A secant piled wall is an embedded pile earth retaining system, consisting of a series of vertical reinforced structural piles designed to resist earth pressures and applied loads. The reinforced piles are formed to interlock with a series of vertical unreinforced non-structural piles, creating a seal.
Secant walls are used on sites with shallow groundwater, or where very soft soils are present within the proposed depth of dig, to minimise the ingress of water, and prevent soil loss into the excavation during the construction process.
Pile diameter, required interlock and reinforcement content are determined by design, relative to the height of soil to be retained, the ground water levels, and any applied forces to be resisted.
Secant piled walls can be designed as embedded cantilevers, or as propped cantilevers with single or multiple layers of props or anchors.
GeMech have experience of designing and installing secant piled walls for deep basement construction, highway support, and in slope stability applications.
A number of our rigs are capable of installing piles within 0.5m of neighbouring buildings, maximising internal space in congested urban sites.
In addition to lateral earth support, secant piled walls can be designed to carry vertical loads from the structure, giving them a dual purpose, with loads applied to the secant piled wall reducing the number of piles required to support the primary structure.
The Secant Piled Wall Process
GeMech typically use continuous flight augered (CFA) methods to form secant piled walls, although on occasion we may revert to rotary bored piling, or percussive drilling.
The process of forming a CFA pile is completed in one continuous operation, using instrumented piling rigs to monitor the pile construction process in real time as the auger is progressed to depth, and whilst concrete is being placed.
The CFA piling method is fast and efficient, generating low levels of noise, and is virtually vibration free.
From start to finish, what is the process of CFA piling?
Step 1 – Prior to the rig arriving to site the client surveys the area for services, prepares a suitable working platform at top of foundation level, and constructs a lightly reinforced sacrificial guide wall at capping beam level. The guide wall is essential in ensuring the desired installation tolerances and pile interlock are achieved.
Step 2 – A health and safety briefing will take place, the rig will be prepared to work, and moved to the first pile position.
Step 3 – The auger is positioned over the setting out point and zeroed at working platform level. The auger is then progressed to a predetermined designed depth, pausing occasionally to bring spoil to the surface.
Step 4 – Readymix concrete will be delivered to site and pumped to the rig using a system of hoses connected to the top of the auger. The rig operator begins extracting the auger string, ensuring a constant head of concrete is maintained within the auger to avoid the risk of borehole collapse during construction. This process is fully computerised in CFA rigs, providing the rig operator a continuous profile of pile diameter during auger extraction.
Step 5 – The attendant excavator clears spoil from piling mat level, exposing the fresh concrete at the head of the pile.
Step 6 – When forming structural reinforced piles, a reinforcement cage meeting the design requirements is plunged into the fresh concrete, to provide structural capacity, and fixity between the piles and reinforced concrete capping beam.
Construction Note – When forming a secant piled wall we will commence with unreinforced piles, and operate on a ‘hit one, miss one’ sequence, alternating daily between the construction of unreinforced and reinforced piles.
After completion of the pile installation work, the piles will be cropped by the groundwork contractor, and a reinforced concrete capping beam formed at the head of the piled wall to ensure piles act uniformly.
How we can help you
GeMech offers a variety of embedded piled retaining wall systems through contiguous, secant and king post piled walls. Our geotechnical expertise enables us to provide in-house design and advise on the most suitable and cost-efficient solution.
Secant retaining wall piles are commonly installed using our fleet of CFA rigs, allowing for fast, quiet and virtually vibration free installation.
We have a number of augered piling machines in our fleet suitable for tackling secant pile walls such as our Comacchio CH 450 for larger open sites, and out Hutte HBR204MP for more restricted applications.
Contact our expert team/engineers for advice on your project and get a no obligation quote