Underground StationsTypical Station Site Plan
Underground StationsGround Conditions: Key Issues
Design Criteria for Shoring System
1. Base Stability and Deformations2. Stability Under Hydrostatic Uplift
Pressures3. Lateral Pressures for Wall Design4. 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 Piles2. Construction of Diaphragm Walls/Soil-
Cement Walls3. Excavation (Including removal of struts)4. Jet Grouting5. Installation of Pile Foundations6. Dewatering7. Vibrations (Steel sheetpiles or foundation
pile driving)8. Extraction of Old Piles
Excavation Induced Deformations
Zone of Ground Movements :Impacts on Adjacent Structures
Mission StreetMission StreetThe Embarcadero
The Embarcadero
Northbound
Northbound
Southbound
Southbound
Howard Street ClosedHoward Street Closed
ConcreteConcreteDeckingDecking
Excavation Adjacent to BuildingMuni Metro Turnback
Settlements of Buildings Caused by Excavations
Muni Metro Turnback
Settlements Caused by Slurry Wall Construction
Mitigation of Impacts on Adjacent Structures
Islais Creek Contract E
Deformations Caused by Jet Grouting and Excavation
SPTC Walls
(a) Inclinometer No. E-87;Indiana Street Segment
(b) Inclinometer No. N-120; Overflow Structure
Lateral Deformations Caused by Excavation and Removal of Struts
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 ProfilesMarket Street and Civic Plaza Stations
Empirical Charts Settlements Caused by Excavation
Stiff Clays
Deformations Caused by Construction of Slurry Walls
DE
PT
H IN
ME
TE
RS
10
20
30
40
50
60
0
SO
IL S
ET
TL
EM
EN
T (
mm
)
Lateral Deformation Caused by Slurry Wall Construction
Muni Metro Turnback
Howard StreetHoward Street
ExcavationsMuni Metro Turnback
Soil Strength CharacteristicsMarket Street Station, San Jose
Comparison of Conventional “Stiff” System with New Shoring Concept
Use of Jet Grouting for Control of Deformations
(b) Inclinometer No. N-76;Overflow Structure
(a) Inclinometer No. E-155;Indiana Street Segment
Lateral Deformations Caused by Jet Grouting
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
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