In this assignment you are to assume that you are working for a Consulting Civil Engineer, Shepherd and Hitch and you have been tasked with preparing a report on the selection of construction for the substructure and drainage for this multi-storey car park the choice of superstructure has been decided in your second assignment. You may use information gained from Unit 7 Geology & Soil Mechanics about the site in the selection of your substructure and you may use Task 2 to illustrate Assignment 2 of Unit 7 Geology & Soil Mechanics.

Tasks

1. Select and describe the type of plant required for these operations.

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2. Describe and illustrate the means of installing piling systems and ground stabilisation activities for the scheme. (P5, P6, M1, M2, M3, D1, D2, D3)

3. Describe and illustrate the construction of the drainage system to the site and provide a simple safety plan for the site. (P7, P12, M2, M3, D2)

4. Describe in outline the provision of health and safety on the site, including the application of the CDM regulations and the role of the CDM coordinator. (P11, P13, M2, D2)

Task 1

The following plants will be used to carry out the construction for the substructure and drainage for this project:

Excavators

Forward tipping dumpers

Vibratory rollers

Excavators

The excavators will be used to excavate the substructure and drainages in this project. This is operate by the boom digging towards the machine in an arc starting at ground level to a position vertically below the rear edge of the machine. Loading is carried out by swinging the boom away from the excavation and depositing the material either on a stockpile or in to haulage vehicles. The maximum depth of excavation depends on the length boom and size of the base unit. (CECA07-4)

Forward tipping dumper

They are designed for carrying bulk material. A dumper is open 4 wheeled vehicles with the load skip in front of the driver. The skip can tip to dump the load. The Forward tipping dumper is usually diesel powered. A towing eye is fitted for secondary use as a site tractor

Vibratory rollers

A vibratory roller is a compactor having a drum (roll or horizontal cylinder) used to densify (compact) soil, asphalt or other materials through the application of combined static and dynamic forces (weight and vibrations) to increase the load-bearing capacity of the surface.

Task 2

Ground stabilization is required mainly for reducing settlement of foundations of roads and structures. Ground stabilization is any process which improves the physical properties of a soil, such as increasing the shear strength, bearing capacity and the resistance to erosion, dust formation, or frost heaving.

It can be achieved by a variety of treatments such as:

Cement stabilization

Lime stabilization

Geotextiles

Vibroflotation

Dynamic consolidation

Cement stabilization.

This process increases shear strength and decreases permeability of the soil. The soils best suited to this treatment are well graded sands and gravels. The amount of organic matter and sulphates present in the soil must be determined as these will inhibit the hardening of the soil/cement mixture. The soil water contains and dry density should be measured. An increase in comp active effort to achieve higher dry density can reduce the amount of cement required.

Different soil types require varying amounts of cement. Generally, the finer the soil the larger the amount of cement required.

The process requires thorough mixing of the soil and cement. There three main types of plant which are used.

Single pass travel mixer

These take up the soil from the formation, pulverized it, add the cement, dry mix the soil/cement, add the water, remix the wet soil/cement, spread it, screed it and compact it.

Central mixing

Soil is excavated and transported to a central mixer using traditional earth moving plant. The mixed material is then taken back to site and deposited by normal paving machine and spread in layers up to 300 mm thick. (Pictures)

Multi pass

The soil is pulverized with a mixer and then spread with a grater .Cement is then deposited over the soil surface and dry mixed into the soil. Water is added (if required) in stages until the desired water content is reached. The soil is wet mixed at each addition of water. Finally the surface is graded again and then compaction must be completed within 2 hours

Lime stabilization

Lime stabilization is applicable to soil types varying from clays to coarse sands.

The overall effect of lime stabilization is to increase the plastic limit of the soil while leaving the Liquid Limit virtually unchanged. This results in a lower Plasticity Index. In addition, water absorption and permeability are reduced giving better frost resistance, increased shear strength and improved stability against swelling and shrinkage.(pictures)

Geotextiles

These are permeable synthetic membranes which are used to enhance the performance of natural materials. They can be used to stabilize subsoil and are used in both temporary and permanent road construction as subgrade.sub base separators and for drainage. For drainage purposes permeability of the textile can be matched to the grading of the soil particles to optimize the flow of water.

Other methods can also be used to consolidate and strengthen the ground, the two most common being are:

Vibroflotation

Dynamic consolidation

These methods of deep compaction can be used on both natural soils and fill materials. Generally these methods are cheaper than grouting for control of settlement. However, they don’t increase the cohesion of the soil particles and so don’t increase stability of excavated surfaces.

Vibroflotation is term used to describe deep compaction methods which make use of vibrating poker.

There are three main techniques

1 Vibrocompaction

Penetration

The Vibroprobe penetrates by vibration and aid of compressed air and water to the required depth.

Compaction

The Vibroprobe is retracted from the maximum depth in 0.5m intervals. The in situ sand or gravel is flowing towards the Vibroprobe.

Backfill

the compaction is achieved either with backfill from the top or with in situ soil only.

Vibro compaction is a deep compaction technique for densifying sandy soils in place, by means of vibration, in that loose sand particles are rearranged into a more compact state. The improved soil will be capable of higher bearing capacity, and to mitigate subsoils liquefaction potential associated with seismic activities.

( See www.laynegeo.com/vibrocompaction_technical)

2 Vibroreplacement

This is a wet process used on soft cohesive soils. The ground can be improved by replacing the weak materials with well compacted columns of granular material. These columns don’t carry all the load however. The effect of compacting the stone also compacts the ground in between.

The vibroflot is jetted into ground to form a hole of the required depth. The vibroflot is then removed. The hole is then filled with with a coarse gravel sized backfill which is compacted using the vibroflot.

3 Vibrodisplacement

On some sites the use of water jetting can cause problems, especially in urban areas where there are difficulties in disposing of the resulting effluent. A dry technique has been developed which uses compressed air instead of water. Stone columns are formed then.

Dynamic consolidation

This process consists of dropping a large weight, 10to 20 tones, on to the ground from a height of 10 to 20 meters. The site is first covered dropping the heavy weight on a widely spaced grid and then covered again with lighter drops at a closer spacing. The effectiveness of this method reduces as the permeability of the soil decreases. Saturated clay soils do not show much improvement while granular materials undergo considerable compaction. Clay fill, however, usually compacts well as it is made up from larger clumps with air voids between them.

The depth of ground which will be stressed by permanent construction will determine the depth to which the treatment is required. In fine grained soils of low permeability it may be necessary to form stone columns in the soil prior to dynamic treatment in order to provide a drainage path for the pore water.

This method is only economic on large sites (in excess of 5000 square meters) which are not close to existing buildings or buried services

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