Underground Stations
Typical Station Site Plan
Underground Stations
Ground Conditions: Key Issues
Design Criteria for Shoring System
1. Base Stability and Deformations
2. Stability Under Hydrostatic Uplift
Pressures
3. Lateral Pressures for Wall Design
4. Lateral Pressures for Design of Bracing
(Prelim Design)
5. Soil Structure Interaction Analysis (Final
Design)
Base Stability
Hydrostatic Uplift
Important Sources
of Ground Deformations
1. Installation of Soldier Piles
2. Construction of Diaphragm Walls/SoilCement Walls
3. Excavation (Including removal of struts)
4. Jet Grouting
5. Installation of Pile Foundations
6. Dewatering
7. Vibrations (Steel sheetpiles or foundation
pile driving)
8. Extraction of Old Piles
Excavation Induced Deformations
Zone of Ground Movements :Impacts
on Adjacent Structures
Excavation Adjacent to Building
Muni Metro Turnback
Howard Street Closed
Mission Street
Concrete
Decking
Settlements of Buildings Caused by
Excavations
Muni Metro Turnback
Settlements Caused by Slurry Wall
Construction
Mitigation of Impacts on Adjacent
Structures
Deformations Caused by Jet Grouting
and Excavation
SPTC Walls
Islais Creek Contract E
Lateral Deformations Caused by
Excavation and Removal of Struts
(a) Inclinometer No. E-87;
Indiana Street Segment
(b) Inclinometer No. N120; Overflow Structure
Risk Management and Cost
Containment
1. Design shoring for deformation control
2. Complete shoring design to eliminate risk
that contractors may underbid the shoring
and then look for excuses to claim extra for
changed conditions
3. Define subsurface conditions to a level that
illustrates clearly the expected variability at
the site. Comprehensive Geotechnical
Investigation is essential
Risk Management and Cost
Containment
4. Perform detailed evaluation of impacts on
adjacent buildings and evaluate the need for
strengthening or underpinning of existing
structures
5. Develop preliminary plans to deal with
potentially unexpected adverse impacts on
adjacent structures; include a budget
contingency
Risk Management and Cost
Containment
6. Provide for a comprehensive monitoring
program to evaluate potential impacts on
adjacent structures. Keep the
instrumentation and the monitoring under
the control of the designer and VTA
Risk Management and Cost
Containment
7. Use soil-cement walls to achieve economy
and safety
•
Higher production rates by a factor of 2
•
Lower cost per sq foot
•
Incorporate soil-cement wall as part of permanent
structure
•
Better control of spoils
•
Lower risk of accidental spill in the traffic lanes
Conceptual Design for Incorporating
Temporary Soil-Cement Shoring Into
Permanent Wall of Box Structure
Risk Management and Cost
Containment
8. Minimize interfaces between designers and
contractors
•
Keep design of shoring under station contract
•
Have one contractor responsible for all excavations
and shoring
Risk Management and Cost
Containment
9. Evaluate instrumentation results promptly to
detect the onset of difficulties as early as
possible, and take steps to minimize
problems
•
Avoid underpinning to the extent possible
•
Have proactive plan to strengthen buildings that can
not tolerate the expected settlement
•
Avoid dewatering
Reasons for Including the Station Shells
in Stations Contract
1. Minimizes interfaces between designers
•
Additional and significant excavations are required
for entrances. Need to have coordinated design to
minimize impacts on adjacent buildings. i.e.
Consideration of cumulative effects
•
Provide single point of responsibility for design and
performance – Avoid mistakes and duplication of
effort
•
Avoid division of responsibility for performance of
the shoring after the excavation is completed
1
Reasons for Including the Station Shells
in Stations Contract
1. Minimizes interfaces between designers
(contd)
•
Avoid having the tunnel designers to design the
base slab of the station which is essential for pulling
the tunneling machine through the station
•
Coordinate shoring design with design of openings
in the station shells for entrances (avoid having to
restrut the excavation to accommodate the
openings)
2
Reasons for Including the Station Shells
in Stations Contract
2. Maintain Construction of all shoring under
one contract
•
Construct the shoring for the entrances and stations
under a single contract for more efficient
coordination – Avoid Division of Responsibility
•
Have the same contractor who installs the shoring
to be responsible for strut removal during
construction of the box structure – Avoid Division
of Responsibility
3
Reasons for Including the Station Shells
in Stations Contract
3. Minimize the time that the excavation
remains open after completion of excavation
4. Flexibility to construct the base slabs in
advance of tunnel break through the station.
5. Minimizes overall risk
4
Stress History Profiles
Market Street and Civic Plaza Stations
Empirical Charts Settlements
Caused by Excavation
Stiff Clays
Deformations Caused by
Construction of Slurry Walls
0
DEPTH IN METERS
20
30
40
50
60
SOIL SETTLEMENT (mm)
10
Lateral Deformation Caused by
Slurry Wall Construction
Muni Metro Turnback
Excavations
Muni Metro Turnback
Howard Street
Soil Strength Characteristics
Market Street Station, San Jose
Use of Jet Grouting for Control of
Deformations
Comparison of Conventional “Stiff” System with New Shoring
Concept
Lateral Deformations Caused by Jet
Grouting
(a) Inclinometer No. E-155;
Indiana Street Segment
(b) Inclinometer No. N-76;
Overflow Structure
Risk Management and Cost
Containment
10.Work under decking to maintain traffic
flowing on Santa Clara Street
11.Construct soil-cement solidified zones
behind the end walls of the box to facilitate
tunnel breakthrough. Minimize interface with
tunnel designer and tunneling contractor