Brace Cut

27/12/2020
Braced Cut Design

Ahmad Safuan A Rashid
Introduction
• Deep excavations with vertical sides require
lateral supports to prevent cave in of the
earth and to protect the adjacent areas
against ground subsidence and lateral
movement of the subsoil.
• When excavations are shallow and ample
space is available, the sides of the excavation
can be sloped at a safe angle to ensure
stability.
• However, in deep excavation, especially in
built up areas there may not be adequate
space for providing safe slopes.
• Moreover it becomes uneconomical to
provide safe slope because of large
quantities of earth involved.
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Introduction
• Excavations which are laterally supported are
Braced.
• The vertical sides of the excavations are
supported by a sheeting and bracing system.
• It consists of relatively flexible sheeting placed
against excavations walls. The lateral thrust on
the sheeting is resisted by the horizontal
members in compression (struts).
• Bracing is provided as the excavation proceeds
and the face of the sheeting becomes exposed.
So, various types of the Bracing systems is
adopted to make the excavation stable.
• Sheet piling is used primarily as a bulkhead to
hold or restrict the lateral movement behind it.
Introduction
• A Gravity Retaining wall is a Permanent Structure, used, when an
excavation is permanent.
• But when excavation is temporary i.e. excavation for buildings or
subway, the excavation is filled with a structure which then permanently
retain surrounding soil/earth. If the temporary excavation is made in
sand, the walls of the excavation must be supported during construction
of the building by a system of bracing.
• Example of constructions - Laying underground pipeline, construction of
bridge abutment, construction of basement, metro railway construction,
construction of subway tunnel.
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Introduction
• Types of braced cut systems:
• Vertical Timber Sheeting
• Steel Sheet Piles
• Soldier Beams
• Tie Backs
Vertical Timber Sheeting

• In this method, vertical timber sheeting consisting of the planks about 8
to 10 cm. thick are driven around the boundary of the proposed
excavation to a depth below the base of the excavation.
• The soil between the sheeting is excavated.
• The sheeting is held in place by a system of Wales and struts.
• The Wales are the horizontal beams running parallel to the excavation
wall.
• The Wales are supported by the horizontal struts which extend from the
side of the excavations.
• However, if the excavations are relatively wide, it becomes economical
to support the Wales by inclined struts, known as rakers.
• For inclined struts to be successful, it is essential that the soil in the
base of the excavation is strong enough to provide adequate reaction.
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Vertical Timber Sheeting

• If the soil can temporarily support itself an excavation of limited depth
without an external support, the timber sheeting can be installed in the
open or in a partially completed excavation.
• Vertical timber sheetings are economical up to a depth of 4 to 6 meter.
Steel Sheet Piles

• Piles, or sheeting, driven in close contact to form a continuous
interlocking wall which resists the lateral pressure of water or earth. In
this method, the steel sheet pile is driven around the boundary of the
proposed excavations.
• A continuous line of pile is driven in advance of excavation. As the soil is
excavated from the enclosure Wales and struts are placed.
• The Wales are made of the steel. The lateral thrust from the sides is
resisted by horizontal members called the struts are placed across the
excavation And wedged against the Wales. The struts may be of the
steel or wood.
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Steel Sheet Piles

• As the Excavations Progresses, another set of Wales and struts is
inserted. The process is continued till the excavation is complete. It is
recommended that the sheet piles should be driven several meters
below the bottom of excavation to prevent local heaves. If the width of
a deep excavation is large, inclined bracing may be used.
• The upper strut is placed when the excavation is shallow and little
lateral yield of soil has occurred to change appreciably the original state
of stress. As excavation proceeds downward the lower part of the face is
freely to yield inward before it could be restrained by the next strut. The
inward yield of soil increase with an increase in the depth of excavation.
Thus problem is analogues to a retaining wall tilting about its top. The
sheeting tilts about its tops.
Steel Sheet Piles
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Soldier Beams
• Soldier beams are H-piles which are driven at suitable spacing of 1.5 to
2.5 m. around the boundary of the proposed excavation.
• As the excavation proceeds, horizontal timber planks called lagging are
placed between the soldier beams.
• When the excavation advances to the suitable depth, Wales and the
struts are inserted. The lagging is properly wedged between piles
flanges or behind thee back flange.
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Tie Backs
• In this method, no bracing in the form of struts or inclined rakes is
provided. Therefore, there is no hindrance to the construction activity to
be carried out inside the excavated area. The tie back is a rod or a cable
connected to the sheeting or lagging on one side and anchored into the
soil or rock out side of the excavation area. Inclined holes are drilled in
to the soil Or Rock, and the tensile reinforcement (tendon) is then
inserted and the hole is concreted.
• An enlargement or bell is usually formed at the end of the hole. Each tie
back is generally prestressed before the depth of excavation is increased
further to cope with the increased tension.
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Lateral earth Pressure

• Rankine's and coulomb earth pressure theory can not be used for the
computation of the lateral earth pressure on sheetings, as those
theories are applicable to rigid retaining walls rotating about the base.
• The sheeting and bracing system is somewhat flexible and rotation takes
place at the top of the wall. Sheeting are placed against the walls of the
excavation when these are shallow. The upper strut is placed when the
excavation is shallow and the lateral yield of the soil has occurred. As
the excavation proceeds downwards, the lower part of the face is free
yield inward before the next strut is placed.
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• The inward yield of the soil increases with an increase in the depth of
excavation. Thus the sheeting tilts about its top. The method of earth
pressure of calculation has been developed by Terzaghi based on the
observations of actual loads in struts in full scale excavations in sand in
BERLIN and in soft clay in CHICAGO.
• Pressure distributions against the sheeting have been approximated on
the assumptions that each strut support the sheeting area. The effect of
various factors is not fully understood. However, the results of the field
studies can be used as a basis for developing earth pressure diagram
required for the design of the bracing system.
• The pressure diagram recommended for design represent an envelope
which encompass the actual pressure distribution diagram obtained
from the field tests. These design pressure diagram are also known as
apparent pressure diagram.
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• The pressure diagram for braced cut in dry or moist sand. The pressure
diagram is uniform with a pressure (Pa) equal to 1.6(Pa/H) or (0.65.γ
.H.Ka).
• Where Ka is Rankine's earth pressure coefficient, given by
• Pa= Total normal active pressure on a wall of height H determined by
coulomb theory .
• The resultant active earth pressure diagram is 28% greater than the
coulomb active pressure for dense sand & 44% greater than for loose
sand. Since, the sheeting can not resist, in general, the vertical shear
forces, the friction and adhesion on them are assumed to be Zero.
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N = stability no =
If , The pressure envelop shown in Fig
(b) is used.
The value of the Pa varies between 0.2.γ .H
to 0.4.γ .H. Average value will be taken (
0.3 . γ.H).
If , the pressure envelop shown in Fig
(c) is used. The Pressure Pa is taken as (
γ.H- 4.C) or ( 0.3 .γ .H).
Apparent pressure diagram Peck (1969) or = Pa=Ka.γ .H = [1-m x ]
(m depends on N. N<4.0 m = 0.6 to 0.8 &
N>4.0 m = 1.0 )
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Example
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Example
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Example
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Example
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Example
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What are the different types of braced cut systems discussed?

How does vertical timber sheeting work?

27/12/2020

Braced Cut Design

Vertical Timber Sheeting

Vertical Timber Sheeting

Steel Sheet Piles

Steel Sheet Piles

Steel Sheet Piles

Lateral earth Pressure

Lateral earth Pressure

Lateral earth Pressure

Lateral earth Pressure

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