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Soil Engineering

Can sheets or geo-grids replace reinforcing elements in soil nails?

Where soil nails are intended for improving the slope stability of existing ground, sheets or geo-grids can hardly replace reinforcing elements in soil nails. Practically speaking, the reinforcing of existing slopes limits the types of reinforcing elements to be adopted. For instance, sheets or geo-grids do not have sufficient bending stiffness to be inserted into exiting slope and they are usually placed in soils as soil layers are built up. The reinforcing element of exiting ground requires steel bars with good tensile strength.

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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.

Can grout be utilized in providing tensile resistance of soil nails?

The passive nature of soils nails requires a small movement for the nails to take up loading. During this process, it is understood that the grout annulus around the nail would crack to allow for these small displacements.

Therefore, the tensile capacity of grout is normally ignored in design and only compressive capacity might be considered.

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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.

Why are steeply inclined soil nails not commonly used in stabilizing slopes?

Based on stress-strain relations, when the angle of soil nails below the horizontal (angle b in the figure below) is small, the soil nails intersect the normal to the potential slope failure surface at a relatively large angle. Tensile stresses can be readily developed in the soils nails. However, if the angle of soil nails below the horizontal is large, the soil nails intersect the normal to the potential slope failure surface at a small angle and little or no tensile stress would be mobilized in the soil nails. In this case, it is ineffective in utilizing its tensile strength to prevent slope failure. In the worst scenario, the soils nail would under the state of compression for even steeper soil nails.

Different inclination of soil nails

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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.

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.

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