In construction, a deadman is a relatively heavy weight, typically a mass concrete block used to provide support or resistance to a load. These blocks are usually embedded firmly in soil; however, some blocks may simply rest on the ground surface.
These types of anchoring systems are used in several temporary and permanent construction applications, including retaining wall design and guyed masts. In the trenchless construction industry, deadman anchors can be used to provide a reactive force during pipe jacking procedures. In this case, it acts as a thrust wall, which braces the jacking rig as the hydraulic jacks push against the installation pipe.
Deadman systems are also used to "hold down" underground storage tanks during periods of high groundwater levels. When used for this purpose, deadman anchors typically consist of a pair of elongated reinforced concrete beams placed on either side of the tank’s exterior. Straps are then run across the tank, fastening the tank to the deadman anchor beams.
The weight of the backfill bearing down on the two anchors also provides an additional downward force, which helps to counteract buoyancy during high water levels.
A deadman is also referred to as a jacking lug.
Trenchlesspedia Explains Deadman
While a deadman may seem simple in concept, it requires engineering analysis to ensure that it is suitable for a given application. A civil engineer will typically look at the various parameters that affect their effectiveness.
How Does a Deadman Anchor Work?
A deadman block or anchor relies mainly on its mass. The weight of the block, and other resistive forces that accompany its weight, must be higher than the forces being applied to it. In other words, the anchor and the forces keeping it in place must be heavy enough to sustain the load.
For example, in some cases, a deadman may depend on other properties, such as friction. In the figure below, two large concrete blocks are used to provide stability to a tilt-up wall. Here, the block resists two main forces: toppling (or overturning) and sliding.
As the wind load is applied to the face of the wall, the tie-back anchor will transfer a force to the deadman block, which has both a horizontal and vertical component. The weight of the block resists the vertical force, thus preventing the deadman from lifting off the ground or overturning.
The horizontal force, on the other hand, is resisted by the friction generated between the deadman block and the ground surface. This friction force is expressed as:
F = µR
F = the resultant friction force
µ = the coefficient of friction between the block and the ground surface
R = The reaction force (which is equal to the mass of the block)
As we can see from the above equation, the heavier the deadman is, the greater the reaction force; thus, the greater the friction force generated.
What Factors Affect the Strength of a Deadman Anchor?
When assessing a deadman for a particular application, several factors need to be taken into consideration. These include:
The surrounding soil properties.
The size and material of the deadman.
The embedment depth of the anchor (if the deadman is buried underground.)
The direction and nature of the applied load.