Barrett Byrd Associates

Inadequate foundations for a new school in Scotland have been firmed up with the installation of 700 minipiles, reports Mike Walter.

Excessive settlement on the site of a new school in Scotland has been corrected by underpinning a steel frame structure with minipiles more commonly used as a means of providing an initial foundation. A total of 700 minipiles have been installed beneath existing pad foundations at the school by the underpinning specialist Roger Bullivant Ltd (RBL).

RBL’s underpinning manager Paul Doyle says: “Vibro stone columns installed beneath the pad foundations were unsuccessful mainly due to the underlying silty soils being much weaker than identified by the site investigation, leading to excessive settlement of the steel frame. RBL was called in to suggest an acceptable method of underpinning the entire structure and we decided to install minipiles.”

Construction of the school was put on hold and RBL was awarded a £300,000 contract to carry out the necessary underpinning works in 10 weeks. Every one of the 130 pad foundations were underpinned with between three and six minipiles, depending on load, and each pile was designed to accommodate a safe working load of 110kN.

Each minipile is comprised of a hollow drill bar, installed to a depth of 10m and flushed with grout to create what is known as a ‘stitch pile’. The stitch piles installed beneath the steel frame provide a competent means of transferring load from the pad foundations, through ground containing loose silty clay, to a firm strata of medium dense sands.

The piles work in compression and rely mainly on skin friction, with an element of end bearing on the medium dense sands. The minipiles are formed from 2m lengths of 40mm diameter hollow bar, that are coupled together with screw fixings as the pile descends into the ground.

RBL’s remedial work began by drilling a 150mm diameter hole for each of the 700 pile positions, through the 600mm deep reinforced concrete pad foundations. A down the hole hammer, driven by one of two 901 series mini piling rigs, provided a clean cut through the existing foundations. The mast of each rig extended to 3m in height and worked safely with sufficient clearance beneath horizontal steel beams.

The same four tonne rigs were then used to install each drill bar through the newly drilled apertures in the pad foundations. The first 2m length of bar contained a sacrificial steel cross-bit cutting head that bored through the silty clay as the bar was rotated into the ground.

Cementitious grout was continually pumped through the centre of each drill bar and was injected into the ground through a pair of holes in the cutting head. This helped to displace material to aid the descent of the bar into the ground and provided a structural outer core of grout to surround each drill bar. RBL’s foundation design specified that each stitch pile must have a body of grout of at least 120mm in diameter along the entire length of the drill bar.

The pile technician operating the rig had to watch carefully as the hollow bar descended into the ground to check that a continual flush of grout could be seen at the top of the pile. If the flow of grout to the surface slowed, the descent of the bar would be stopped and grout would continue to be pumped to fill the ground with sufficient cementitious material.

Grout was mixed on site and around 300kg was injected into every pile position to create a stitch pile. The grout also provided a secure bond between the drill bars and the existing concrete bases, effectively turning what were pad foundations into pile caps spanning at least three piles.

“The use of drill bars in this way to underpin a structure is fairly unusual,” says Paul Doyle. “Drill bars come in a range of diameters and are typically used to provide a foundation for buildings as opposed to a remedial solution. Also, we do not often get to underpin a building that has not been fully constructed.”

Further underpinning on site included the installation of a stitch pile every 1.5m beneath ground beams that linked between the pad foundations.

Underpinning using rotary bored drill bars provides a vibration free and quieter alternative to driving piles, as well as a more speedy substitute to other systems, adds Doyle. “We considered installing bottom driven minipiles but established that would have taken twice as long,” he says. “In addition we were concerned about obstructions which could have impeded the installation of a driven pile.”

BOX ONE: Paul Doyle, underpinning manager

Underpinning is a very specialist component of the foundations market, according to Paul Doyle of Roger Bullivant Ltd. The 30 year old underpinning manager has been with RBL for three years and in that time he has helped to develop the company’s minipiling and underpinning business north of the border.

Paul studied civil and structural engineering at Aberdeen University and quickly developed an interest in soil mechanics. He first worked for a specialist contractor that builds retaining walls before joining RBL in 2002.

“Underpinning buildings is a great challenge and often involves working with very small rigs in confined spaces, such as in basements. It is a good job that I don’t suffer from claustrophobia!”

BOX TWO: William Redfern, piling foreman

Piling foreman William Redfern is a familiar figure on RBL’s underpinning sites in Scotland. He has developed a reputation as a very capable operator of mini rigs and describes the underpinning of the school as being one of the more unusual jobs he has been called to in his 12 months with the company.

“Working around a structure that is half finished is not something I am used to. I am usually called to underpin houses or conservatories where there is very little room to manoeuvre.

“I find that working in small, tight areas means that it is more important than ever to keep our sites very tidy. It is not just for my safety but for my colleagues on site.“

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