Upload
others
View
6
Download
0
Embed Size (px)
Citation preview
Slope Stabilisation for the Thirlmere Aqueduct at Nab ScarDavid Preece, Design Manger, Bachy Soletanche, UK
International Society for Micropiles, 23 September 2010, Washington DC
Introduction
• History of Thirlmere Reservoir and Aqueduct• Original construction of conduit at Nab Scar• Aqueduct issues at Nab Scar• Requirements & restrictions
S l t d l ti• Selected solution• Design of spaced piles• Effectiveness of worksect e ess o o s
International Society for Micropiles, 23 September 2010, Washington DC 2
History of Thirlmere Reservoir and Aqueduct
• Man made reservoir• Water to support industrial revolution in Manchester
International Society for Micropiles, 23 September 2010, Washington DC 3
History of Thirlmere Reservoir and Aqueduct
• Connection established in 1894• Mass unreinforced concrete• 95 miles in length95 miles in length• 180m @Thirlmere to 110m @Manchester• Gradient 450mm to 1km (1 in 2200)• 220 million litres of water per day220 million litres of water per day• Water speed 3 to 5 kph
International Society for Micropiles, 23 September 2010, Washington DC 4
Original construction of conduit at Nab Scar
International Society for Micropiles, 23 September 2010, Washington DC 5
Original construction of conduit at Nab Scar
International Society for Micropiles, 23 September 2010, Washington DC 6
Original construction of conduit at Nab Scar
International Society for Micropiles, 23 September 2010, Washington DC 7
Original construction of conduit at Nab Scar
International Society for Micropiles, 23 September 2010, Washington DC 8
Aqueduct issues at Nab Scar
• Inspections & investigations since 2005• Series of spiral crack observed within the
conduit & around arch barrel• Opening of joint between slab and walls• Torsional movement considered due toTorsional movement considered due to
the movement of ground over top of conduit
International Society for Micropiles, 23 September 2010, Washington DC 9
Requirements & Restrictions
• No direct loading of the conduit• Minimal vibration techniques• No slope loading• Monitoring of conduit and slope
throughout works:throughout works:• Hillside surface & ground at depth• Conduit position• Conduit internal cracksConduit internal cracks
International Society for Micropiles, 23 September 2010, Washington DC 10
Selected Solution
• All about construct-ability
International Society for Micropiles, 23 September 2010, Washington DC 11
Design of spaced piles
• Determination of required stabilising force• Determination of pile loads to provide required stabilising resistance
force• Layout of stabilisation works• Development of load on A-framesDevelopment of load on A-frames• Analysis results considering different models• Micropile & cap dimensions
International Society for Micropiles, 23 September 2010, Washington DC 12
Design of spaced piles: determination of stabilising resistance force
• Slope stability analysis
• Circular slip and infinite slope analysis
• Back calculate soil Stabilising forceand ground water parameters
• Establish required
Tunnel spoil
Stabilising force
qrestoring force to provide increase stability FoS = 1.3
Intact Volcanic Tuff
FoS = 1.0; Slip depth: 2.5m, ’ = 45o, ground water: 1m b.e.g.lFoS = 1 3; restoring force = 250kN per m run of slope
International Society for Micropiles, 23 September 2010, Washington DC 13
FoS = 1.3; restoring force = 250kN per m run of slope
Design of spaced piles: determination of stabilising resistance force
• Infinite slope & finite slope eqns
saturated soil unit weight t 20 kN/m3
• FoS = 1.0 for existing case• FoS = 1.3 with 250kN/m
restoring force
saturated soil unit weight sat 20 kN/munit weight of water w 10 kN/m3
angle of friction 45 ºslope angle to horizontal 34 ºcohesion c 0 kPa
d th t li f 2 5restoring force depth to slip surface, z 2.5 mdepth to groundwater surface, zw 1 m
slope length, l 38 mRestoring force parallel to slope, H 250 kN per m
F = [c/(sat.z.sin.cos)]+[(sat-{w.m})/sat].[tan/tan] m = (z-zw)/z 0.6
F = c.l + sat.l.z.cos2.m.z.l.cos2tan + Hsat.l.z.sin.cos sat.l.z.sincos
International Society for Micropiles, 23 September 2010, Washington DC 14
sat sat
Design of spaced piles: Determination of pile loads to provide required stabilising resistance force
• Structural frame model & elastic continuum model using Piglet
-1.5
-1
-0.5
0
-10 -8 -6 -4 -2 0 2
oord
inat
e
Lateral H
Plane
0.5
1
1.5
Y C
o
Pile tip levelPile cap levelAssumed ‘pin’
Lateral force HCross-section sketch of proposed up-slope stabilisation piles
R l d
34º
Compression CTension T
Resolvedvertical component559kN
Resolved horizontalcomponent: 829kN
34º
Compression CTension T
Lateral H
40º
16º
Lateral H
International Society for Micropiles, 23 September 2010, Washington DC 15
Design of spaced piles: Layout of stabilisation works
• Pairs of spaced ‘A-frame’ micropiles to provide restoring force
• Targeted permeation groutingTargeted permeation grouting
Section
Plan
International Society for Micropiles, 23 September 2010, Washington DC 16
Design of spaced piles: Development of load on A-frames
• Arching check & development of load on spaced A-frames inspaced A-frames in response to hillside movement
• Function of:• Function of:• Spacing• Cap width/pile diameter• ’; c’; slip depth z
Rock outcrop• ; c ; slip depth z
• Output: lateral load on pile cap & piles
International Society for Micropiles, 23 September 2010, Washington DC 17
Design of spaced piles: Analysis results considering different models
Analysis Tension pile force
Compression pile force
Individual pile bending moment / shear force
Pile cap moment
Based on providing required restoring force of 250kN per m run:
Slope restoring force structural frame model
541kN 628kN -- --
Slope restoring force elastic continuum model (Piglet)
531kN 635kN 27kNm / 20kN 784kNm
Based on applied slope movement loads according to Ito & Matsui:
Slope movement induced forces on pile caps:
623kN 401kN -- --
structural frame model
Slope movement induced forces on pile caps: elastic continuum model (Piglet)
623kN 422kN 25kNm / 18kN 663kNm
International Society for Micropiles, 23 September 2010, Washington DC 18
continuum model (Piglet)
Design of spaced piles: Micropile & cap dimensions
• 225mm diameter micropiles 6.5m rock
k tsocket• 168.3 x 10mm CHS from
cap to 1m into
competent rock• Tension pile: 50mm
Gewi bar 7No. 350mm dia
4No. 400mm dia B16 helical at 55mm centres & 3No. 350mm dia B16 helical at
• Compression pile: 40mm pre-stressing bar
• 40N/mm2 grout Main reinforcement omitted for
B16 helical at 55mm centres
B16 helical at 55mm centres
• 40N/mm2 grout Main reinforcement omitted for clarity comprises: 4No. B25 bars top and bottom 5No. B25 bars to each side face B12 link bars @ 130mm c/c
International Society for Micropiles, 23 September 2010, Washington DC 19
Effectiveness of works: Strain gauge results from inside conduit
2000
1000
1500
120m
m
500
mic
rost
rain
ove
r
-500
0
24/09/09 14/10/09 03/11/09 23/11/09 13/12/09 02/01/10 22/01/10 11/02/10 03/03/10 23/03/1024/09/09 14/10/09 03/11/09 23/11/09 13/12/09 02/01/10 22/01/10 11/02/10 03/03/10 23/03/10
Date
start on site completion of down-slope piles, caps & perm grouting
completion of up-slope piles & caps
International Society for Micropiles, 23 September 2010, Washington DC 20
Finished works
International Society for Micropiles, 23 September 2010, Washington DC 21
Conclusion
• A vital section of water infrastructure has been stabilised• Early indications show the works to be effective• Water continues to flow at Queen Victoria Jubilee Fountain in
Manchester
International Society for Micropiles, 23 September 2010, Washington DC 22