Shoring a Continuous Beam to Cut Out End
Most of the parking requirement of tall building is provided by basement floor. UGWT, STP and other building services are also situated in basement. Number of basement floor below ground is depends on the above requirement. The construction of basement structure is coming with cost of deep excavation. There should be system, to help retain earth, ground water and adjacent structure such that excavation and construction of these basement can be carried out efficiently. Often such system is called Shoring.
The two main component of shoring system that need to be design which include earth retention system (Piles or Wall) and support system (internal or external bracing such as rakers, structs and tiebacks) that support the earth retention system. The design includes various checks such as earth-water pressure, sliding, rotational, bearing stability check. Detail design calculation can be done using integrated platform of structural and geo-technical analysis.
As a structural engineer, we should be able to decide which type of shoring is suitable for construction projects that we are about to design. Selection of shoring for deep excavation is depend on following factors which includes - Allowable construction period, Construction budget, Condition of adjacent structure, Type of structure and foundation structure and Construction site location and soil condition. These factors need to consider before choosing the shoring system. This article will briefly explain various types of shoring usually used in construction of various residential and commercial buildings.
1. Soldier Pile
A soldier pile is common retaining wall system which include H, or I shaped steel beam and lagging walls. First, these H shaped beams (piles) are drilled or driven deep into earth at regular interval of 6 to 10 feet. Then excavating is done in small stages up to 5 feet while placing the horizontal support known as lagging walls. These walls are used to fill the gap between vertical piles as shown in figure 1. Most commonly precast concrete panels, steel girders or pressure treated timber planks are used as lagging walls. Finally back filling and compacting is done to fill the space behind the lagging walls.
Figure 1: Soldier Pile (Image by- TPUB)
Soldier piles can be designed as cantilever system up to 12 feet excavation height. If deep excavation required tieback bracing should be provided.
These shoring systems can be used for temporary projects as well as permanent solution for earth retention. This system is used where benched or sloped excavation is not possible due to site restrictions. Some of the advantages of soldier piles which includes cost effective, easy and flexible to construct makes them favorable for many projects. High water table and soft soil condition is not suitable for this type of shoring system.
2. Contiguous Pile
A series of adjacent piles constructed to develop an earth retaining structure as a cantilever or a propped cantilever wall. Contiguous piled walls consist of a row of successive unconnected cast-in-situ concrete piles constructed with 100 -150 mm gaps between the adjacent piles. These piles are drilled in ground and can be used for excavation depth up to 18 meters.
A guide beam is constructed first to alignment of piles. Then cast in situ pile is driven by either rotary bored or CFA technique (Continuous Flight Auger) techniques. Capping is then installed to tie piles together as shown in figure 2.
Figure 2: Contiguous Pile
This shoring is system can be used in various soil conditions such as granular soils, cohesive soils, soft layers of rock. However, the contiguous wall is unsuitable for structures retaining fine granular soils and soft clay with a high-water table. It is also unsuitable in hard clay, hard rocks.
3. Secant Pile
A secant piled wall is a shoring system constructed for earth retention prior to excavation. It consists primary known as female pile (low strength un-reinforced concrete pile) and secondary knows as male pile (reinforced concrete pile). The wall is formed by alternate cast-in-situ concrete piles.
With the help of guide beam primary piles are installed first, once it gains enough strength secondary piles are drilled in between primary piles to form a continuous wall as shown in figure 3. Overlap of these two piles are usually ranges from 100 mm to 150 mm. Secant pile wall can be used up to 24-meter height of excavation.
Figure 3: Secant Pile (Image by- ENRWest)
It can be installed in difficult ground like cobbles/boulders. Also, it can used where temporary resistance to ground water is required. In some cases, it can used as load bearing walls along with earth retaining system.
4. Sheet Pile
Sheet piles are sections of sheet with interlocking edges that are driven into ground to provide earth retention and excavation support. Most commonly sheet piles are made of steel. However, it can also be formed using reinforcement concrete or timber.
It is construction by driving prefabricated Z section steel sheet into ground using vibro hammer. Connection is done by interlocking and welding sheets to form a single wall as shown in figure 4. The cantilever sheet piles system is used for height up to 6 meters. For greater depth shoring support anchoring is done to sheet pile for lateral stability.
Figure 4: Sheet Pile (Image by- Civilengineeringbible)
As it is light in weight, making and handling is easy. It is recyclable and reusable in case of temporary system. It is more effective in very high ground water condition. It is not preferred in hard rocky soil condition.
5. Diaphragm Wall
A diaphragm wall is a reinforced concrete wall constructed in deep excavation using cast-in-situ or precast concrete panel. These walls are constructed panel by panel with interlocking each other to ensure stability and water tightness.
Excavation is carried out in section by rope suspended mechanical cranes using cutters, with the help of stabilizing fluid (bentonite slurry), between previously installed guide walls. When desired depth is achieved, temporary or permanent stop ends are placed in trench, stabilized fluid is removed from trench and reinforced cages are installed. Then concreting is done as shown in figure 5.
Figure 5: Diaphragm Wall (Image by- Geo-technical Engineering)
The diaphragm walls can have a thickness ranging from 500 mm to 1500 mm with width of 2 to 4 meters. This wall can be used for excavation depth over 50 meters. This shoring system can be used in all type of soil. It is also suitable in high water table condition. It makes more efficient in deep excavation because of low cost and speed of construction.
Source: https://www.linkedin.com/pulse/shoring-system-deep-excavation-sandeep-k
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