Pre-cast techniques in correctional facilities

by Andreas Alexiou,
Associate Director of Architecture, TPS

Until procurement of correctional facilities went down the PFI route, prisoner living accommodation was constructed using traditional insitu concrete techniques. Indeed this was the Prison Service’s preferred and only approved method of construction. Systems employed included either steel shuttering for individual cell walls or bespoke steel moulds where paired prison cells, complete with their associated service duct, could be cast at one time.

With the demand for prisoner places at an all time high, utilisation of pre-cast or other volumetric units is now the only acceptable means of construction, as reducing construction time has become such a key imperative for the Ministry of Justice.

The “Design Construct Manage and Finance” methodology of the first prison PFIs required prospective bidders to demonstrate innovation in design operation and construction. The constructor team which won the opportunity to construct and the prison facility at Fazakerley, Liverpool, highlighted the need to minimise construction risk by using a variety of construction methodologies for the various prison components and challenged the convention of constructing the prisoner living accommodation by using pre-cast modular concrete techniques.

A precast concrete modular cell solution was chosen in preference to an equivalent flat pack solution as modules offered a range of benefits such as a reduced number of construction joints (a security consideration), a faster construction programme and a reduction in temporary works requirements.

The brief required the provision of some 600-prisoner places, with prisoners housed primarily in single cells. There was also a limited requirement for two-person cells and the provision of cells for wheelchair-bound prisoners. This effectively generated three standard cell types.

The prison operator demanded that living units did not exceed fifty as this scale of unit allowed for positive interaction between custodial staff and prisoners whilst still offering flexibility and future-proofing, should overcrowding become a requirement.

The section through a typical wing is relatively simple, with a two-storey high top-lit atrium space where prisoners are free to associate at the ground level with cells located either side.

The living units located either side of the central core are generally precast cast in a block of four and were effectively designed as five-sided boxes, located so that the 5-side of the box forms the ceiling of the ground floor cell and the floor of the cell over. All ground floors were constructed conventionally in insitu concrete.

The module of four cells, which weighed approximately forty tonnes, was established as the optimum design since it was possible to transport these modules without recourse to special transport measures, while the weight of the units posed no unique issues in terms of site cranage needs (where generally crawler cranes were used to off-load and locate cell modules).

Though CCTV is used extensively within the prison environment, clear sight lines are essential to operational efficiency. Columns supporting the balconies that provide access to those cells located at the first floor level would have compromised vision so these concrete balconies were cantilevered off the internal wall of the modular cells instead.

The facilities required within the living unit cores are not repetitive and therefore do not lend themselves to modular construction. Pre-cast concrete wall units, which are either ‘L’ or ‘T’ shaped, were used in lieu in these locations.

The perceived benefits of the modular concrete system used are:

• Highly cost-effective production from repetitive manufacture of identical or similar cell units
• Quality standards secured through construction of modules within a factory environment
• Reduction in the number of potential defects
• Offsite construction reducing the number of site operations and consequently the number of site operatives thus minimising congestion on site.
• Use of a material with inherent fire resistance qualities
• Highly durable material with a minimal maintenance requirement (essential within a prison environment)
• Modular construction techniques minimising the number of joints between concrete elements, which both reduces the amount of grouting on site while also meeting Ministry of Justice security requirements to avoid joints within construction as these can offer opportunities to conceal items
• Reduced wastage both on and off site, contributing significantly to sustainability
• A significant reduction of the need for temporary supporting structures since units are self-supporting there is
• Swifter construction progress, allowing for early occupation of units while reducing the potential exposure to inclement weather
• Improved health and safety due to the reduction of site operations, the reduced number of lifts and the elimination of temporary works
• Assistance in minimising overheating, since concrete as a construction material has inherent thermal mass

The initial precast modular units developed for the Fazakerley project (HMP Altcourse) were purely concrete boxes without any element of fit-out. However the design of units have evolved over time with security grilles and windows now cast in. In addition there is a first fix of mechanical and electrical services within the paired vertical service risers associated with each pair of cells.

Brickwork or blockwork facings materials are preferred within the robust environment of a prison, at least at the ground floor levels, which are readily accessible to prisoners. Consideration has now been given to introducing brick facings within the factory environment and to a limited extent brick-faced concrete panels have been used on the latest expansion of prison facilities at HMP Altcourse, a project which was completed at the beginning of last year.

Further innovations with the modular system which have either improved the quality of the product or contributed to speed of construction, include the introduction of precast concrete ground floor slabs; modular roofing systems with roof structure and roof cladding fully integrated; the provision of radiant panels and under-floor heating in lieu of the traditional low temperature heating coils, and modularised plant rooms.

In terms of satisfying BREEAM requirements the latest expanded living accommodation at HMP Altcourse met the required standard of BREEAM Excellent.

Andreas Alexiou is an Associate Director or Architecture at TPS, part of Carillion plc. The modular units first used on the HMP Altcourse prison project at Fazakerley Liverpool were developed by Precast Cellular Structures Limited (PCSL) as were the subsequent expanded facilities which were completed last year. HMP Altcourse was built by Tarmac Construction and the expanded facilities by Carillion. In both cases TPS’s multidisciplinary design team was responsible for developing the conceptual and the detailed designs. The Prison operator is G4S.

Links

  TPS Law and Order sector

  back to 2010 news