Question

a)Explain and Identify the earthwork activities carried out when performing, deep excavation and cuttings. The works must be identify the location for public utility given by public utility provider. You must illustrate and explain the different type of plants used during excavation, supporting system and falsework required to provide in safe working environment in various sub-soils condition.

b)Describe different type of methods for controlling groundwater. You must describe a range of methods of groundwater control during the deep excavation, for example sump pumping, dewatering and land drainage.

Answer 1

Answer (a)

Brief description of different types of plants used for excavation are-

1. Face shovel excavators

It can be of cable or hydraulic operated, mounted on wheel or track .They are fitted with bucket which faces away from the machine. They are used for loosening, excavating vertical or near-vertical soil above the machine base level.

2. Backhoe

It is used for below ground level excavation. The bucket acts downwards and drag towards the machine and tilted upwards to hold the loads. They are used mainly as trench or large scale open excavation, but sometimes they are also used as loading machines.

3. Bulldozers

It is used for grading materials to levels over relatively smaller area ,to cut small tree ,remove surface vegetation or hard surfaces etc .The max cut is about 40Omm below base of the machine.

4. Tractor shovel

This machine is similar to a bulldozer but has a hydraulic operated bucket in place of the blade. Materials above the base of vehicle can be lifted and unload onto a dump truck or onto a spoil heap .

5. Clamshell excavator

It is like a crane, usually track mounted, and hanging a wire operated clamshell at the jig. It is used to handle or load soft soil on site.

6. Powered shovel or drill

It is used for cutting of larger boulders or rock. Usually the drill is pneumatically operated and mounted on a tracked base.

7. Grader

A grader does not excavated but it levels and grades out to fine loose or deposit materials. A centrally mounted blade much narrower and f latter than a bulldozer's serves the purpose.

8. Scraper

It works similar to a grader but it has a container to hold the surplus soil after scarifying .The container which is filled with soil can also serve the purpose of backfill of hollow ground.

9. Dumper

This is a smaller vehicle with a tipping hopper or skip designed to carry material within a site. The hopper is usually front mounted to provide better control by the driver. It is easy to manoeuvre on uneven and rutted ground. It helps in transporting the soil from one place to other.

10. Dumper truck

It is designed for large-capacity loads to be carried over a long distances .

Normal capacity ranging from 6 to 15m3 , some even up to 40 m 3 or above.

Following are the supporting system used in deep excavation-

1. Dumpling method

   This is used where there are buildings or street in the proximity. The method is to construct a series of retaining wall in trench, section by section, around the site perimeter , leaving a centre Called "dumpling" When the perimeter walls are in place, excavation may start at the centre of the dumpling, until exposing a section of the wall. Then the wall may be side supported by struts, shoring or soil anchor etc., again section by section in short length, until the excavation is all completed.

2. Diaphragm walling

This method need to construct a R.C. retaining wall along the area of work. Because the wall is designed to reach very great depth, mechanical excavating method is employed. Typical sequence of work includes:

i) Construction of a guide wall

ii) Excavation for the diaphragm wall

iii) Excavation support using bentonite slurry

iv) Inert reinforcement and concreting Construct a guide wall – guide wall is two parallel concrete beams running as a guide to the clamshell which is used for the excavation of the diaphragm wall.

3. Using cofferdams

A cofferdam may be defined as a temporary box structure constructed in earth or water to exclude soil or water from a construction area, such as for foundation or basement works. Interlocking type steel sheet pile is used and can use for excavation up to 15m.

Sheet pile in this case acts as a cantilever member to support the soil therefore adequate depth of pi le or suitable toe treatment may be required. In addition, cofferdams are need to be braced and strutted or anchored using tie rods or ground anchors. Making use of sheet pile to form a cofferdam to support excavation

Sheet Steel Piling Steel, amongst other materials such as timber, is most effective to be used as sheet pile due to its high tensile as well as their interlocking ability. It can be used as timbering to excavation in soft or waterlogged soils especially in congested site where there is no enough space for complicated shoring.

Answer (b)

Different type of methods for controlling groundwater are-

1.Sump pumping-

It is the most common and economical method of dewatering as gravity force plays main role. Sump is created in the excavated area into which the surrounding water converges and accumulates facilitating easy discharge of water through heavy solid handling pumps. It is used to the areas where soil is either gravelly or sandy or cohesionless.

2.Dewatering

Just like the well point method, wells are drilled around the excavated area.. In this case, diameter of wells kept around 200mm. By making deep wells around the vicinity, the groundwater is made to fall into them under the influence of gravity force. Casings of diameter fitting to wells may be used in order to retain the wells.

3.Deep well drainage

Deep well drainage system consists of deep wells and submersible or turbine pumps which can be made outside the zone of construction operations and the water table lowered to the desire level. Deep wells are usually spaced from 10 -80 meters depending upon the level to which water table must be lowered, permeability of the sand stratum, source of seepage and amount of submergence available. Deep well system is suitable for lowering the ground water table where the soil formation is pervious with depth; the excavation extends through or is underlain by coarse-grained soils. This method is also suitable when a great depth of water-lowering is required