Construction in Urban Areas

Introduction

With the increase in infrastructure and the subsequent decrease in available land, especially in the more over populated areas such as the CBD, there has been a greater call for construction to move below ground. This has a number of advantages for the property owner such as providing parking or simply increasing the floor area of an existing building, but with this comes a number of challenges. It is a huge task to perform such large excavations without damaging of effecting the structural integrity of the building in question as well as the surrounding buildings.

However construction in urban areas encounter numerous difficulties, not only the potential damage to surrounding buildings, the lack of space above ground for machinery and workforce poses a major threat especially when excavations need to be executed.

Support systems need to be implemented to add to the strength of the soil and prevent the collapse of the excavated surface, various methods used for this include: braced walls, sheet pile walls, contiguous or secant pile walls, RCC retaining walls and diaphragm walls. For the purpose of this assignment focus will be put on diaphragm walls and the various steps taken in the construction of these walls as well as the equipment used in this process, as well as analysis of the bentonite slurry used in the implementation of diaphragm walls.

 

Definition

Robert Ratay described the term diaphragm wall as a reinforced concrete wall constructed below ground level, using the slurry method of trench stabilization. Once fully completed and lateral bracing installed making the wall a retaining or foundation wall depending of the requirements of the building.

 

History

Ratay further adds to the explanation of the history of diaphragm walls and the use of bentonite slurry walls. Diaphragm walls is a fairly recent construction method being developed in the early 1950s, originally called cast-in-place wall. While the bentonite slurry has been used in the industry for rotary drilling majorly in the oil-drilling industry since 1901 and then subsequently being introduced shortly after, with the necessary modifications, to the drilled pile foundation method. The use of the slurry in rectangular excavations was not used until the late 1950s and early 1960s in North America, however the USA was still reluctant to use this method due to the scepticism of the local engineers at the time. It was not until the late 1970s that the diaphragm wall was widely accepted and used throughout the country.

 

Sequence of Construction

Step-1: Construction of Guide Wall

For the installation of the guide wall the working platform needs to be established beforehand as the guide wall is usually an extension of the working platform. The ‘guide wall’ is effectively two parallel concrete walls constructed to establish the location of the proposed diaphragm wall. The guide wall allows excavation in the beginning stages to be more controlled, as well as assisting to control the bentonite slurry as it is pumped into and removed from the trench.

Step-2: Excavation

The excavation of diaphragm walls is done in panels or sections to prevent the load being too great on the shoring, leading to collapse or failure. The panels are divided carefully with considerations made for the maximum load capacity of the excavation. These panels will subsequently be linked later on in the process at the top of the panel, to increase strength.

In most cases, as soon as the excavation is performed the bentonite slurry is placed in the space of the earth with the slurry pump attached to the excavation plant.

Step-3: Installation of Reinforcement Cages

After the excavation of the panel and the settlement of the bentonite slurry a steel reinforcement cage is lowered into the trench with the use of a crane. The reinforcement cage is usually made on site, but can also come pre-fabricated and they are formed into three dimensional cages, one cage for each panel excavated.

In order to ensure that the reinforcement is situated in the centre of the panel and not favouring one side concrete block roller spacers or in some cases steel plate spacers are used.

Step-4: Concreting

For the concreting in diaphragm walls the tremie pipe method is used. The steel reinforcement cage is designed to accommodate the size and number of the pipes depending on the size of the panel excavation.

The concrete is introduced into the trench through hoppers and travels to the bottom of the trench through the pipes by gravity flow. Due to the concrete density being much greater than that of the bentonite slurry, the bentonite slurry will be displaced at the top of the panel.

Step-5: Necessary Repetition

This process is repeated for the necessary number of panels until the entire wall in cast.

 

Bentonite Slurry Detail

Properties

Bentonite is a special type of clay that comes in the powder form; the slurry is then made when water is added to this powder. The resultant bentonite slurry has the ability to mix with water many times its weight in the powder form this allows it to occupy the large spaces in the excavated panel. The most phenomenal quality of the bentonite slurry is its ability form a gel like material when allowed to settle and rest, the agitation of the mixture will then cause it to revert back to its liquid state.

The exact properties of the slurry are difficult to define as the properties of the slurry can be altered using polymers and chemical additives to suit the site specific needs of the project. Ratay mentions that generally a good mix is obtained by mixing water with 5-6% bentonite powder (by weight), but again this can easily altered to suit the requirement s on a specific site.

Some of the key properties that are essential to the use of the slurry mixture are that they can control the underground water flow, separating contaminated water from the natural ground water, and is effectively water tight.

 

Preparation

One of the major costs of using the bentonite slurry which is often underestimated by inexperienced contractors is the establishment and removal of the bentonite farms. The set-up of a bentonite farm requires a large portion of land on site and often a problem where space is limited. The bentonite farm is the area where all the mixing of the slurry is undertaken as well as the cleaning and storage of the material.

The slurry mixture is mixed in a high speed mixer in the bentonite farm. Water is introduced into the tank under pressure the bentonite powder is then added to the mixture and re-circulated until the mixture is the desired consistency. Once the contractor is satisfied with the mixture the slurry is the sent to storage for full hydration. It is extremely essential to the whole process that the slurry mixture is correct. If the slurry mixture is too thick it will interfere with the flow of the concrete during the concreting process, alternatively if the mix is too weak it will lack strength and integrity risking collapse of the excavation.

After to use of the slurry it must be cleaned and de-sanded, this process is essential to the re-use of the material, due to the fact that the slurry mixes with the sand from the excavated material and therefore harms the bond and integrity of the bentonite slurry. This process removes the sand and allows the slurry to be used in another panel excavation; this process will be explained in more detail later in the report.

 

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