What are braced cuts?

The braced cut is a method of providing lateral support to the soil in foundation trenches, to prevent the trenches from deformation by the caving in of soil and resisting it from moving laterally. During shallow excavation in firm soil, generally, slopes are provided to resist the caving in of soil. During shallow excavation in soft soil and deep excavation, the chances of caving in of soil increase. Hence, it becomes necessary to make a provision to resist the deformation of trenches due to caving in of soil. The braced cut is the method used for such conditions. The excavation using braced cuts is a bit uneconomical as compared to the traditional open-cut excavation method but provides better safety than the cut excavation method.

Sheeting and bracing system

The sheeting and bracing system is the most commonly used system for braced cuts. In this method, vertical planks or sheets are inserted running down along the sides of the excavation, known as sheeting. To support the sheeting, horizontal beams known as wales are provided and to support the wales, struts are provided. For varying soil types as per geotechnical analysis, various methods can be used. The provision of braced cut cancels down the chances of soil caving to a large extent. The various types of sheeting and bracing methods are given below-

• Vertical Timber Sheeting
• Steel Sheet Piles
• Soldier Beams
• Tie Backs

Vertical Timber Sheeting

Vertical Timber sheeting is one of the most common types of sheeting and bracing technique used for braced cuts. The vertical timber sheeting includes timber planks of about 80 mm to 100 mm thick, wales, and struts. The planks are first driven into the soil around the boundary of the trench to be excavated. Excavation of soil between the planks is then done. To hold the planks in place, assembly of wales and struts is provided. Wale runs horizontally parallel to the excavated trench and the struts are placed perpendicular to the two wales (opposite to each other), throughout the trench at specified distances. The vertical sheeting is provided in stages below the ground. Each stage is limited to a depth of 3 meters and different sheeting is provided for every stage.

Steel Sheet Piles

Steel sheet pile is also one of the most common types of sheeting and bracing technique used for braced cuts. In Steel Sheet Pile, vertical sheet piles are used. The sheet piles are made of steel and are driven into the soil by the piling method around the boundary of the trench to be excavated. Excavation of soil is then done and the wales and struts are placed to support the sheet piles in place. The wales are made of steel and the struts can be made of steel or wood. If the width of excavation is large, inclined bracings are provided to support the assembly of sheet piles, wales, and struts.

Soldier Beams

The soldier beams method is a bit expensive but reliable type of sheeting and bracing technique for braced cuts in deep excavations. Soldier beams, also known as H-piles are driven into the soil by the piling method around the boundary of the trench to be excavated. The soldier beams are mostly used in the excavation of hard soil or rocks due to the good durability and driving characteristics of the beams. The soldier beams are driven at the spacing of 1.5 m to 2.5 m around the boundary of the trench to be excavated. Horizontal timber planks called lagging are inserted between the soldier planks, as the excavation proceeds. After reaching a suitable depth, wales and struts are provided to support the assembly.

Tie Backs

The tieback method is one of the advanced types of sheeting and bracing technique used for braced cuts in deep excavations. A tieback is a cable or rod connected to the lagging, sheeting, or sheet pile on one side and is anchored to the soil or rock on the other side outside the excavation area. For this purpose, inclined holes are drilled into the soil, and tendons are inserted through the holes. The holes are then concreted. Every time before advancing the excavation to greater depths, the tendons are prestressed to sustain the increased tension. The advantage of providing the tiebacks is that it cancels out the need of using inclined bracings. As inclined bracings create difficulties in construction, tiebacks can be used instead.

Earth pressure on a braced cut

The members of braced cut, specially sheetings and sheet piles experience lateral earth pressure. The lateral earth pressure is an important parameter of geotechnical engineering. Generally for rigid structures like dams, experiencing maximum moment at bottom of the structure, Rankine's and Coulomb's earth pressure theories are applicable. But for the structures like sheetings, the application of Rankine's and Coulomb's theory cancels out as the sheetings are flexible structures, and maximum moment and rotation take place at the top. The method for analyzing the lateral earth pressure for structures facing maximum moment at the top was developed by Terzaghi. As per the theory of Terzaghi, it is assumed that each strut supports the sheeting area. Later, Peck, in the year 1969, suggested the lateral earth pressure distribution diagrams for sheet pile walls piled in the sand and undrained clay. From the pressure distribution diagrams, the following formulae were calculated-

For sand, the pressure is given as, 

$P_a = 0.65. K_a. \gamma . H$

Where

$K_a$ is the Rankine's earth pressure coefficient

$\gamma$ is the density of soil

$H$ is the height of sheet pile wall structures

For the analysis of clay, the stability number needs to be determined. The stability number is given as,

$N = \gamma . H / C$

Where

$C$ is the undrained cohesion of soil

$H$ is the height of the sheet pile wall.

If $N<4.0$, the clay is treated as sand and $P_a = 0.65. K_a. \gamma . H$

If $N>4.0$, the clay is treated as the clay itself, and pressure is given as, $\gamma .H - 4.C$ or $0.3 \gamma .\hspace{0.17em}H$

In general, the formula for lateral earth pressure for clay is given as, $P_a = K_a. \gamma . H$ = $(1-m). 4.C / \gamma .H$

Where

for $N<4.0$, $m = 0.4 to 0.6$ and for $N>4.0$, $m = 1.0$

Procedure for designing carious components of Braced cut

Struts

There are the following general procedures for designing struts:

• Drawing pressure envelope for braced cuts.
• Determining reactions for cantilever beams and non-cantilever, that is simple beams.
• Calculation of strut loads and choosing proper section from the steel manual.
• Strut loads can be calculated as-
  • $P_a = A.S$
  • $P_b = (B_1+B_2). S$
  • $P_c = (C_1+C_2). S$
  • $P_d = D.S$

Where $P_a$, $P_b$, $P_c$, and $P_d$ are the strut loads at four levels $a$, $b$, $c$, and $d$. $A$, $B_1$, $B_2$, $C_1$, $C_2$, and $D$ are the reactions calculated, and $S$ is the horizontal spacing.

Sheet piles and wales

These are the following general procedures for designing sheet piles:

• Calculating maximum bending moment.
• Obtaining section modulus of sheet piles and wales using the formula,

$Section Modulus =\frac{Maximum B.M.}{Allowable flexural strength of sheet pile}$

• Choosing sheet pile and wales having section modulus greater than or equal to required section modulus.

Context and Applications

The braced cuts are useful for students undergoing the following courses:

• Bachelors in Civil Engineering
• Masters in Civil Engineering

Practice Problems

1. Braced cuts resist the soil from moving in which direction?

1. Laterally
2. Vertically
3. Diagonally
4. Cyclic

Answer: Option a

Explanation: Braced cuts resist the soil from moving laterally.

2. Which of the following is not a type of sheeting and bracing method?

1. Vertical Timber sheeting
2. Soldier Beams
3. Steel sheet piles
4. Steel planks

Answer: Option d

Explanation: Steel planks are not a type of sheeting and bracing method.

3. Which of the following is not a component of the sheet and bracing system?

1. Plank
2. Wale
3. Lintel
4. Strut

Answer: Option c

Explanation: Lintel is not a component of the sheet and bracing system.

4. What is the other name for soldier beams?

1. H-piles
2. Cofferdams
3. C-Piles
4. Angular Beams

Answer: Option a

Explanation: Soldier beam is also known as H-piles.

5. What is the formula for stability numbers?

1. ${\gamma }^3. H / C$
2. ${\gamma }^2. C / H$
3. $H / \gamma . C$
4. $\gamma . H / C$

Answer: Option d

Explanation: The formula for stability number is $\gamma . H / C$