Shoring
A shoring system is necessary so as to ensure safety for the workers. The basement construction process will involve excavation near the neighboring building. The neighboring building is delicate because it was constructed long before the Australian standards of footing were set up. If shoring is not done, the workers will be at danger of a collapsing building. Shoring will help to offer support to the building so that it does not collapse in the process of constructing the basement. Shoring will also help in speeding up the excavation process. Secant pile walls and sheet piling systems are two good ways of shoring. Secant pile walls are, however, more suited for areas with high water levels, and for deep excavations. The excavation in this case is not deep because it involves digging out only 3.50m of soil. The excavation level is supposed to be reduced up to the level of 6.60m. It is, therefore, best to use sheet piles. Secant piles are costly compared to sheet piles, and cannot achieve maximum waterproofing. The other advantage of sheet piles is that they can be reused.
The tools needed to install sheet piles include using vibratory hammers and impact hammers. Impact hammers will be used in places where the earth
is dense or hard. In areas where vibrations are to be avoided, the pile sheets will be pushed to the ground hydraulically. Sheet piles are the best option for shoring because they are constructed using recycled steel, and they can be reused in future.
An image showing a section of the sheet pile system
Anchors
The excavation level of the basement is on soft sandstone and the depth will surpass that of the neighboring building. These factors make it necessary to use rock anchors so as to increase stability. The rock anchors will be tied to the shoring system so as to increase lateral support. The support is necessary so as to enhance the safety of the workers. Anchors offer support and stability by transferring tension loads via steel strand tendons or steel bars. The process involves drilling a hole to the bond zone or rock through the soil. A casing may be used if it is necessary. In case a casing is used, it should be extracted after drilling the hole. The steel strand or bar that is on top of the bond zone should be covered using a bond breaker. Covering that length helps in the elimination of load transfer. Finally the head of the anchor should be tensioned and then connected to the system that requires the support.
A diagram showing a steel bar connecting the anchor to the shoring
A diagram showing rock anchor
Dewatering Process
A dewatering system is necessary before the excavation process begins. The water table is at 8.30m and the excavation level is to be at 6.60m. Therefore, the water table has to drop from its level of 8.30m to a level that is lower than 6.60m. That is why the dewatering system is required. The soil at the level 6.60m is very soft sandstone, therefore, it is more appropriate to use well-points all round the perimeter of the excavation. These well-points will help in lowering the water table.
Well-points are tubes with small diameters. These tubes have slots towards the bottom, and are inserted to the ground. Water is drawn out of the ground using a vacuum that is generated using a dewatering pump. The dewatering process will be performed by slotting spears to the ground. It is also important to add gravel as the spears are stuck to the ground so as to create a barrier. After slotting the spears to the ground, all of them will be attached to header pipes of the dewatering pump. The pumping process will then be started, and will operate by creating a vacuum that draws out the water from the soil. The excavation process on the dry soil will then be started as the water table falls. Creation of additional barriers upon reaching the original level of the water table is necessary so as to prevent problems of immersion later.
A diagram of a dewatering system
Source: http://www.pumphire.co.nz/dewatering.html
Underpinning System
Underpinning is a necessary strategy given the fact that the neighboring building is fragile. The neighboring building was constructed in the year 1968. During that time, the Australian standards that address design of residential footings had not yet been set. Underpinning will offer support to the structure as well as preventing the collapse of the soil during the deepening process of the basement. The underpinning system that is the best for this basement construction process is Jet Grout. Jet Grout involves adding a fluid known as grout to the holes that have been drilled.
Jet grout is recommended for areas that have loose soils. In our case the soil in the excavation level is very soft sandstone. Therefore, jet grout is the best to use for this basement construction. The system is also good because the construction of the basement does not require interference with the normal functioning of the neighboring building.
The process of carrying out the underpinning system involves grout tests. The grout tests are necessary so as to ensure that the basement is strong enough to withstand the whole construction process. The next step is to choose the best grout system depending on the type of soil. The soil at the excavation level is very soft sandstone; therefore, the triple jet set-up grout will be used. A hole will be drilled to a depth of 3.50m to reach the excavation level of 6.60m and then a fluid will be pumped at a high pressure. The process will be repeated until all the sheet piles have been served.
A diagram showing jet grouting
Steps to be followed in basement construction
The steps to be followed by the workers while constructing the basement are as shown below:
- Dewatering so as to lower the water table to a level beneath the excavation level.
- Underpinning using jet grout.
- Breaking the ground.
- Excavation and removal of soil.
- Boring holes for shoring columns along the perimeter.
- Installing columns for shoring.
- Installation of anchors for additional support.
References
Hayward Baker, 2014. Geotechnical Construction: Sheet Piles. [Online] Hayward Baker. Available at: http://www.haywardbaker.com/WhatWeDo/Techniques/EarthRetention/SheetPiles/default.aspx [Accessed 02 November 2014]
