A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
11. What is arching effect in soils? (RW9)
Fig. Wires of gabion walls
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.
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.
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