter embankment with 2 mm thick LLDPE geomembrane Geocomposite drainage layer connected to a piped drainage system

23 Proposed design Design Philosophy

24 Overview Background Site details Design philosophy Design issues Construction issues Conclusions

25 Design Issues Proposed design Since material was limited to on-site sources, the quality would be variable Side slopes would be steep in order to achieve capacity Geosynthetic reinforcement, geotextile rather than geogrid to aid dissipation of pore pressures Primary geotextile reinforcement Secondary geogrid reinforcement Granular material used for foundation layer Cohesive material used for most of embankment but granular material used for upper section

26 Proposed design Design Issues

27 Proposed design Design Issues

28 Design Issues Reinforcement details Layer Material Reinforcement No. of Layers Basal layer 1 75 mm crushed concrete (min Tensar 160RE 1 50 kn 10 % fines) Basal layer 2 Crushed concrete fines Tensar 120RE 1 Embankment Class 6I, 6J, 7B, 7C or 7D Stabilenka 100/50 3 material Fornit 20/20 4

29 Design Issues Settlement Final top of embankment = 7.5m AOD Constructed to = 8m AOD due to anticipated settlement

30 Design Issues Material parameters Material γ b (kn/m 3 ) c (kpa) φ (deg) r u Additional information Embankment fill Foundation layer Old waste Beneath main slope Old waste Beneath northern embankment Alluvium to m v = 0.2m 2 /MN 0.8 k = 1 x 10-8 m/s

31 Design Issues Value Engineering Time constraints Design optimisation during construction Supported by field trials where possible ECC form of contract

32 Design Issues Value Engineering NEC/Engineering and Construction Contract Six Main Options Option A - Priced with Activity Schedule Option B - Priced with BQ Option C - Target with Activity Schedule Option D - Target with BQ Option E - Cost Reimbursement Option F - Management Contract

33 Design Issues Contract NEC/Engineering and Construction Contract Six Main Options Option A - Priced with Activity Schedule Option B - Priced with BQ Option C - Target with Activity Schedule Option D - Target with BQ Option E - Cost Reimbursement Option F - Management Contract

34 Design Issues Stability A key factor in the design of the perimeter embankments is their stability Internal Inside face and outside face External Global failure and siding Major issue is the development of pore water pressure in the subgrade due to embankment construction Slope/w and Seep/w used to examine the effects of pore water pressure on stability Results indicate that the rate of build up and dissipation was critical to stability

35 Design Issues Stability Piezometers used on site to monitor pore water pressures Construction of toe berms would improve short term stability Factor of safety >1.3 if r u <0.7 Toe berms allow increase in r u to 0.8

36 Stability Design Issues

37 Design Issues Stability 700mm thick reinforced foundation layer: 300mm drainage layer 400mm stability layer

38 Design Issues Stability Typical output: kpa 3 Layers of Stablenka Top Layer of Fornit 20/20 Top Layer of 120RE Lower Layer of 160RE

39 Design Issues Groundwater Geotextile reinforcement allows dissipation of pore pressures in embankment fill Geocomposite allows dissipation of pore pressures beneath the lagoon 75mm crushed concrete layer allows dissipation of pore pressures beneath the embankment

40 Design Issues Environmental considerations Leachate containment 2mm LLDPE geomembrane GCL UV protection geotextile Underdrainage Complete system required to remove gases beneath lining system 12mm geocomposite

41 Overview Background Site details Design philosophy Design issues Construction issues Conclusions

42 Construction Issues Construction works Groundwater monitoring Settlement monitoring

43 Construction Issues Construction works Poor ground conditions

44 Construction Issues Construction works Generally, a flat lying area in a floodplain

45 Construction works Construction Issues

46 Construction Issues Construction works Relatively flat formation level (3.0mAOD) required: Removal of bushes/trees Re-profiling of hummocky areas Elevated ground in NE was excavated Low area in south backfilled Existing leachate trench backfilled Dewatering, where required Installation of piezometers Construction of cut-off trench

47 Construction works Construction Issues

48 Construction Issues Construction works Basal layer Separator geotextile on existing ground 9 mm Drainage geocomposite 300 mm thick drainage layer Separator geotextile on drainage layer 400 mm thick stability layer including two layers of geogrid reinforcement

49 Construction Issues Construction works Foundation layer, first layer of geogrid

50 Construction Issues Construction works Foundation layer, second layer of geogrid

51 Construction Issues Construction works Foundation layer

52 Construction Issues Construction works Reinforced embankment Compacted general fill material (Class 6I, 6J, 7B, 7C or 7D) Geotextile primary reinforcement Geogrid secondary reinforcement

53 Construction Issues Construction works Lining system Drainage geocomposite Geomembrane Perforated pipe Soil retention geocomposite

54 Construction works Construction Issues

55 Construction Issues Construction works Installation of perforated pipe at toe of inside slope

56 Construction Issues Construction works Embankment starting to be constructed

57 Construction Issues Construction works Embankment works continuing

58 Construction Issues Construction works Embankment works continuing, inside face

59 Construction Issues Construction works Embankment works continuing, outside face

60 Construction Issues Construction works Embankment works, becoming difficult with cohesive fill

61 Construction Issues Construction works Trafficability concerns

62 Construction Issues Construction works Embankment works, revised fill

63 Construction Issues Construction works Embankment works, revised fill, impact on geotextile reinforcement

64 Construction Issues Construction works Embankment works, revised fill, impact on geogrid reinforcement

65 Construction Issues Construction works Embankment works, working in the winter

66 Construction Issues Construction works Embankment works, geocomposite and geomembrane deployment

67 Construction Issues Construction works Embankment works, uv protection geotextile and tyre wave wall

68 Construction Issues Construction works Embankment works, soil retention geocomposite

69 Construction Issues Construction works Basal works, prior to geocomposite deployment

70 Construction Issues Construction works Basal works, deployment of geocomposite, GCL and geomembrane

71 Construction Issues Construction works Basal works, deployment of geocomposite

72 Construction Issues Construction works Basal works, deployed GCL

73 Construction Issues Construction works Basal works, deployment of geomembrane

74 Construction Issues Construction works Basal works, hydraulic bund

75 Construction Issues Construction works Perimeter leachate trench

76 Construction Issues Construction works Tested to capacity for Reservoir Act sign off, 2008

77 Construction Issues Construction works Completed lagoon, 2008

78 Groundwater monitoring Construction Issues piezometers installed at a number of locations to monitor PWP

79 Groundwater monitoring Construction Issues leachate level in lagoon

80 Groundwater monitoring Construction Issues r u typically between 0.2 and 0.3

81 Construction Issues Settlement monitoring Temporary vertical and horizontal stations at toe of embankment Permanent stations at top of embankment

82 Settlement monitoring Construction Issues

83 Construction Issues Settlement monitoring Approximate elevation of top of bund between 7.6m and 8.0mAOD Level (m aod) Jan-03 1-Apr-03 1-Jul-03 1-Oct-03 1-Jan-04 1-Apr-04 1-Jul-04 1-Oct-04 1-Jan-05 Leachate level in lagoon Elevation of foundation layer

84 Construction Issues CQA Quality assurance and conformance testing of all materials: Separator geotextiles Reinforcing elements Lining elements All soils In accordance with the approved CQA Plan

85 Construction Issues

86 Overview Background Site details Design philosophy Design issues Construction issues Conclusions

87 Conclusions Review of the design issues relating to the construction of a leachate storage lagoon has been presented Geotechnical and geoenvironmental applications of geosynthetics have been used to ensure the short and long term stability of the perimeter embankment of the lagoon and to maximize containment capacity Design revised during construction as part of a Value Engineering approach Lagoon now tested by filling to capacity

88 A range of geosynthetics was used in the project Conclusions Geosynthetic Material Geotextile Geotextile Geocomposite Geocomposite Geogrid Geopipes Geomembrane Geosynthetic Clay Liner Function Reinforcement and Drainage UV protection Drainage Soil retention Reinforcement Drainage Primary component of composite liner Secondary component of composite liner This project could not have been constructed without the use of these materials

89 Acknowledgements Veolia Environmental Services (formerly Cleanaway) Steve Smith (now Golder Associates) John Bowers Nick Sinclair Panel Engineers Michael Kennard (retired) George Hallowes (deceased) Chris Hoskins

90 Thank you for your attention Conclusion slide