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Soil Engineering

What are the rationales of Observational Method in geotechnical works?

The idea of Observational Method was first discussed by Peck in the Rankine Lecture in 1969. The Observational Method is commonly adopted in geotechnical works in construction phase, though it is also feasible in design stage.

In essence, in the conforming design by engineers during planning stage, the design is usually based on over-conservative approach or most unfavourable conditions owing to a lack of precise and actual site information. During subsequent construction, with precise site information and condition available the Observational Method is adopted in which the original design is revised based on most probable conditions with instrumentation monitoring. If the monitoring results show that performance of the revised design approaches the limit of acceptable level of risk, then it shall be reverted to planned modification which is based on most unfavourable conditions and hence the level of risk is lowered back to the original design. Otherwise, the revised design shall continue and this results in cost reduction without comprising safety of works.

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However, care should be taken in implementing Observational Method when rapid deterioration of the site may occur so that there is insufficient time for carrying out the planned modification. For instance, rapid deterioration can result from development of high pore water pressure in heavy rainfall or burst watermain.

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 measures to reduce the effects of soil liquefaction?

To reduce the effect of soil liquefaction, it is intended to reduce the pore water pressure induced during earthquake shaking. This can be achieved by providing better drainage in soils (e.g. wick drains, sand drains etc. ) and densification of soils (e.g. vibroflotation, dynamic compaction etc.).

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Liquefaction hazards can be reduced by improving the drainage ability of the soil. If the pore water within the soil can drain freely, the build-up of excess pore water pressure would be reduced accordingly.

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.

Does liquefaction occur to sand only?

In liquefaction, the pore water pressure builds up steadily and eventually approaches a value equal to the confining pressure. In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is trapped and this avoids the soil particles from moving closer together. Consequently, this results in an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil. Eventually, soils particles lose contact with each other and behave like a liquid.

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Hence, the type of soils which is susceptible to liquefaction is the one like sand whose resistance to deformation is mobilized by frictional forces between particles under confining pressure. In case the soil is fine grained, cohesive forces tends to develop between these fine particles and it is difficult to separate them. Therefore, sand with increasing content of fines tends to increase its resistance to liquefaction.

The consequence of liquefaction is that the subsequent settlements after liquefaction may damage the overlying structures. Moreover, for sloping ground lateral flow may result which is undesirable. Liquefaction only occurs to saturated soils.

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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 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.

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.

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