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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.

Is soil nail head an essential feature of soil nails?

Soil nail head is an essential feature of soil nails which help prevent active zone failure. Active zone failure involves shallow failure in the outer part of slope within active zone and it fails by separating and sliding down the slopes and sols nails remain in place.

Soils nail heads also enhances the mobilization of tensile resistance of soil nails and increases the factor of safety of slopes.

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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.

What is the basic mechanism of soil nails in improving soil stability?

Soil nails improve slope stability by:

(i) Partial increase in the the normal force and shear resistance along potential slip surface in frictional soils.

(ii) Direct reduction of driving force along potential slip surface for cohesive and frictional soils.

(iii) Soil nail head and facing provides containment effect to limit the deformation near slope surface.

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Soil nail head serves to provide reaction in mobilizing tensile force in soil nail. Moreover, it provides confinement to soils in active zone behind soil nail head and avoids the occurrence of local failure between adjacent 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 arching effect in soils?

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.