ARCHITECTURAL TECHNOLGY 4 ADVANCED CONSTRUCTION TECHNOLOGY BASEMENT CONSTRUCTION YEAR 3 SEMESTER 1 AIDAN WALSH R Lecturer: Jim Cahill

Transcription

1 ARCHITECTURAL TECHNOLGY 4 ADVANCED CONSTRUCTION TECHNOLOGY BASEMENT CONSTRUCTION YEAR 3 SEMESTER 1 AIDAN WALSH R Lecturer: Jim Cahill 29 th November 2011

2 TABLE OF CONTENTS SUMMARY 1 MAIN BODY SECTION 1: BOTTOM UP CONSTRUCTION 2 TOP DOWN CONSTRUCTION 3 SECTION 2: WATERPROOFING METHODS 4 DETAILS 5 REFRENCE PAGE 9

3 Summary As part of the advanced construction module we were required to complete a written report on basements beneath commercial buildings. For this report I had to investigate two types of basement construction: 1. Bottom up construction 2. Top down construction We also had to investigate several waterproofing methods which I will talk about in detail throughout the report. I had to decide as to which was the best method of construction for my studio project and what waterproofing I was going to use. As a result of my research I have found that bottom up construction is economical when you are constructing most buildings. However I have found that the top down construction is more economical when constructing super structures. I have also discovered internal and external tanking is more likely to be used in a single floor small basement. And that Impervious aggregates are more likely to be used in a situation where the basements is very large and two or more story s high. Finally I discovered that the drained method is most suitable where there is a free draining site with a low water table. Before choosing my method of construction for the basement I had to decide as to which type of grade the basement was going to be and after researching I decided that it was in fact grade 1. Grade 1:Basic Utility Car parking Mechanical plant rooms. >65% relative humidity15 32 C. Temperature Minor wet seepage and visible damp patches maybe acceptable. For my studio project I have chosen to use the bottom up construction method. I have chosen this method as it is a relatively low building and a small sight and therefore this would be the most economical method. This would also be the fastest method of constructing the basement and that is why I choose this method. I am going to use sheet piles to form my basement which will consist of a raft foundation, 215 mm block on flat, protection to tanking, tanking, 100 mm cast in-situ concrete. The sheet piles will be removed when the basement is complete and the extracted land filled in. I have chosen to internally tank using twenty millimetre asphalt tanking for my studio project. My reason for choosing this method is because it is quick and easy to install. It also satisfies the requirements for waterproofing the basement.

4 Main Body Section 1: Bottom up construction: 1. The first thing you must do is form the perimeter walls. These become the structural walls of the basement on completion and will be deeper than the lowest floor of the basement. 2. The internal area is then excavated to the required level. As the earth is removed temporary props and bracing is used to support the walls. 3. When the required level is achieved the location of any intermediate columns supports are set out and pad/pile foundations are provided for them. 4. The lower sections of the columns are formed 5. The floor slab is cast 6. The work then continues upwards towards the ground level with the columns and upper floors being formed. Under this method, after the construction of pile and diaphragm wall, slurry pile or sheet pile surrounding the construction works, the contractor will conduct open-cut excavation to certain depth and then proceed installation of the strutting system (Bracing System) to support the basement walls during excavation and construction of the basement. Depending on the depth of foundation mat, structure design may require one or more different layers of struts to ensure sufficient resistance against pressure of soil + ground water outside the project impact on the basement walls. After installation of strutting system is completed and ground is excavated to bottom level of foundation, the contractor will construct foundation, basement, superstructure of the building upward from the bottom in accordance with normal procedures.

5 For my project I will be using the sheet pile method. The sheet piles will be dug to a depth of about 9 metres and then the area inside will be excavated to the required depth for the basement. The raft foundation and walls will then be constructed and the gap between the sheet piles and basement walls is then filled in. the sheet piles are removed at a later stage and the extracted area is filled in. Advantages: It s a faster method of construction especially for smaller projects. Its much easier to construct and much less awkward. Disadvantages Unlike the top down method the basement must be complete before you can start the rest of the construction. Temporary supports are required during the construction process which can lead to increased cost. Top down construction: 1. Form the perimeter walls. 2. Form the intermediate support columns which will provide support for the floors of the basement. The columns are formed using a method called the plunge method. 3. Once these are formed excavation takes place to the upper most floor of the basement and the slab is cast. 4. Holes are left in the floor slab at convenient places eg. Lift shafts, stairs, services, etc.. These holes will allow the below floor slab to be excavated. 5. Excavation continues to the next floor level which is cast and work proceeds downwards towards the finished floor level.

6 The top-down method of construction enables a high-rise superstructure and its sub-basement to be built simultaneously. Perimeter foundation walls are constructed using the concrete diaphragm or secant wall methods. Load-bearing elements or drilled shafts are installed and the building columns brought up to grade level. Ground level and first basement slabs are poured, with access holes left to allow excavation beneath. As each subsequent subgrade level is completed, the floors act as lateral bracing for the perimeter wall system. Above grade construction can proceed while the subgrade work is on-going since the building s structural support is already in place. Advantages: The floor acts as a restrainer which therefore eliminates the need for costly support required for the bottom up method. Another advantage is the fact that you can build up and down at the same time. This is very useful and cost saving for superstructures. Disadvantages: Working conditions are poor below the floor slab. Artificial lighting and ventilation will be required therefore leading to an increased cost.

7 Section 2: Water proofing methods: There are three methods of waterproofing for basements: Internal tanking- where the waterproof material is applied internally to the structural wall. External tanking- where the waterproof material is applied externally to the structure wall. Impervious aggregates- where the basement wall is waterproof. Drained- where there is a drain running through the centre of the car park. Internal/ external tanking: Involve the utilisation of an impervious material applied to the structure, internally or externally, to keep out moisture. The selected waterproofing element must be capable of withstanding hydrostatic pressure from ground water and must be able to accommodate any other loadings required. The structural walls of the basement to which the impervious materialism applied may be formed in reinforced concrete or masonry. The waterproofing material may be applied externally during the construction process, in which case it would be considered external tanking. Alternatively, it may be applied to the interior face of the walls after construction, and in this case it would be considered internal tanking. Since the internal environment of the basement is totally reliant on the effectiveness of the applied waterproof membrane, any defect arising within the membrane will have a serious effect internally. The ability to gain access to the membrane to rectify any such defect is hindered by the adoption of external tanking systems. Internal systems are more readily accessible following completion of the basement, but they are at greater risk of damage due to penetration by fixings for shelving systems etc. As the function of my basement is a car park this won t really be an issue.

8 For internal and external tanking I have looked at two types of materials. 1. Asphalt 2. Bonded membrane Asphalt: This is usually about twenty mm thick and can be placed either internally or externally. It is a quick and easy method of waterproofing. It is also very effective. Bonded membrane: There are many different types of bonded membranes in the construction industry and these can again be used both internally and externally. This is again a quick and easy way of water proofing. Impervious aggregates: Utilise waterproof materials within the structure of the walls, considered as structurally integrated protection. The waterproofing is provided by the structural walls themselves, which are almost always formed from reinforced or pressurised concrete designed such that it is inherently water-proof. The waterproofing of the basement is reliant on the integrity of the external shell or enclosure. The composition of the structural concrete and the detailing at construction joints must ensure that the enclosure remains impervious to ground water. However, even where this is achieved, there is still the possibility of water vapour passing through the shell. The formation of construction joints must be carefully considered, as it is in these locations that there is the greatest risk of leakage. Joints normally incorporate water stops made from materials such as neoprene. Alternatively, they may utilise hydrophilic technology, which introduces a crystallising agent that reacts in the presence of water to seal the joint in the event of leakage. Drained: Allows for ground water to pass through the walls, but provide a mechanism for it to be drained away. A drained cavity is formed in the basement structure, which facilitates the collection of groundwater that seeps through the external basement wall. In order to be effective the drainage system must be able to cope adequately with the quantity of water that is likely to penetrate the external walls. They can present problems in situations where there is a high water table if there is any failure in the mechanical pumping systems that must be used to remove the water, or if the drainage channels become blocked for any reason, such as the accumulation of soil fines or silt. In these cases flooding of the basement would inevitably occur

9 Reference page &bih=923&tbm=isch&tbnid=7S1RKS5ik6MAxM:&imgrefurl= tos/uchicagolibrary/sets/ /detail/&docid=l10104hmzhhwom&imgurl= TvDHApO6hAegruzjDQ&zoom=1&iact=rc&dur=406&sig= &pag e=2&tbnh=184&tbnw=234&start=32&ndsp=21&ved=1t:429,r:1,s:32&tx=104&ty= g=en Home Bond. House Building Manual.6 th edition.