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What is arching effect in soils?

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Arching occurs when there is a difference of the stiffness between the installed structure and the surrounding soil. If the structure is stiffer than the soil then load arches onto the structure. Otherwise, if the structure is less stiff than the soil then load arches away from the structure.

For instance, if part of a rigid support of soil mass yields, the adjoining particles move with respect to the remainder of the soil mass. This movement is resisted by shearing stresses which reduce the pressure on the yielding portion of the support while increasing the pressure on the adjacent rigid zones. This phenomenon is called the arching effect.

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The principle of soil arching can be easily illustrated by buried pipes. If a rigid pipe is installed in soils, soil columns on both sides of the rigid pipe are more compressive than the soil columns on top of the rigid pipe because of the higher stiffness of rigid pipes when compared with soils. As such, soil columns on both sides tend to settle more than the soils on top of the rigid pipe and this differential settlement causes a downward shear force acting along the sides of soil columns on top of the rigid pipe. As such, the load on the rigid pipes becomes larger than the sole weight of soil columns on its top. Similarly, if a flexible pipe is adopted instead, the above phenomenon shall be reversed.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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