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
5. What is the relation of bearing pressure on soil nail head to the
ratio La/Lb, where La is the length of soil nail before the potential slip
circle while Lb is the length of soil nail beyond the potential slip circle?
(SN2)
The unstable soil mass before the potential circular slip is resisted by two
components: soil nail head bearing pressure and friction of soil nail in the
unstable soil mass. Therefore, the longer is the length of soil nail before
potential slip circle La, the higher is the proportion of forces being resisted
by frictional forces and hence the smaller amount is to be resisted by soil
nail head. Hence, the smaller the ratio La/Lb, the greater is the resistance
provided by soil nail head.
Fig. La & Lb in slopes
6. Soil nails are mainly designed for stabilization of major slips. How
should designer cater for the stability of minor slips?
There are some methods to treat minor slips:
(i) Adoption of smaller diameter size bars at closer spacing;
(ii) Installation of tie beams at the same horizontal levels;
(iii) Provision of steel wire meshes in-between soil nails; and
(iv) Provision of short soil nails in combination of long soil nails.
7. How do soil nails help to improve the stability of slopes? (SN1)
It is commonly believed that with the introduction of soil nails to slopes, this
new combination of elements possesses higher shear strength than the
original soils. In the context of Rankine’s active state, soil nailing serves to
provide horizontal restraint to guard against active failure.
318