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DIAPHRAGM WALLS AND PILES

A diaphragm wall is the most cost-effective retaining solution for providing strong soil and hydraulic support to control and protect the surrounding environment against settlement, water drawdown, etc.

  • Lisboa Palace
    Construction of a large diaphragm wall for a new hotel and casino complex.
    Hong Kong - 2014 read more

    Lisboa Palace

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  • Shatin to Central Link contract 1106
    Diaphragm wall and barrette construction for the extension of Diamond Hill Station.
    Hong Kong - 2014 read more

    Shatin to Central Link contract 1106

                     DOWNLOAD PROJECT REFERENCE(S)
  • Wai Yip Street
    Construction of a diaphragm wall and associated works for a commercial building development.
    Hong Kong - 2014 read more

    Wai Yip Street

                     DOWNLOAD PROJECT REFERENCE(S)
  • New Doha Port
    Construction of a 6,000m-long cut-off wall for a mega harbour project.
    Qatar - 2014 read more

    New Doha Port

                     DOWNLOAD PROJECT REFERENCE(S)

VSL's Ground engineering services are provided by its subsidiary


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VSL’s Ground engineering services at a glance

VSL’s diaphragm walls – a very cost-effective solution

Diaphragm walls are retaining, supporting and/or waterproofing walls. A trench is dug using a cable-suspended grab or ‘hydromill’ cutter. A reinforcement cage is then installed and the trench is concreted.

Project sites are subject to ever more stringent conditions and so the limits of diaphragm wall techniques are today being pushed even further. Thicknesses now range from 600mm to more than 1,500mm; single panel volumes can exceed 1,000m³ ; and diaphragm walls can reach depths of more than 100m below the surface. VSL’s ground engineering division Intrafor has already succeeded in meeting these challenges and is geared up to embrace new challenges that will push the limits still further on new projects.

A diaphragm wall is the most cost-effective retaining solution for providing strong soil and hydraulic support, especially where watertightness and a stiff retaining structure are needed to control and protect the surrounding environment against settlement and water drawdown. For these reasons, diaphragm walls are becoming increasingly popular. Around the world, Intrafor’s portfolio of experience in the construction of diaphragm walls includes a large number of different structures ranging from small-diameter shafts on small sites to large metro stations, from shallow basements to deep underground structures, and from construction within a greenfield open space to busy metropolitan centres.

Intrafor offers all of the ground engineering techniques that may be required as part of civil engineering or building projects such as metro stations, tunnels, roads and bridges, land reclamations, dams, deep basements, car parks and high-rise buildings. This includes retaining structures (diaphragm walls, sheet piling, contiguous and secant pile walls), foundations (barrettes, bored piles, continuous auger piles, micropiles, H-piles, pre-bored piles), soil improvement (grouting, jet grouting, vibro-compaction, stone columns, soil mixing), environmental protection (slurry walls, curtain grout, curtain wall by ground freezing) and soil investigation (sub-horizontal and directional drilling and coring, ground investigation, laboratory testing).

VSL Services

Intrafor builds underground walls and deep foundations up to 120m in depth for civil engineering and building projects, including metro stations, tunnels, bridges and other structures.

Intrafor offers the full range of services from design consulting to execution of the works, using its own equipment pool to achieve the best result in terms of safety, time and costs.

Associated Services

Monitoring

A full range of techniques to monitor the behaviour of the ground or of a structure, during underground works or afterwards

Ground improvement
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LMC project in HK, soil freezing to construct cross-passage between the 2 tunnels

A combination of techniques to improve the ground properties for both temporary and permanent works to enable construction of underground or elevated structures. VSL can implement a wide variety of these techniques including soil freezing, vibro-flotation, grouting, jet-grouting…

Ground investigations a combination of techniques to investigate and assess the ground strata (layers, faults, voids, water level) and soil properties (chemical and mechanical) prior to starting construction works.

VSoL® Retained earth walls system. Precast concrete or preformed mesh facing panels anchored to galvanised steel mesh or polymeric strips to reinforce soil and create retaining structures with a variety of shapes, surface textures and colours. Uses include abutments, ramps and road or rail projects.

Ground anchors (or tie-backs). Used for retaining walls, buildings, bridges, dams, unstable slopes and to prevent water-table uplift. Systems include drilled and grouted, post-tensioned (stressed) passive or active ground reinforcement to provided resistance to uplift and lateral forces. They can take the form of strands or bars and the system may be temporary (during the construction phase) or permanent. VSL has developed a complete electrical insulation system for permanent anchorages.

Advantages of Intrafor's Solution

Experienced staff

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Intrafor is convinced that its people are its main and most critical asset. Intrafor’s project teams are organized around a permanent core of highly experienced and mobile staff, guarantor of the highest quality and safety standards and problem solving.

 

Cutting edge equipment

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Intrafor owns its pool of specialized equipment composed of some of the most powerful machines available on the market. This allows Intrafor to be very reactive and able to mobilize the project right upon award which often proves to be a key advantage.

Value engineering

As experienced ground engineering specialist, Intrafor can use innovative geotechnical techniques to provide value engineering for its clients’ projects at an early stage in the design. Intrafor proposes construction schemes or alternative approaches. The project details and interfaces are then developed with the client’s team to produce the most cost effective and safe solutions. Intrafor aims to be “Your Foundation Partner”, providing clients with safe and economic ground engineering solutions.

In-house design capabilities for better optimisation and risk assessment

Intrafor’s design teams are experienced and conversant with design calculation methods and standards, as well as - and especially - with construction processes and risk management. These aspects often prove to be decisive factors in the success of a project. In addition, Intrafor’s designers work closely with estimators and operational staff at all phases of the project, from the tender to the final details of executing the work. This allows selection of the most suitable methods together with the definition and optimisation of the construction sequences and programme.

Designers remain involved in the construction processes and in the risk management throughout the execution of the work. Thus, they can refine the methods and the design as required to take into account any changes on the site, whether due to ground conditions or other factors. This makes Intrafor’s design expertise one of the company’s key strengths.

Contributing to sustainable solutions

Limiting the impact on the environment

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By carefully choosing the best method to improve the soil, ground improvement contributes to limit the impact on the environment.

Founded on safety

Intrafor’s employees demand, and actively participate in, the establishment of a safe environment. They are proactive in identifying and communicating their concerns. All employees are given the authority and responsibility to stop works if they feel safety compromised.

Identifying hazards ahead

Hazards need to be carefully managed. Intrafor is well versed in safe working methods and makes safety a top priority, as demonstrated by gaining OHSAS 18001 certification in 2003.

Example of a sustainable solution

Good practice in environmental impact assessment
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Lok Ma Chau Spur Line, Hong Kong - 2007

The Lok Ma Chau Spur Line is a 7.4km railway from the existing Sheung Shui station on East Rail to a new terminal at Lok Ma Chau. A tunnelled alternative was developed to pass beneath the Long Valley without disturbing an environmentally sensitive wetlands habitat, which is home to a number of rare bird species.

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After the construction of 3.2km of twin-bored tunnels 8.75m in overall diameter, 12 cross-passages were then built. These connect the two tubes at regular intervals and allow passenger evacuation from one tunnel to the other. They were built in saturated, mostly decomposed, rock and were therefore very unstable, requiring ground treatment to ensure safe excavation. Construction of three of the passages had to be carried out from the tunnel without any access from the surface.

Intrafor worked with the contractor and engineers to develop a ground treatment scheme based on ground freezing to stabilise the ground during excavation. It was Hong Kong’s first use for tunnelling works of this environmentally friendly technique, which does not introduce any foreign materials into the ground, nor does it require any work above ground. The process involved pumping coolant through a series of pipes installed in the soil to chill it to -30°C. The ground freezes at this very low temperature, gaining a significant increase of strength as well as becoming watertight, thus making it completely safe for excavation.

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How it works

A diaphragm wall creates a barrier that provides strong soil and hydraulic support to control and protect the surrounding environment against settlement and water drawdown. The wall is either self-standing or can be temporarily or permanently supported by other means as required.

Diaphragm walls have the following advantages

  • they can form part of the permanent structure;
  • they allow for the top-down method of construction;
  • their stiffness limits wall movement during the excavation – particularly of use next to sensitive structures;
  • they are well suited to the construction of underground structures in built-up areas;
  • they bring overall programme savings.

Main components and equipment

Cutter

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Ground excavation by cutter: hard teeth mounted on wheels excavate the trench while materials are taken away by circulating bentonite (see below). A state-of-the-art cutter can meet extremely tight tolerances as well as reducing excavation times by up to 40%, when properly deployed by a trained operator.

Typical set up for cutting

Clamshell grab

A clamshell grab is rope-suspended and operated either by another rope or hydraulically. It can dig trenches of short or medium depth but is less productive than a cutter.

Bentonite Slurry
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Bentonite testing

Bentonite Slurry is created by mixing water and bentonite powder. Bentonite It is a thixotropic material with a density higher than water and is used at every step of diaphragm wall construction to keep the excavation faces in place and prevent collapses until the permanent concreting works are carried out. Bentonite is used to transport material removed from the bottom of the trench to the surface, where it is filtered in a desander to separate out the excavated materials. The bentonite slurry must be monitored at all times as its quality and cleanliness are critical at every step of the construction from excavation to concreting. The quality is affected by the nature of the excavated material and so chemical additives may be required to maintain it.

Desanding plant
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The desanding plant is used to separate the excavated material from the bentonite slurry and to filter it. The excavated material is then taken away. The bentonite slurry is recycled for reuse in the operation.

Stop ends
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Stop end at Lai Chi Kok, Hong Kong

Stop ends are used along the sides of the first panels to be concreted, prior to digging and building the adjacent ones. The stop end is shaped to allow the formation of keys between the panels. It also allows a water stop joint to be secured in position between adjacent panels to ensure that the wall is watertight.

Concrete and reinforcement

Reinforcement can be made either of steel or glass fibre, which is used in situations such as where a tunnelling machine has to bore through a diaphragm wall at an underground station.

The quality of the concrete has to be designed to resist the aggressive environment of the surrounding water.

Construction sequence

Diaphragm wall construction stages

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Diaphragm walls are composed of individual panels that are cast in sequence to form the overall wall. Panels are formed with stop ends at their edges to interlock the panels.

The exact shape of the panel depends on its position within the construction sequence.

Underground walls (to serve either as a cut-off wall or as a retaining wall)

Diaphragm walls are generally suitable for:

  • Granular or cohesive soils, but not for rock;
  • Deep basement excavation;
  • Integrated structures, providing permanent retaining solutions in situations such as underground metro stations, water and sewerage shafts or building basements.

Underground cut-off walls

Underground cut-off walls are not always exposed, in which case their sole purpose is to control the movement of underground water, acting as a water cut-off wall. Such a solution is more specifically required in mitigating water migration under a dam or in protecting the environment by isolating an area, for instance to confine contaminated land. It creates a watertight barrier to allow works to proceed without affecting the ground water table of the surroundings.

Diaphragm wall

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Retaining, supporting and/or waterproofing wall e.g. for a basement. Structural examples of diaphragm wall applications include:

  • Circular shafts: The ground and water pressure created by the soil is taken in compression by the shaft structure. Attention has to be paid to the water pressure that may be encountered inside the shaft at the point of excavation.
  • Self-standing wall: This type of retaining structure is designed to withstand earth and water pressure at it rear once excavation has been completed, thanks to the use of a socket that is dug deep enough below the inside surface to stabilise the wall.
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  • Diaphragm walls that require support: Diaphragm walls may be designed so that their stability relies on temporary or permanent supports, such as ground anchors, steel struts, or the floors of parking or building structures.

Site Insight

Low-headroom diaphragm wall in Singapore
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Cities are expanding and going underground. In Singapore, Intrafor has constructed diaphragm walls and deep barrettes in restricted headroom of less than 6.5m using an in-house designed rig and clamshell together with an MBC30 mini-cutter.

Small section installation of cages and stop-ends presented challenges that the Intrafor team overcame by applying its specialist know-how. Intrafor had implemented the same techniques on previous projects in Thailand, Australia and Hong Kong.

Intrafor ensures the timely delivery of projects by using some of the best performing equipment, such as heavy-duty cable grabs and the most sophisticated and up-to-date hydraulically operated trench cutter, capable of reaching depths of up to 120m.

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Sheet piling Segmental retaining walls made of steel - usually used in soft soils and tight spaces. Interlocking steel sheets of specific shapes are driven into the ground.

Soldier pile or Berliner wall. Soldier pile walls are generally constructed of wide-flange steel H-sections driven prior to excavation. As the excavation proceeds, horizontal lagging is installed within the H-pile flanges. Soldier piles are particularly suited to overconsolidated clays, soils above the water-table if they have some cohesion, and free-draining soils where dewatering is often required.

Continuous bored piles or secant pile walls. Secant piled walls are constructed by alternating 'female' piles with the subsequent construction of 'male' piles. Construction of 'male' piles involves boring through the concrete in the 'female' piles in order to key 'male' piles between them. The male pile is generally the only one where steel reinforcement cages are installed.

Nailed wall. Retaining wall using anchors for support.

Bentonite-cement slurry wall. Typically used to guarantee ground stability or to seal permeable ground prior to excavation e.g. for diaphragm walls.

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