What Is a Braced Cofferdam?

Tips & Advice

Braced cofferdams are systems used in urban areas to support the walls of an excavation project, or the prevent water from entering an excavation. Braced cofferdams are comprised of a sheeting and bracing system and are frequently used in the construction of bridge piers and abutments.

What Are the Components of a Braced Cofferdam?

Braced cofferdams include the following components:

  • Laggings – Laggings are horizontal timber planks that are set by hand as the excavation is underway.

  • Sheet piling – Sheet pilings are timber, steel or concrete sheet piles that are installed into the earth by vertical pile driving.

  • Soldier beams – Soldier beams are vertical beams made of timber or steel that take reaction from laggings. When sheet piles are used, soldier beams are not required.

  • Wales – Whales are horizontal beams used to transfer reaction from laggings or sheeting to the struts.

  • Struts – Struts are compression member that are used to carry reaction from one side to other side of excavation.

Steel beams: wide flange, h-oile, and more

What Are the Types of Braced Cofferdam?

Braced cofferdams come in three main types:

1. Braced Cofferdams with Wales and Soldier Beams

Braced cofferdams with wales and soldier beams are best used during small excavation projects since no heavy equipment is required for construction. They can be used for excavations with depths up to 4.5 meters.

2. Braced Cofferdams with Wales and Without Sheet Piles

Braced cofferdams with wales and without sheet piles are made of soldier beams that are driven into the earth prior to the start of excavation. Laggings are placed between the soldier beams as the excavation is underway. As soon as the excavation reaches the level of each tier, wales and struts are installed.

Braced cofferdams with wales and without sheet piles is best used for deep excavation projects. This is because the soldier beams allow for stability of the system and prevent the bottom from heaving. Soldier beams are sometimes left in place and used as support of the permanent wall.

3. Braced Cofferdams with Sheet Piles

Braced cofferdams with sheet piles are the final type of braced cofferdam. The sheet piles are driven prior to excavation to the proposed depth. As the excavation project continues, wales and struts are installed. The excavation is continued and another set of wales and struts are installed. The benefits of braced cofferdams with sheet piles is that there is no danger of bottom heave; no danger of loss of ground in granular soils; the sheet pilings work well as cutoff wall; the cost of de-watering is reduced; and piping, boiling and blow up of soil is prevented.

Generally, sheet pile cofferdams are only made large enough to allow for the construction of the project within. Because of this, the physical dimensions of the structure govern the shape in plan. However, it should be noted that when conditions permit, circular cofferdam may be preferable. This is because circular cofferdams wales can act as compression rings without requiring struts, thus eliminating the cost of struts and also providing a large clear working space within the cofferdam.

Common Sizes for Cofferdam Pile Beams

Cofferdam pile beams come in a wide range of sizes, including both wide flange piles and H-piles. Some common sizes include:

  • W8: W8x18, W8x21, W8x24, W8x28, W8x31, W8x35, W8x40, W8x48, W8x58, W8x67

  • W10: W10x22, W10x26, W10x30, W10x33, W10x39, W10x45, W10x49, W10x54, W10x60, W10x68, W10x77, W10x88, W10x100, W10x112

  • W12: W12x26, W12x30, W12x35, W12x40, W12x45, W12x50, W12x53, W12x58 ,W12x65, W12x72, W12x79, W12x87, W12x96, W12x106, W26x120, W12x136, W12x152, W12x170, W12x190, W12x210, W12x230, W12x252

  • W14: W14x22, W14x26, W14x30, W14x34, W14x38, W14x43, W14x48, W14x53, W14x61, W14x68, W14x74, W14x82, W14x90, W14x99, W14x109, W14x120, W14x132, W14x145, W14x159, W14x176, W14x193, W14x211, W14x233, W14x257, W14x283

  • W16: W16x26, W16x31, W16x36, W16x40, W16x45, W16x50, W16x57, W16x67, W16x77, W16x89, W16x100

  • W18: W18x35, W18x40, W18x46, W18x50, W18x55, W18x60, W18x65, W18x71, W18x76, W18x86, W18x97, W18x106, W18x119, W18x130, W18x143, W18x158, W18x175, W18x192, W18x211, W18x234

  • W21: W21x44, W21x50, W21x57, W21x58, W21x55, W21x62, W21x68, W21x73, W21x83, W21x93, W21x101, W21x111, W21x122, W21x132, W21x147, W21x166, W21x182, W21x201, W21x223, W21x248, W21x275

  • W24: W24x55, W24x62, W24x68, W24x76, W24x84, W24x94, W24x103, W24x204, W24x117, W24x131, W24x146, W24x162, W24x176, W24x192, W24x207, W24x229

  • W27: W27x84, W27x94, W27x102, W27x114, W27x129, W27x146, W27x161, W27x178, W27x194

  • W30: W30x90, W30x99, W30x108, W30x116, W30x124, W30x132, W30x148

  • W33: W33x118, W33x130, W33x141, W33x152, W33x169

  • W36: W36x135, W36x150, W36x160, W36x170, W36x182, W36x194, W36x210, W36x232, W36x256

  • HP8: HP8x36

  • HP10: HP10x42, HP10x57

  • HP12: HP12x53, HP12x63, HP12x74, HP12x84, HP12x89, HP12x102, HP12x117

  • HP14: HP14x73, HP14x89, HP14x102, HP14x117

  • HP16: HP16x88, HP16x101, HP16x121, HP16x141, HP16x162, HP16x183

W-beams

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