CONSTRUCTION OF BASEMENT OF A TWENTY STOREY BUILDING IN GOLDCOAST, AUSTRALIA

 

CONSTRUCTION OF BASEMENT OF HIGH RISE BUILDING

Construction of Basement of High Rise Building

Introduction

The foundation is the first section of a building to be constructed and creates a base for the rest of a house’s components. The foundation carries and transfers the load of the building onto the ground and is usually located below the ground surface level. The bearing capacity of the ground is the amount of load that can be supported by the ground on which the building rests. The following are the different forces that the foundation encounters:

  1. Dead loads- the constant weight of the building arising from the walls, floors, roof etc.
  2. Live loads (imposed loads) - this is the weight of items like furniture, goods and people and they are movable. These also include weight resulting from rain and snow.
  3. Wind loads- these are the loads caused by the wind. These may be positive of negative.

The foundation is supposed to support the building and safeguard it from lateral and vertical geological actions. High rise buildings require a firmer foundation because of the high center of gravity.

In Australia, some of the types of foundations in use for the construction of buildings are

-        Piled foundation

-        Raft foundation

-        Piled raft foundation

-        Non-piled raft foundation

-        Slab foundation

-        Slab-pile foundation

-        Basement foundation

When a structure is constructed on the ground, it exerts pressure on the top soil. Due to the flexibility of the top soil, settlement occurs. The extent of settlement depends of among other factors:

-        The thickness of the subsoil layer.

-        The distribution of the loads on the foundation.

-        The depth of the foundation.

-        The pressure at the contact points between the foundation and the ground.

-        The flexibility of the building structure.

In order to minimize settlement and make the building firm, upper layers of the soil are removed so that the foundation of the building rests on the firmer bedrock. This may require many meters’ depth of soil to be excavated. In most cases, the basement is constructed by digging out large quantities of earth and then structuring the space created with concrete and steel in order to create room which can be occupied.  In most cases, this room is used for car parking or storage purposes. According to Newton’s third law of motion, the downward force must be equal to the upward force for the system to be in equilibrium. If the downward force arising from the weight of the building exceeds the upward force, then the building will sink and the reverse of this creates a heave on the building.

Engineers in Australia adapt new technologies from time to time in order to reduce the cost of construction for the benefit of both the contractor and the owner of the building. One of the most popular types of foundation used in construction of high rise buildings in Australia is the basement foundation. Basement has the advantage of being deep enough so that it rests on the solid bedrock and hence making the building more stable and form. It safeguards the building from both vertical and lateral geological movements. Another advantage of basement is that it creates room which can be used for many purposes. The space created when the ground is excavated would be very expensive to fill with concrete and therefore constructing a basement is actually and economical advantage. The fact that basements are usually deep and rest on solid rocks in Goldcoast makes the settlement of the building almost negligible and very low probability of tilting of the building. This paper will present a case study of a high rise building basement construction, using concrete reinforced with steel structures, by looking into all the procedures involved in its construction.

In the construction of a basement, the foundation best foundation is chosen and then constructed in such a way that there is underground room in the building. This room is usually used for saunas, disco halls, hotel rooms, car park etc.  After excavation, the foundation then constructed (Chew and Chew, 2001).

Main Body

a) Excavation

The building in this case study is twenty stories tall in Goldcoast. In this region, the soil is sandy clay and has low water content. The basement is excavated below the permanent water table and is constructed for a car park. The initial step was to carry out soil tests on samples of soil from the site in order to determine the composition and the water content. The site is originally level ground and 27000 cubic yards of earth was excavated. During the excavation, care had to be taken in order to avoid the collapsing of the walls by:

i)                    Shoring. This is the use of piles concrete and steel vertical structures in order to prevent the walls from collapsing during the period of excavation and construction. The type of soil in Goldcoast is sandy and the walls of the pit collapse easily. Shoring was therefore very necessary. Shoring was done using closely placed together concrete piles.

ii)                  Ground battering. This is another way in which collapse of the walls of the basement during excavation was prevented. This involves digging vertically downwards in a series differing steps and slopes. In this case, differing slopes were used because the steps could not be used as the soil consists of a high content of sandy material.

Excavation was done by an excavating machine which was fastened to a steel platform. After the depth went below the water table, water had to be drained out using electric pumps to give way for excavation.

b) Construction of Basement Using Concrete

Concrete is a mixture of gravel and sand held together in a matrix of Portland cement.   The mixture draws its strength from a reaction that takes place between the cement and the water used to make the mixture, which must be in a specific ratio in order to enhance the quality and strength. If more water is used, the mixture will be weaker and the concrete wall may leak after completion of the construction.

c) Piled Raft Foundation

This is a type of foundation used in the construction of medium and high rise buildings in most parts of Australia. Studies show that the piled raft foundation is most suitable in regions of sandy subsoil. It is imperative in Australia that building structures withstand geological activities like earthquakes. The structure is a combination of a raft

and a pile foundation. The structures are constructed such that room for basement is left.

d) Steel Reinforcement

Steel is a very strong material but has almost zero flexibility. This makes it crack under pressure. In order to prevent this, concrete is usually reinforced with steel. In our case, the pressure from soil around the walls of the basement would cause the concrete to crack. This is why steel reinforcement was necessary. The reinforcement is done by first making the steel structure and then pouring in the concrete mixture. The figure below illustrates the placement of steel (Chew and Chew, 2001).

e) Foundation Drainage

The building in this study has its basement under the permanent water table. Under normal circumstances, water will collect along the walls of the basement and will eventually leak into the basement. It is therefore superlative to have a drainage system to drain away the water. In this building, drain tiles were placed under the inside walls, slanted at an angle so that the water flows away from the building. On top of the tiles, a layer of gravel was placed to ease the flow of water. In order to prevent clay particles from flowing into the gravel and blocking the water passages, the gravel was covered with hay. In very wet areas, a provision is made for the water to be drained out by use of an electric pump. In the study building in this paper, use of draining tiles was sufficient to take care of the problem of drainage.

f) Wall Coating

Concrete has small pores on it which makes it porous. To seal these holes, liquid asphalt compound is applied to the outside walls. The liquid may be applied cold, though it is more effective if sprayed hot onto the outside walls of the basement. Praying the hot liquid is more expensive, but the results are better. This method was used in the case building.

g) Backfill

After construction, the soil is replaced at a slopping angle away from the building in order to help with draining the water away. Care must be taken to avoid throwing in large boulders as they might cause cracks in the walls of the foundation.

h) Concrete Floors

In order to allow for water drainage, a layer of gravel is placed below the floor. For most purposes, the tradition is to use a four inch floor. However, for our case, the floor was double this practice due to the consideration that the basement was to be used as a car park (Gregerson, 2006)

Conclusion

Construction of a basement of a high rise building can present a problem if the necessary analysis is concerning the soil and the seismic activity of the region is not properly carried out. The main problem however becomes the method used to drain away the water. Nowadays, provision is made in most buildings to drain the water by use of electric pumps in case of abnormal meteorological occurrences.

In Australia in general, and in Goldcoast in particular, the water table is high but also the solid bedrock is high. This makes the construction relatively cheap.

Bibliography

Chew, M.Y.L. and Chew, Michael. (2001). Construction Technology for Tall Buildings. Singapore: Singapore University Press.

Gregerson, Chris, (2006). A High-Rise Building and Construction Cranes. San Diego:  Calib Publishers.


 

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