Soil Engineering

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.

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.

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.

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.

(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.

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.

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 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.

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.

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.

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.

Gabions are wire mesh boxes which are filled with stones and they are placed in an orderly pattern to act as a single gravity retaining wall. Lacing wires or meshes are designed to hold the gabion boxes together. Most of the wires are zinc-coated or PVC coated to prevent the steel wire from corrosion. Moreover, it is common that the wires are fabricated in hexagonal patterns with doubly twisted joints to avoid the whole gabion mesh from disentanglement in case a wire accidentally breaks. Owing to the nature of gabion filling materials, they are very permeable to water.

They have particular application in locations where free water drainage has to be provided. Moreover, gabions are capable of accommodating larger total and differential settlements than normal retaining wall types so that they are commonly found in locations where the founding material is poor.

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.

Ground water flow into excavations constructed by sheet pile walls should be minimized in order to save the cost of the provision of pumping systems or well points to lower the water table inside the excavation.

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Counterforts are used for high walls with height greater than 8 to 12 m. They are also used in situations where there is high lateral pressure, i.e. where the backfill soils are heavily surcharged.

The counterforts tie the base slab and wall stem together and they act as tension bracing which strengthen the connection between wall and base slab. The counterforts help to reduce bending moment and shear forces induced by soil pressure to the retaining wall. Moreover, it also serves to increase the self-weight of the retaining wall which adds stability to the retaining wall.

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.

A typical brick wall structure normally contains the following components:
(i) a coping on top of the brick wall to protect it from weather;

(ii) a firm foundation to support the loads on the brick wall; and

(iii) a damp course near the base of the brick wall to avoid the occurrence of rising damp from the ground.

Bricks are bedded on mortar which serves the following purposes:
(i) bond the bricks as a single unit to help resist lateral loads;

(ii) render the brick wall weatherproof and waterproof; and

(iii) provide even beds to enhance uniform distribution of loads.

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.

For thick and rigid footings, the pressure distribution under the footings is normally assumed to be linear. If uniform and symmetrical loadings are exerted on the footings, the bearing pressure is uniformly distributed. However, if unsymmetrical loads are encountered, then a trapezoidal shape of bearing reaction would result.

For flexible footings on weak and compressible soils, the bearing pressures under footing would not be linear. As such, a detailed investigation of soil pressures is required in order to determine the bending moment and shear forces of the structure.

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.