Why and when they are the best option for your job.
Worried about water? Low headroom or difficult ground conditions?
A Secant pile wall is a retaining wall constructed for ground retention prior to excavation.
Primarily used in ground conditions that are not suitable for other systems or where there is a high-water table, it is a commonly used cost-effective solution when short-term water retention is required.
Secant pile walls can be used to form a permanent or temporary continuous watertight wall and is a popular choice for the construction of basements and underground car parks where headroom restrictions are a concern. The additional benefits of an almost silent and vibration-free installation plus its increased wall rigidity compared to sheet piling make this the favoured solution in built up areas too. Secant walls are also frequently used to as an alternative to diaphragm walls where obstructions are anticipated.
How it works
Secant pile walls are formed by constructing reinforced (usually with steel rebar or steel beams) concrete piles that interlock to form a continuous impervious structure. Typically, primary (female) piles are cast in situ leaving space in between them, this is followed by secondary (male) piles being cut into the primary piles creating a secant and forming a continuous wall with a typical interlock of around 150 mm.
This provides a closed structure to act as a barrier in water bearing soils, and to prevent the ingress of soil between the piles. In comparison to diaphragm walls they can be flexible in shape and offer a relatively compact operation in comparison to the equipment required to support bentonite holding and circulation equipment.
Secant piled walls can be:
‘Hard/Soft’ which incorporates a bentonite/cement slurry in the primary piles.
‘Hard/Firm’ which incorporates high reinforcement/low strength concrete in the primary piles.
‘Hard/Hard’ which incorporates a full-strength structural concrete in the primary piles.
Capability
The development of powerful high torque drilling equipment has led to an increase in the range of ground conditions and obstructions that can be penetrated as well as the wall thicknesses that can be constructed. Secant walls are an easy option in the most challenging of urban environments.
Depending upon ground conditions, the pile walls can be constructed from 600mm to 1180mm diameter and up to a depth range of 24m for the primary pile and 60m for secondary pile (male) subject to technique.
Secant piling can also go through underground obstructions such as steel, heavily reinforced concrete, granite and masonry while at the same time avoiding any risk of construction induced settlements to neighboring structures.
Cased Secant Piling
The Cased Secant Piling (CSP) system has been developed to combine the cost-effective CFA technique with heavy duty casings that are more traditionally associated with rotary bored piling methods. The CSP method can be used to construct deep secant or contiguous piled walls depending on the actual ground conditions and water table.
How it works
CSP piles are constructed by drilling hollow stem augers encased within a string of casing to the required depth. A twin rotary drive system rotating in opposite directions allows soils extraction and casing installation simultaneously. The leading casing and auger are armed with tungsten tipped cutting teeth which means the system is suitable for constructing piles in hard soils. Piles arisings are removed from the top of the auger and brought down to ground level via a chute acting as a safe system of work. The pile is then concreted using the normal CFA technique of pumping concrete through the auger stem under pressure in a controlled manner as the auger returns to ground level all of which is monitored inside the rig cabin by computer (capturing the same level of detail collected using the CFA system) in order to maintain quality. At this point the steel reinforcement cage is plunged into the concrete.
Whilst the process does have limitations with respect to diameter and depth, it will generally give a better assured product in terms of vertical tolerance and visual appearance. As it is a cased system and almost vibration free it reduces the risk of distress to adjacent structures in certain soils like sands or gravels for which it is particularly suitable. Cased CSP has the methodology to be used as bearing piles, through its effectiveness of drilling through rock.