Soil Engineering

For the construction of diaphragm walls adjacent to buildings, previous experience showed that excess slurry head above groundwater level had to be maintained to limit the ground settlements during the construction of diaphragm walls. In fact, the excess slurry head can be achieved by the following methods. The first one is to construct a ring of well points to lower the piezometric level to achieve a higher excess slurry head in diaphragm walls. Alternatively, guide walls may be raised above ground level to accommodate the slurry column.

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.

The functions of diaphragm walls are as follows:

(i) It is designed to retain soils during the construction of underground structures.

(ii) It helps to control the movement of ground during construction.

(iii) It is intended to take up high vertical loads from above ground structures during construction (e.g. top-down approach). In addition, during the servicing of the completed structures, the diaphragm walls, internal piles and basement raft act together as a single unit to perform as piled raft.

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.

The use of tiebacks in deep excavation allows uninterrupted earth moving within the excavation zone owing to the absence of interior obstructions. The spacing of tieback should not be placed too close as this may impair the capacity of tieback because of the interference between adjacent grouted zones.

A tieback is made first by drilling a hole by a drill rig, followed by placing a bar in the drilled hole. Concrete is then poured in the hole and the connection of tieback with wall is made lastly. A tieback anchor consists of an anchorage located in a bearing layer and the anchor is tensioned at the front face of the wall. The portion of the anchor which transmits the force to the surrounding soil is called the “fixed length”. One the other hand, the “free length” of tieback transfers the force from the fixed length through the anchor head to the 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.

When excavation work is carried out in grounds with highly permeable soils, other than the installation of well points to lower down the groundwater table, consideration may be given to the injection of grout to the soils.

The purpose of the injection of grout is to fill the pore spaces and cavities of soils with grout and to reduce the permeability of soils. The method of grouting is effective in coarse soils but not for sands. In essence, “grout curtain” is constructed around the excavation by installation of several rows of injection holes for grouting.

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.

Diagonal strutting is sometimes used at the corners of excavation to leave a large working space at excavation level. Sometimes, raking struts are observed in long flying shores across excavation and these struts serve to decrease the span length of struts.

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.

Preloading in struts in braced excavation helps eliminate the potential movement. The application of preload decreases the shear stress in soils established previously by excavation. As a result of stiffening of soils, the soil movement is declined accordingly.

However, the use of very high preloads in struts may not be desirable because the local outward movement at struts may cause damage to nearby utilities.

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.

The use of active and at-rest theory is not applicable in braced excavation. In essence, upper struts tend to be more heavily loaded while lower struts appear to be less loaded when compared with active pressure theory.

Peck then measured the bracing loads which were converted back to soil pressures. For example, the pressure envelope for non-cohesive soils is 0.65rHK_a
where
r=soil density
H=height of excavation
K_a=active pressure coefficient

Some engineers consider r as total soil weight without applying any water pressure. However, Peck has said “the earth pressures are essentially effective active pressures multiplied by a factor and redistributed as a
rectangle or a trapezoid.” Hence, effective weight of soils should be used for r with water pressures added separately.

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.

In braced excavation, sheetpiling is constructed at the first place, followed by the installation of struts as excavation proceeds. Following the installation of first row of struts, the depth of excavation is small so that there is no major yielding of soils. However, as further excavation takes place, soils yield before the installation of nth row of struts. The first row of struts prevents yielding near the ground surface. As such, deformation of wall increases with depth with the smallest at the ground level. Owing to the effect of construction method of braced excavation, it differs from the deformation condition of Rankine’s theory. This is attributed to arching effects in which there exists upward redistribution of loads. The upper part of braced excavation is in the state of elastic equilibrium while the lower part is in the state of plastic equilibrium.

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 free earth support method, the soils at the lower part of piling is incapable of inducing effective restraint so that it would not result in negative bending moments. In essence, the passive pressures in front of the sheet piles are insufficient to prevent lateral deflection and rotations at the lower end of piling. No passive resistance is developed on the backside of the piling below the line of excavation.

For fixed earth support method, the piling is driven deep enough so that the soil under the line of excavation provides the required restraint against deformations and rotations. In short, the lower end of piling is essentially fixed.

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.

Bleeding is a form of segregation in which a layer of water migrates to the surface of the grout during the initial stage of cement hydration process. Later on, some of the floating water is re-absorbed into the grout due to further hydration reactions. Even without the problem of bleeding, there is a total reduction of volume of grout after hydration action when compared with the total initial individual volume of cement and reacted water. Bleeding tests should be carried out for grout because of the following reasons:

(i) During bleeding, the upflow of water from grout mixture leads to the formation of channel paths inside the grout mix. These channels act as potential paths for aggressive materials to pass through as these
channels would not be closed during further hydration of the grout.

(ii) The loss in volume by bleeding generates voids inside the grout mix which affects the properties and performance of the grout. Moreover, it increases the chance of corrosion of steel elements protected by the grout. (e.g. tendons)

(iii) In bleeding test, there is a usual requirement of total re-absorption of water after 24 hours of grout mixing because for some cold countries, this layer of water may cause severe freezing problem leading to frost damage.

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.