geotechnical engineering in the urban environment engineering
TRANSCRIPT
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 • [email protected]