BLOCKING AND BRACING Blocking & Bracing 1 Blocking and Bracing.
Lecture 12 - Bracing
42
CVEN4102 – Operations and Projects Bracing Instructor: Dr X Shen Date: 22 October 2014
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Bracing
Transcript of Lecture 12 - Bracing
CVEN4102 – Operations and Projects
Bracing
Instructor: Dr X Shen
Date: 22 October 2014
Dr X Shen©2014
OUTLINE
• Bracing Systems
– Struts
– Rakers
– Tiebacks
• Soil Nailing
• Design of Bracing Systems
2
Dr X Shen©2014
Bracing Systems
• The purpose of the bracing system is to
provide support for and prevent movement
of the retaining elements which are in
direct contact with the soil
• Commonly used bracing systems
– Trench: struts or jacks
– Wider excavation: rakers and tiebacks
3
Dr X Shen©2014
Bracing Systems
• Example:
Timber
Shoring
4
(Courtesy: Nunnally 2011)
(Strut)
Dr X Shen©2014
Bracing Systems
• Wales
– Continuous horizontal members usually
employed in bracing systems
– contacting with the earth-retention system and
transfer the earth loads to the braces
• Main purpose of the wales is to permit the
braces to be placed far enough apart
5
Dr X Shen©2014
Internal Bracing
• Struts
– Most appropriate for narrow excavations
– When installing cross-lot struts, one end is
welded to the wale and the other end stressed
with plates and wedges
– Pipe sections are used for high loads or long
braces
– Cross-lot struts are not feasible for very wide
excavations
6
Dr X Shen©2014
Internal Bracing
7
(Courtesy: SkyscraperPage)
Dr X Shen©2014
Internal Bracing
8
(Courtesy: Pont Cornwall Bridge)
Dr X Shen©2014
Internal Bracing
9
(Courtesy: Mabeyhire)
Dr X Shen©2014
Internal Bracing
• Rakers
– Raker bracing is used for very wide
excavations
– The support for the rakers are installed at the
bottom of the excavation
– Berm: The earth below the raker can be
excavated on a slope to support the sheeting
10
Dr X Shen©2014
Internal Bracing
11
Raker Bracing (Courtesy: Ratay 1996)
Dr X Shen©2014
Internal Bracing
12
(Courtesy: Shorwall)
Dr X Shen©2014
Internal Bracing
13
(Courtesy: RCJ Construction)
Dr X Shen©2014
Tieback Systems
• Tiebacks or Anchors: The structural
system acts in tension and receives its
support in earth or rock
• Tiebacks eliminate obstructions in the
excavation inherent in struts or rakers
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Dr X Shen©2014
Tieback Systems
• Tieback systems consist of:
– The earth or rock providing ultimate support
– A tension member or tendon transferring the
load from soil-retention system to the earth or
rock, e.g. high strength steel
– A transfer agent transferring the load from the
tendon to the soil or rock
– A stressing unit engaging the tendon and
permitting the tendon to be stressed
15
Dr X Shen©2014
Tieback Systems
16
(Courtesy: Ratay 1996)
Dr X Shen©2014
Tieback Systems
17
(Courtesy: Ground Support PLLC)
Dr X Shen©2014
18
(Courtesy: Geo Structures)
Dr X Shen©2014
Earth Anchors
• Earth anchors are usually installed at a
angle of 10 to 20 degree down from
horizontal
• Advantages: less vertical loads introduced
into the sheeting or soldier piles
• The slight depressed angle aids in
placement of the anchor and grout
19
Dr X Shen©2014
Earth Anchors
20
(Courtesy: Ratay 1996)
Dr X Shen©2014
Rock Anchors
• Rock anchors are used in situations where
excavations extend through soil into rock
• The depressed angle for rock anchors is
about 45 degree
• Great care must be exercised in protecting
the rock face under the vertical members
due to the large vertical loads
22
Dr X Shen©2014
Rock Anchors
23
(Courtesy: Ratay 1996)
Dr X Shen©2014
Drilling and Grouting Anchors
• Key factors for selecting the equipment:
– The type of soil to be drilled through
– The depth to the supporting stratum
– The level of groundwater
25
Dr X Shen©2014
Drilling and Grouting Anchors
• Drilling equipment: truck or crane mounted
drills
• Most common materials used to transfer
the loads from the tendon to the soil or
rock are concrete, grout and epoxy glue
26
Dr X Shen©2014
Drilling and Grouting Anchors
27
(Courtesy: vwj.org)
Dr X Shen©2014
28
(Courtesy: Lifetime Developments)
Dr X Shen©2014
How to Choose A Bracing System
• Key Factors:
– Geotechnical conditions
– Excavation geometry
– Cost and time of installation
– Possible consequence to structures or utilities
outside of the excavation due to deflections of
retaining structure
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Dr X Shen©2014
Soil Nailing
• Soil nailing is an earth retention technique using grouted tension-resisting steel elements (nails) that can be designed for permanent or temporary support
• The walls are generally constructed from the top down
• An array of soil nails are installed in a grid that functions to create a stable mass of soil
30
Dr X Shen©2014
Soil Nailing
• Advantages:
– Rapid and economical in the right soil
– Least disruptive to excavations
• Soil nailing requires an unusual amount of
hand work, craftsmanship and
geotechnical knowledge to construct
31
Dr X Shen©2014
Soil Nailing
33
(Courtesy: Geo Structures)
Dr X Shen©2014
Soil Nailing
34
(Courtesy: Gunform)
Dr X Shen©2014
Soil Nailing
35
(Courtesy: Earthwork)
Dr X Shen©2014
Earth Pressure
36
(Courtesy: OSC Shoring Manual 2011)
- Horizontal stress
- Unit weight of soil
- Earth pressure coefficient
- Lateral earth load
- Depth of wall
Dr X Shen©2014
Lateral Earth Pressure for Braced Wall
37
H - Exposed height of
anchored wall
D - Embedded depth
Trapezoidal Shaped Apparent Earth Pressure Distribution
Dr X Shen©2014
Cohesionless Soils
38
Single Braced/Tieback Walls
𝜎𝑎 - Maximum ordinate
T - Horizontal anchor
force
P - Total lateral load
𝜎𝑎 = 1.3𝑃
23
𝐻
𝑃 =1
2𝛾𝐻2𝐾𝑎
Dr X Shen©2014
39
Dr X Shen©2014
Design Procedures
1. Determine the earth pressure coefficients
2. Convert the active earth pressure above the excavation line to a trapezoidal earth pressure
3. Take moments about the tieback to calculate embedment depth D
4. Set summation of forces equal to zero in horizontal direction to calculate tieback/brace force T
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Dr X Shen©2014
Example
• Draw the pressure
loading diagram with
each load indicated
41
Anchor tie rod
9 m
D
3 m
Water Table
Sand
Sand
γdry = 16.8 kN/m3
ϕ' = 32°
γsat = 18.9 kN/m3
ϕ' = 32° γwater = 9.8 kN/m3
20°
Water Table2 m
1 m
Earth Pressure Coefficient
𝐾𝑎 = 𝑡𝑎𝑛2 45 −∅′
2
𝐾𝑝 = 𝑡𝑎𝑛2 45 +∅′
2
Dr X Shen©2014
42
Questions?
