Essential Design Criteria for Safe Braced Cuts in Excavation Projects

Prepared by
Ziyad Assad
Ismail Mokhlis
Ahmad Karim
paywand Shakir
Mohammad Jalal
ERBIL POIYTECHNIC UNIVERSITY
ENGINEERING TECHNICAL COLLEGE
CIVIL ENGINEERING DEPARTMENT
Lecturer: Dr. Zina M. Dawood
Assist Lecturer: Mr. Deedar Hussein
2023-2024
Design Criteria of Braced Cuts
Foundation Engineering and Piles

Outline
• Introduction
• Objectives
• Structural Design
• Construction Considerations
• Pressure Envelope for Braced-Cut Design
• Cuts in Clay
• Pressure Envelope for Cuts in Layered Soil
• Example
• Summary
• Conclusion
• Reference

Introduction
Braced cuts refer to a construction method used in excavations to prevent soil collapse. The design involves
selecting and configuring structural elements, such as soldier piles, sheet piles, or reinforced concrete walls, to
support the excavation's sides. Factors like soil type, depth, and adjacent structures influence the design. The goal
is to create a stable and safe excavation by providing structural support against lateral soil pressure. Professional
engineers use geotechnical principles and structural analysis to develop effective and safe braced cut designs for
specific projects.

Objectives
The objectives of braced cuts design in excavation projects are
primarily focused on ensuring safety, stability, and efficiency.
Here are the key objectives:
2.1.Preventing Soil Collapse:
The primary purpose of braced cuts is to prevent the collapse of
soil or rock walls during excavation. Bracing systems provide
lateral support to the excavation, reducing the risk of cave-ins
and ensuring the safety of workers.
2.2.Maintaining Stability:
Bracing systems help maintain the stability of the excavation site
by resisting the lateral pressures exerted by the surrounding soil
or water. This is crucial for ensuring that the excavation remains
structurally sound throughout the construction process.

Objectives
2.3.Protecting Adjacent Structures:
Braced cuts help prevent damage to nearby structures, utilities, or infrastructure
by controlling the movement of the soil during excavation. This is particularly
important in urban areas where excavations may be close to existing buildings,
roads, or other infrastructure.
2.3.Facilitating Construction Activities:
By providing structural support to the excavation walls, braced cuts enable
construction activities to proceed smoothly and efficiently. This is especially
important in deep excavations where the risk of soil collapse is higher.
2.4.Cost-Effectiveness:
While the installation of bracing systems incurs additional costs, it can be cost-
effective in the long run by preventing accidents, delays, and damage to
property. The investment in braced cuts contributes to the overall success of the
construction project.

Structural Design
The structural design of braced cuts involves determining the appropriate system to provide support and stability
to the excavation walls. The design considerations aim to prevent soil or rock collapse, ensure the safety of
workers, and maintain the integrity of adjacent structures. Here are key aspects of the structural design of braced
cuts:

3.1.Soil Analysis:
Conduct a thorough analysis of the soil properties at the excavation site.
This includes determining soil types, cohesion, angle of repose, and
groundwater conditions. Understanding these factors is crucial for
selecting the appropriate bracing system.
3.2.Depth and Configuration of Excavation:
Consider the depth and configuration of the excavation. Deeper
excavations or irregular shapes may require different bracing systems or
additional support. The design should account for variations in soil
pressure with depth.
3.3.Bracing Systems:
Choose the appropriate bracing system based on the site conditions.
Common types of bracing systems include:
1-Sheet Piles: Interlocking sheets driven into the ground.
2-Soldier Piles with Lagging: Vertical piles with horizontal support.
3-Diaphragm Walls: Continuous walls constructed in panels.
4-Tiebacks or Anchors: Horizontal supports extending into the stable soil.

3.4.Loadings and Forces:
Analyze the loads and forces acting on the excavation walls. This includes earth
pressure, water pressure, surcharge loads, and any loads imposed by adjacent
structures. The design should account for both static and dynamic loads.
3.5.Global and Local Stability:
Assess the global stability of the entire excavation and the local stability of
individual elements of the bracing system. Ensure that the braced cut remains stable
under various loading conditions.
3.6.Deflection Limits:
Establish deflection limits for the braced cut to prevent excessive movement that
could lead to wall failure. Deflection criteria should consider the impact on adjacent
structures and ensure the safety of workers.

4.Construction Considerations
The construction of braced cuts involves careful planning, execution, and
monitoring to ensure the safety of workers, protect adjacent structures, and
maintain the stability of the excavation. Here are key construction considerations
for braced cuts:
4.1.Site Investigation:
Conduct a thorough site investigation to understand soil conditions, groundwater
levels, and any potential obstructions. This information is crucial for selecting the
appropriate bracing system and construction methods.
4.2.Bracing System Selection:
Choose the most suitable bracing system based on site conditions, excavation
depth, and project requirements. Common bracing systems include sheet piles,
soldier piles with lagging, diaphragm walls, and tiebacks.

4.3.Construction Sequence:
Develop a construction sequence that considers the phased installation and
removal of bracing elements. The sequence should prioritize safety and stability
throughout the excavation process.
4.4.Excavation Methods:
Select excavation methods that align with the chosen bracing system. This may
involve using excavators, clamshell buckets, or other specialized equipment.
Careful excavation is crucial to prevent damage to the bracing system and
maintain stability.
4.5.Water Control:
Implement effective dewatering and water control measures, especially if
groundwater is present. Excessive water can increase hydrostatic pressure on the
excavation walls, affecting stability.

4.6.Shoring and Tieback Installation:
Install shoring elements such as sheet piles, soldier piles, or diaphragm
walls according to the design specifications. Ensure proper alignment
and embedment depth. Install tiebacks or anchors if required for lateral
support.
4.7.Monitoring and Inspection:
Implement a comprehensive monitoring and inspection program to
assess the performance of the bracing system during construction. This
may involve the use of inclinometers, settlement gauges, and other
monitoring tools.
4.8.Safety Measures:
Prioritize safety measures for workers involved in the excavation. This
includes the use of personal protective equipment (PPE), barricades,
warning signs, and regular safety briefings.

Pressure Envelope for Braced-Cut Design
the lateral earth pressure in a braced cut is dependent on the type of soil, construction method, and type of
equipment used. The lateral earth pressure changes from place to place Each strut should also be designed for
the maximum load to which it may be subjected.
Using the procedure just described for strut
loads observed from the Berlin subway cut,
Munich subway cut, and New York subway cut,
Peck (1969) provided the envelope of
apparent-lateral-pressure diagrams for design
of cuts in sand. This envelope is illustrated in
the Figure, in which

Cuts in Clay
Peck (1969) provided the envelopes of apparent-lateral pressure diagrams for cuts in
soft to medium clay and stiff clay in a similar manner. The pressure envelope for soft
to medium clay is shown below
Here ɣH/c is a dimensionless number that is used to separate soft to
medium clay from stiff clay. The pressure, бa, is the larger of
The pressure envelope for cuts in stiff clay is shown in Figure, in which is applicable
to the condition ɣH/c ≤ 4. Keep the following points in mind when using the pressure
envelopes just described:

1. They apply to excavations deeper than about 6 meters.
2. They are predicated on the assumption that the water table is below the cut's bottom.
3. Sand is assumed to be drained with zero pore water pressure.
4. Clay is assumed to be undrained and pore water pressure is not considered

Pressure Envelope for Cuts in Layered Soil
When constructing a braced cut, layers of both sand and clay may be
encountered. Peck (1943) proposed that an equivalent value of cohesion be used
in this case.
( ϕ = 0) . should be determined according to the following formula
The layers' average unit weight can be expressed as
where ɣc is the saturated unit weight of clay layer.
average undrained cohesion becomes

Example
The cross section of a long braced cut is shown in the Figure.
a. Draw the earth pressure envelope.
b. Determine the strut loads at levels A, B, and C.
c. Determine the section modulus of the sheet-pile section required.
d. Calculate the design section modulus of the wales at level B.
( In the plan, the struts are spaced 3 m apart, center to center.) Use

Solution
Part a/ We are given that ɣ = 18 kN/m2 , c = 35 kN/m2 , and H = 7 m. So,
Thus, the pressure envelope will be like the one in the Figure . The
envelope is plotted in Figure a with maximum pressure intensity, бa, equal
to 0.3ɣH = 0.3(18)(7) = 37.8 kN/m2.
Part b To calculate the strut loads, examine Figure b. Here, the strut–
sheet-pile connection at B is assumed to be a hinge, so the pressure
diagram is separated at B into two blocks. Taking the moment about B1,
we have ∑MB1 = 0, and

Part c At the left side of Figure b, for the maximum moment, the
shear force should be zero. The nature of the variation of the shear
force is shown in Figure c. The location of point E, where the
maximum moment occurs, can be given as

Because the loading on the left and right sections of Figure b is the same,
the magnitudes of the moments at F and C (see Figure c) will be the same
as those at E and A, respectively. Hence, the maximum moment is 27.03
kN.m/meter of wall.
Part d
Part b has thus calculated the reaction at level B.

In designing braced cuts, it is necessary to know the lateral earth pressure distribution along the vertical walls.
The lateral earth pressure distribution along the walls depends on several factors, including the soil type, the
method of construction, and the type of equipment used. Due to the difficulties associated with determining
the lateral earth pressure distribution, we use lateral earth pressure envelopes, which conservatively include
all possible scenarios. Three pressure envelopes for sand, soft-medium clay, and stiff clay proposed by Peck
(1969) are commonly used in the design of struts, wales, and the sheet-pile sections. While the struts resist
compressive axial loads, the wales and the sheet piles undergo bending stresses. In clay, the braced
excavation may fail by bottom heave in spite of properly designed struts, wales, and sheet piles. This is more
common in soft clay, where the clay from the sides is pushed into the excavation through the bottom. This
was treated as a bearing capacity problem, and an expression for the factor of safety was suggested.
Summary

Conclusion
In conclusion, braced cuts stand as a fundamental engineering solution that seamlessly integrates safety, stability,
and environmental consciousness in excavation projects. Functioning as a bulwark against soil collapse, these
structures prioritize the well-being of construction personnel through the provision of structural support, assuaging
the risk of accidents and ensuring a secure work environment. The meticulous structural design, encompassing
considerations of soil properties, groundwater dynamics, and load distributions, underscores the commitment to
achieving both global and local stability. Notably, the adaptability of braced cut construction to site-specific
conditions, the implementation of effective environmental controls, and strict adherence to regulatory standards
showcase a comprehensive approach to responsible construction practices. Furthermore, the successful execution
of braced cuts hinges on thoughtful construction sequencing, continuous monitoring, and a commitment to open
communication among project stakeholders. As a result, braced cuts emerge not merely as excavation support
structures but as a testament to the harmonious integration of engineering expertise, safety imperatives, and
environmental stewardship in the construction landscape.

Reference
 Terzaghi, K. (1943). Theoretical Soil Mechanics, John Wiley & Sons, New York. U.S. Department of the
Navy (1971). “Design Manual—Soil Mechanics, Foundations, and Earth Structures.” NAVFAC DM-7,
Washington, DC
 Swatek, E. P., Jr., Asrow, S. P., and Seitz, A. (1972). “Performance of Bracing for Deep Chicago Excavation,”
Proceedings of the Specialty Conference on Performance of Earth and Earth Supported Structures, American
Society of Civil Engineers, Vol. 1, Part 2, pp. 1303–1322.
 Chang, M. F. (2000). “Basal Stability Analysis of Braced Cuts in Clay,” Journal of Geotechnical and
Geoenvironmental Engineering, ASCE, Vol. 126, No. 3, pp. 276–279

Essential Design Criteria for Safe Braced Cuts in Excavation Projects

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