What is the relation of bearing pressure on soil nail head to the ratio L_a/L_b?

Where L_a is the length of soil nail before the potential slip circle while L_a is the length of soil nail beyond the potential slip circle

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 L _a/L _b, the greater is the resistance provided by soil nail head.

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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