geotechnical engineering in the urban environment engineering

Geotechnical Engineering in the Urban Environment

Engineering Consulting

Services, LTD

Karl A. Higgins, III, P.E.,

Senior Principal Engineer

2014 Annual Meeting

Geotechnical Engineering Challenges in Urban construction.

• Greatest challenge is adjacent structures and deep excavations, followed by…

• Evaluating SOE system designs and• Monitoring movement

These are today’s learning objectives

Introduction

Topics for Presentation

1. Deep Support of Excavation (SOE) Systems (several types discussed)

2. Lateral Earth Pressures

3. Monitoring Movements

4. Adjacent Structures

Support of Excavation (SOE) Systems

What is it, why is it needed?

Topic 1

• Needed to provide maximum use of site

• Many buildings adjoin on property line or extend outward toward the right-of-way limits

SOE Systems

New BuildingExisting Mather Building

Property Line

Project: 900 G St NW, Wash, DC

A A’

Support of Excavation Systems

Historic Mather Building Adjacent to 900 G St NWSection A – A’

Mather

Property Line

900 G St.

Note Building Elevation Differences

Property LineProperty Line

Mather

Property Line

900 G St.

Mather

Property Line

Mather

900 G St.

Mather

Property Line

900 G St.

Mather

The are several types:

• Cross Lot or Corner Bracing• Traditional Soldier Pile and Wood Lagging• Tangent/Secant Piles • Slurry Walls• Deep Soil Mix Walls

Support of Excavation Systems

Project: 900 G St NW, Wash, DC

MatherBuilding

Cross Lot Steel Bracing Used for SOE

Cross Lot Steel Bracing Type

Support of Excavation SystemsWhen Tiebacks are not feasible

Examples of Cross Lot SOE Bracing

Support of Excavation Systems

GWU Sq. 55 Project in Washington, DC

Traditional Soldier Pile and Wood Lagging

Underpinned Adjacent Structure on Bracket Piles

Support of Excavation Systems

Soil Anchor (Tieback)

Not Absence of Raker or Cross Lot Bracing

Makes working in Excavation less difficult

Traditional Soldier Pile and Wood Lagging

Support of Excavation Systems

Secant/Tangent Pile Walls

Support of Excavation Systems

Secant/Tangent Pile Walls

Supported with Earth Anchors

Secondary Pile Reinforcement(H piles are also often used)

Support of Excavation Systems

Support of Excavation Systems

Permanent Slurry Walls

Capitol Visitors Center Completed in early 2000’s

Support of Excavation Systems

Sequence of ConstructionGuide Wall Forms

Panel Wall Reinforcing

Permanent Slurry Walls

Support of Excavation Systems

Sequence of Construction

TiebacksCross Lot Bracing

Permanent Slurry Walls

Support of Excavation Systems

Sequence of Construction

Deep Soil Mix (DSM) Walls

Similar to Slurry Walls but not a permanent exterior wall, rather a temporary support one

Used for the African American History Museum Currently under construction

Support of Excavation Systems

Deep Soil Mix (DSM) Walls - Equipment

Tieback Drill Rig

Used to Support a Variety of SOE wall Types

Earth Anchor Support

Support of Excavation Systems

Example of Earth Anchor SOE Bracing, aka “Tiebacks”

Most Desirable Type

Number of Arrows indicates number of tieback levels/tiers

Support of Excavation Systems

Example of Earth Anchor SOE Bracing, aka “Tiebacks”

Excavated SideTiebackTier 1

Tier 2

Tier 3

Look for Utility Impacts and Conflicts

Adjacent Structures, Utilities, and Easements may affect their use

Tieback Stressing

Hydraulic Jack Stresses/Elongates Tendon

Mechanics of Tiebacks

Mechanics of Tiebacks

Bonded Anchor Portion

Unbonded Anchor Portion

** note location of Failure Surface

Mechanics of Tiebacks

Lateral Earth Pressures

Topic 2

Lateral Earth Pressures

Lateral Earth Pressure (LEP) Diagrams

LEP for Multiple Tiers of SOE support

≠

LEP for Permanent Basement Wall

Lateral Earth Pressures

SOE Stability Failure Modes (4 types)

Global Stability

Evaluation of Lateral Earth Pressures

SOE Stability Calculations

Factors of Safety as Low as 1.2 are often acceptable as every anchor is tested

Lateral Earth Pressures

LEP Soil Design Parameters

Density, soil strength (phi and c), and layer thickness are all important parametersMay need multiple LEP values for one site – avoid oversimplification

Lateral Earth PressuresMust consider adjacent Structures when

choosing design parameters:• Active Soil Conditions when movement of SOE wall is acceptable• At-Rest Soil Conditions when little to no movement is desired

Adjacent Building supported on Bracket piles attached to SOE. Active or At-Rest Condition?

SOE Movements

H

D = 0.005H

D

Typical expected or “allowable” movement of SOE walls design to Active soil pressures. Ex. 40 foot excavation, D = 0.005 * 40 ft = 0.2 ft = 2.4 inches. This movement would be too much for bracket piles or underpinning pits but probably OK for streets.

Topic 3

Typical Active Soil Allowable Movement

SOE Movements

900 G Street, NWMather Building Monitoring Points

Lateral Movement - South Face

-1.50

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

2/2

8/2

013

3/1

4/2

013

3/2

8/2

013

4/1

1/2

013

4/2

5/2

013

5/9

/2013

5/2

3/2

013

6/6

/2013

6/2

0/2

013

7/4

/2013

7/1

8/2

013

8/1

/2013

8/1

5/2

013

8/2

9/2

013

9/1

2/2

013

9/2

6/2

013

10/1

0/2

013

10/2

4/2

013

11/7

/2013

11/2

1/2

013

12/5

/2013

12/1

9/2

013

1/2

/2014

1/1

6/2

014

1/3

0/2

014

2/1

3/2

014

2/2

7/2

014

3/1

3/2

014

3/2

7/2

014

4/1

0/2

014

4/2

4/2

014

Late

ral

Dis

pla

cem

en

t (i

n)

- T

ow

ard

Excavati

on

(E

) +

Aw

ay f

rom

Excavati

on

(W

)

M-1

M-2

M-3

M-4

M-5

M-6

LimitingValue

1/10/2014 1/13/2014 1/15/2014 3/25/2014M-1 -0.16 -0.16 -0.13 -0.15M-2 -0.16 NA NA NAM-3 -0.22 -0.21 -0.20 -0.22M-4 -0.19 -0.28 -0.27 NAM-5 -0.34 -0.36 -0.36 -0.36M-6 -0.36 -0.34 -0.33 -0.36

At-Rest Soil Design

Previous Mather Building Example

Lateral Movement Threshold set at 3/8 inchFor Building

Vertical Movement was near zero

Lateral Movement Threshold set at 3/8 inchFor Building

900 G Street, NWWest Pile Line

Lateral Movement

-1.50

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

7/19

/201

3

8/2/

2013

8/16

/201

3

8/30

/201

3

9/13

/201

3

9/27

/201

3

10/1

1/20

13

10/2

5/20

13

11/8

/201

3

11/2

2/20

13

12/6

/201

3

12/2

0/20

13

Late

ral D

ispl

acem

ent (

in)

- To

war

d E

xcav

atio

n (E

) + A

way

from

Exc

avat

ion

(W)

NA NA NAPile 60 PB PB PBPile 64 PB PB PBPile 68 PB PB PBPile 72 PB PB PBPile 76 PB PB PBPile 80 PB PB PB

SOE Movements

At-Rest Soil Design

Previous Mather Building Example

Lateral Movement Threshold set at 3/8 inchFor SOE

SECANT PILES USEDProject was success, no excessive movement of historic building

Example of Excessive SOE Pile Movement

Active Design Next to Road

Damage to Utilities and Roads Occurred

Example of Excessive SOE Pile MovementActive Design Next to Road, Pepco Vault and WMATA Tunnel

Supplemental Rakers were ultimately used due to excessive movement

Vertical movements were similar

Example of Excessive SOE Pile Movement

Supplemental Rakers Installed –impacted slab pours but not columns

Movement Monitoring Plans

• Monitoring Points on Adjacent Structures (measuring both Lateral And Vertical Movement)

• Monitoring Points on streetscape to evaluate vertical movement (settlement)

Movement Monitoring Plans

Monitoring Points on SOE piles (measuring both Lateral And Vertical Movement)

Monitoring Devices

How we monitor movement:

1. Traditional Survey Equipment2. Vertical Inclinometers3. Tiltmeters4. Crack Monitors5. Groundwater Piezometers

Traditional Survey Equipment

Highly precise total stations measuring angle and distance

Capable of Robotic Monitoring and real-time data acquisition

Example of Inclinometer Plot

Example of Tiltmeter

Alarm Levels

Example of Crack Monitors

Automatic Electronic Data recording for Real Time Measurements

Epoxy Installed for Manual Reading

Example of Ground Water Monitoring Devices

Typical Monitoring Well

Can be measured manually or automatically

SOE Movements

Architect of the Capitol Building in DCSlab Cracking (12 in. vert and horiz.)

Summary Conclusions for SOE Designs /Movements:

1. Peer review design and soil pressure assumptions,

2. Develop a Movement Monitoring Plan

3. Report movement data and react to adverse trends if they exceed thresholds

Topic 3 - End

Adjacent Structures

Metro Tunnel Lies Beneath Planned Building Development

The Wharf Project at Southwest Waterfront, DC

Topic 4

Adjacent Structures

The Wharf Project at Southwest Waterfront, DC

New transfer girder supported by piles

ZOI

Strategy – support new loads on piles below tunnel

Adjacent Structures

The Wharf Project at Southwest Waterfront, DC

Strategy – support new loads on piles below tunnel. Metro asked ECS to evaluate influence of piles on adjacent tunnel.

-50

0

50

100

150

200

250

300

0 50 100 150 200 250

Augercast Pile LocationsStress PointDriven Pile Locations

Edge of Tunnel

Adjacent Structures

The Wharf Project at Southwest Waterfront, DC

Strategy – support new loads on piles below tunnel. Metro asked ECS to evaluate influence of piles on adjacent tunnel.

-60

-58

-56

-54

-52

-50

-48

-46

-44

-42

-400 50 100 150 200 250

Stress Induced (PSF)

Elev

atio

n (F

eet)

Adjacent Structures

9 ft

Metro’s ZOI

20,000 psf on footing

George Washington University Sq. 55

Strategy – determine if new loads can be accommodated by tunnel

Adjacent Structures

Sometimes you can utilize Hand Calculations to evaluate stress impacts

George Washington University Sq. 55

Adjacent Structures

900 G Street Washington, DC

Planned Building Development Directly Adjacent to Metro Tunnel/Station

Adjacent Structures

900 G Street Washington, DC

Strategy -Building loads impacted Metro Tunnel so new building supported on deep foundation

No stress analysis needed

Engineering Consulting Services, Ltd.

14026 Thunderbolt Place, Suite 100

Chantilly, Virginia 20151

703-471-8400www.ecslimited.com

Karl Higgins, P.E., Vice President/Sr. PrincipalBS Civil Va. Tech, M.Eng. Geotechnical Univ. of Missouri703.471.8400 • khiggins@ecslimited.com