The maximum settlement associated with trenching is equal to. 0.045% for both shallow and deep foundations. â The maximum settlement due to pit excavation ...
Citation: Ahmed, S.A. & Fayed, A.L. (2015) "Mitigation of Risks Associated with Deep Excavations: State of the Art Review", Industry Academia Collaboration (IAC 2015), Cairo, Egypt, 6-8 April, 2015.
Mitigation of Risks Associated with Deep Excavations: State of the Art Review By
Sayed M. Ahmed
&
Ayman L. Fayed
Structural Engineering Dept., Geotechnical Engineering Group Ain Shams University
CONTENTS I. INTRODUCTION II. RISKS ASSOCIATED WITH DEEP EXCAVATIONS III. GEOTECHNICAL AND GEOLOGICAL ASPECTS IV. FACTORS AFFECTING GROUND DEFORMATIONS V. RISKS OF BUILDING DAMAGE VI. RISKS ASSOCIATED WITH GROUNDWATER VII. OBSERVATIONAL METHOD AND MONITORING VIII. RISK MANAGEMENT AND MITIGATIONS IX. SUMMARY 2
I. Introduction
“Sustainability is Not Possible without Infrastructure and that, often, the best form of infrastructure involves the underground.” Harvey W. Parker
3
I. Introduction
Sustainable Urban Development
Utilization of Underground Space (Underground Master Planning) Basements
Underground Garages
Cut & Cover Tunnels
Large Water Tanks 4
I. Introduction
Utilization of the Underground Space in Egypt
Cairo Vision 2050
The Greater Cairo Metro 6 Lines
5
II. RISKS ASSOCIATED WITH DEEP EXCAVATIONS
“A road exhibiting no failures or at least deficiencies along its alignment is overdesigned and hence too costly” Karl Terzaghi 6
II. RISKS ASSOCIATED WITH DEEP EXCAVATIONS
Risks Associated with Deep Excavation
Failures
Aesthetical and serviceability problems
Shanghai, China, 2009
Nicoll Highway, Singapore, 2004
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III. GEOTECHNICAL AND GEOLOGICAL ASPECTS
“Every soil does not bear the same fruit” Latin Proverb 8
III. GEOTECHNICAL AND GEOLOGICAL ASPECTS
Peck (1969)
Bentler (1998) The maximum horizontal wall deflection for excavations In sand or hard clays is 0.19% H In soft to stiff clays 0.45% H, where H is the depth of excavation. The average of the maximum settlement is 0.22% H in sands/hard clays and 0.55% H in soft-stiff clays. 9
III. GEOTECHNICAL AND GEOLOGICAL ASPECTS
Nile Alluviums
Abdel-Rahman & El-Sayed (2002) El-Sayed & Abdel Rahman (2002) Abdel-Rahman & El-Sayed (2009)
The maximum settlement associated with trenching is equal to 0.045% for both shallow and deep foundations. The maximum settlement due to pit excavation is about 0.11% of the excavation depth for shallow foundations and 0.03% of the maximum depth of excavation for pile foundations The extent of the settlement troughs was found to reach up to a distance equivalent to 3.5 of the depth of excavation in alluvial soils for both shallow and deep foundations. Most of the settlement of buildings on pile foundations occurs 10 during the trenching stage
IV. FACTORS AFFECTING GROUND DEFORMATIONS
“Geotechnical Engineering is a Science, But it’s Practice is an Art. ” Karl Terzaghi 11
IV. FACTORS AFFECTING GROUND DEFORMATIONS
Wall Deformation Patterns Clough & O’Rourke (1990)
Spandrel-type settlement trough (Ou et al., 1993)
Concave settlement profile (Hsieh & Ou, 1998)
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IV. FACTORS AFFECTING GROUND DEFORMATIONS
Wall stiffness and stability
Goldberg et al., (1976)
Manna & Clough (1981)
Clough et al. (1989); Clough and O’Rourke (1990) Flexible systems
Rigid systems
13
IV. FACTORS AFFECTING GROUND DEFORMATIONS
Time-Dependent Effects
The negative water pressure dissipates with time generated by the excavation at the base of the excavation which causes loss of some passive resistance that occurs immediate after excavation. This leads to time-dependent deformations in the wall and the soil behind the wall.
Excavation Geometry and Three-Dimensional Effects Corner effect (Ou et al., 1996)
Parallel distribution (Finno & Roboski, 2005; Roboski &Finno, 2006)
14
IV. FACTORS AFFECTING GROUND DEFORMATIONS
Wall Installation Effects Clough and O’Rourke (1990)
Gaba et al. (2003)
15
IV. FACTORS AFFECTING GROUND DEFORMATIONS
Building Weight and Stiffness
The effect of building weight is was small as long as a high factor of stability of the wall was maintained. The stiffness of the building reduces the differential settlement in sagging deformation. In hogging mode, such a restraint is not provided and the structure behaves more flexibly leading to higher degrees of damage.
Workmanship
Peck (1969)
16
V. Risks of Building Damage
“Our practical experience can be very misleading unless
it combines with it a fairly accurate conception of the mechanics of the phenomena under consideration. ” Karl Terzaghi
17
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V. Risks of Building Damage
iii- Deep Beam Model
ec= 0.05% (Polshin & Tokar, 1957) ec =0.075% (Burland & Wroth, 1974)
V. Risks of Building Damage
Burland & Wroth (1974 & 1975); Burland et al. (1977)
Sagging mode, E/G = 2.6
Hogging mode, E/G = 2.6
Boscardin & Cording (1989)
19
V. Risks of Building Damage
iii- Effect of horizontal strains
Son & Cording (2005)
20
V. Risks of Building Damage
iv- Three-phases building damage assessment
Mair et al. (1996) Son & Cording (2005) 21
VI. RISKS ASSOCIATED WITH GROUNDWATER
“Don’t think there are no crocodiles because the water is calm.” Malayan Proverb 22
VI. Risks Associated with Groundwater
Failure due to soil erosion due to improper dewatering
Leaking joints
Excessive settlement due to improper filter design
Failure due to seepage through the wall 23
VI. Risks Associated with Groundwater
Dewatering and Groundwater Control
El-Nahhas (2006)
24
VII. OBSERVATIONAL METHOD AND MONITORING
“No theory can be considered satisfactory until it has been adequately checked by actual observations.” Ralph Peck 25
VII. OBSERVATIONAL METHOD AND MONITORING
Deformations
Stresses
Surface points
Piezometers
Extensometers
Strain gauges
Inclinometers
26
VII. OBSERVATIONAL METHOD AND MONITORING
A Reflectorless Robotic Total Station (RRTS) measuring RSPs and prisms (Tamagnan & Beth, 2012)
Terrestrial Laser Scanning (TLS)
27
VIII. RISK MANAGEMENT AND MITIGATIONS
“There can be no great accomplishment without risk” Neil Armstrong 28
VIII. RISK MANAGEMENT AND MITIGATIONS
Abdel Rahman (2007)
Risk source Contingency plan of action Excessive lateral movement of the wall and Increase the number of lateral supports ground settlement Instability of the grout plug Refill the excavation pit with water up to the level that adequately re-stabilize the situation, or perform heavy dewatering to lower the water table as needed. Insufficient drawdown to the water below Increase the number of wells excavation level Lateral leaking from the support system Inject grout columns behind the leaking locations
29
IX. Summary
IX. Summary
Deep excavations structures.
occasionally
cause
failures
of
adjacent
Deep excavation often produce serviceability problems to nearby buildings.
The deformations patterns associated with deep excavations depend of the mode of the wall deformations.
The induced deformations depend on diverse aspects.
A well-designed support systems for deep excavations do not only ensure the stability of the excavation itself but also warrant that the excavation will not cause damage to the adjacent buildings and utilities.
Many damage criteria have been set to assess the effect of the ground deformations induced by deep excavations.
Monitoring programs and risk management are powerful tools in the observational approach to allow construction to proceed smoothly in the face of the abundant risks associated with deep excavation projects, particularly the risks associated with 30 unforeseen geotechnical conditions or construction problems.
We are engineers, not magicians
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