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Soil Engineering

How do soil nails help to improve the stability of slopes?

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.

Moreover, when the soils inside the zone of failure are improved, block failure based on Coulomb is still feasible. By installation of soil nails, it helps to defer the original failure planes of slopes to a greater depth inside the slope, which it is normally of high stability condition with less ingress of surface rainwater.

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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.

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;

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(iii) Provision of steel wire meshes in-between soil nails; and

(iv) Provision of short soil nails in combination of long soil nails.

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.

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 La is the length of soil nail beyond the potential slip circle

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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.

La and Lb in slope

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.

What is the purpose of loading and unloading cycles in pull-out tests of soil nails?

In carrying out pull-out tests for soil nails, it normally requires the loading and unloading of soil nails of several cycles up to 80% of ultimate tensile strength of soil nails. The principal function of soil nail tests is to verify the design assumptions on the bond strength between soil and grout which is likely to exceed the design values based on past experience. In addition, the ultimate bond strength between soil and grout can be determined and this information is helpful as a reference for future design.

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Then someone may query the purpose of conducting load/unloading cycles of soil nails as it does not provide information on the above two main purposes of soil nails. In fact, loading and unloading soil nails can provide other important information on their elastic and plastic deformation behaviour. However, as stress levels in soil nails are normally low, the knowledge on elastic and plastic performance may not be of significant value. On the other hand, the creep and slippage performance of soils nails can also be obtained which may be useful for some soils.

typical-pull-out-test-result

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.

What are the main reasons for conducting pull-out tests for soil nails?

There are mainly four reasons for this test:

(i) To check and verify the bond strength between soil and grout adopted during the design of soil nails. This is the main objective of conducting soil nail pull-out test.

(ii) To determine the bond strength between soil and grout for future design purpose. However, if this target is to be achieved, the test nails should be loaded to determine the ultimate soil/grout bond with a upper limit of 80% of ultimate tensile strength of steel bars.

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(iii) To check if there is any slippage or creep occurrence.

(iv) To check the elastic and plastic deformations of test nails. This is observed during the repeated loading and unloading cycles of soil nails.

Note: Pull-out tests are carried out by applying specified forces in an attempt to pull out the constructed soil nails.

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.