underground stations typical station site plan. underground stations ground conditions: key issues

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