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The project to construct a 2 storey 4 bedroom duplex on a plot that is about 400 meters from breaking surf necessitates a study of the soil and an assessment of its type.
The Australian Soil Classification System is relied on to define the soil conditions on site based on appearance, tactile feel and physical properties.
A visual check indicates the soil falls in the Brown Kandosol classification. There is a marked absence of strong structure and open minor cliff face close to the site shows a uniformly brown appearance with no strong contrast. A study shows that soil has clay in it and there are no clear or abrupt textural B horizons. There is no indication of the soil being calcareous since test with dilute HCl gave negative results. Rainfall in this region is below 1400 mm and soil appears to be well drained, brown in color with sparse vegetation around, a typical characteristic of Kandosols.
Soil classification type determines the construction plan, especially influencing the foundations, support structures and outer walls.
Construction method for concrete slab
- The plot is about 400 meters from the coast and at a level about 1.5 lower than the main road, located 100 meters away from the plot.
- There is a gradient allowing water to naturally run off.
- Design states garages are located at ground level. A concrete slab with footings is chosen as appropriate and cost effective.
- Construction method involves boxing for the slab with TMT bar reinforcements. Footing space is cleared and prepared to be poured at the same time as the main slab.
- The site is dug, vegetation and rocks removed, surface is leveled and compacted with a roller, a grid of steel bars is put into position for the slab as well as the footing and covered with a strong wire mesh for additional strength.
- The area of the concrete slab is surrounded by wooden board frame with exterior supports, taking care of the footing at the same time. Excavation is done for the footing perimeter, filled with gravel; wooden boards for footings are held in place by stakes and braced by supports that will stay in place until concrete cures.
- Torsteel bars project from the slab upwards to a height of 1.5 m to allow a perimeter wall to be put up and for retaining walls for the garage.
Design considerations are influenced by soil type, drainage properties, structure to be built and estimated load of the building, load of vehicles as well as frequency of usage and desired durability. Thinner cross section bars can be used since galvanized TMT is chosen. Wire mesh improves strength as well as prevents cracking in the long run. Footer provides additional strength and stability. The footing skirts the perimeter of the slab. Special marine grade cement with waterproofing additives to prevent moisture ingress is chosen for the concrete to resist corrosive saline atmosphere. Galvanized steel bars and galvanized wire mesh are chosen for the same reason. Heavy duty compactors will further compact and create a solid concrete slab structure.
Special requirements of building
- Building needs to withstand hurricane forces. 12 mm TMT bars projecting from the slab to a height of 1.5 m allow for building the retaining walls to the garage area.
- Retaining walls are to be built using galvanized torsion bar reinforcement and special marine grade cement with admixtures to protect against moisture ingress and provide corrosion resistance in a highly saline moisture laden environment.
- The finished slab as well as structure is to be covered with approved polymer coating to further act as a barrier against moisture and corrosive salt laden air in accordance with approved BCA procedures and norms. Distance of 500 m. from breaking surf does not absolutely necessitate the use of stainless steel in construction.
Method to construct a party wall between each unit and justification for use
- Brick masonry wall is preferred as it offers long term solution for the party wall between each unit. It is ideal to isolate the two units.
- Cost permitting and subject to availability, special bricks can be used that are resistant to efflorescence and with a denser structure. Cement contains additives for better impermeability characteristics, holding characteristics and resistance to saline environment. Brick wall of at least 9 inches thickness is used for the party wall, built form the ground up, allowing for a staircase for access to the upper floor.
- Brick walls add stability and strength to the structure while having inherent acoustic, thermal and fire insulating properties.
- As compared to fiberglass, steel or synthetic pre-built wall units, brick walls also serve to anchor down the building in case of strong winds.
- Brick walls also offer greater security and protection against vandalism and break-in.
Sketch/Information on construction of the Brick Party Wall
- Brick walls will be erected on the concrete slab, rising to a level where the first floor concrete slab can be laid and then projecting upwards for the first floor construction.
- Brick wall isolates the two units completely, providing a sound barrier too.
- Brick wall construction is also used for the side load bearing walls for complete security and for protection against the weather.
Basic construction methods:
- Ground slab for garage and footing: Concrete mix with galvanized torsion bars as well as net. Perimeter footing for added support and strength. Footing rests on gravel bed.
- Termite Control: Prior to laying the concrete slab, holes are drilled into the area and termite control chemical like chlorpyriphos is injected in the recommended quantity to completely saturate the subsoil beneath the slab. All organic materials like deadwood, etc. are removed prior to compaction process.
- Frame System: Brick-mortar is the preferred long term solution for the party wall as well as the frame running around the building from the ground floor up to the first floor.
- Roof System: Roof is designed with a slope to minimize wind force on the structure. RCC roof with galvanized torsteel bars is chosen for best long term protection and corrosion resistance. Roof will be appropriately covered with marine grade polymer coating for additional corrosion resistance and water proofing.
- External Cladding: Other than corrosion resistant decorative paint, masonry walls will not need any external cladding.
- Windows are provided in recessed bays provided with a concrete slab awning to shade against bright sunlight and protect against rain. Single hung windows are chosen since these will stay shut and external wind force will only serve to keep them tightly closed. When opened, they allow plenty of ventilation.
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