Recent development work in Kent has called for innovation of conventional grouting techniques to meet the demands of local ground conditions.
Engineers at Keller UK have had to adapt one of the company’s established grouting techniques to deal with a geological problem peculiar to Kent in order to meet the needs of building work currently underway there.
In the Medway Valley the complex geology can lead to the formation of cavities, which are known as gulls, as a result of cambering where the competent Hythe Beds overlie the Atherfield Clay. The resulting gulls can present a hazard to development but Keller’s new grouting technique has been designed to remove the risk for one development in the area.
According to Keller, the risk develops where the harder beds sit on the clays and curve down towards the valleys leaving vertical fissures – often in the shape of a seagull in flight – parallel to the contours of the cambered slope. These fissures are often quite small but can be deep and infilled with loose material.
When a site investigation is undertaken in the area it is not uncommon to carry out extensive probing to rule out the presence of gulls. Sometimes weak material can be found at a significant depth because the probing or boreholes have been undertaken straight into a fissure. These fissures alone are not particularly troublesome if located at depth, but they could provide future risk to foundations if inundation from leaking drainage or flooding occurs.
Keller has dealt with many sites underlain by voided soils including Chalk and Coal Measures and, when a site investigation for the development in Maidstone revealed infilled features at depths of up to 12m, it turned to the principles of compaction grouting.
This technique can compact the void infill when it is granular but, when there is cohesive content, there is some relaxation of pore water pressures post-grouting making it is less effective.
At the Maidstone site, it was the presence of voids which presented the greatest risk and so a method had to be developed that would ensure these were infilled.
Normally for filling voids a 1:20 fluid to pulverised fuel ash mix would be adopted but at this site a hybrid approach was used.
“Think of it as like root canal work in dentistry only we have to get down to the cavity first,” says Keller contract manager Javier González Hernández. “When we achieve the depth, we then start pumping a special grout mix that we can apply a pressure of 10 to 12bar. The stiff hybrid mix allows us to compact any granular material within the void and ensure the feature is filled.
“By carefully lifting the drill string, we pump large charges of grout sometimes as much as 1m3 until we can get no more in within our pressure bands. During the process we watch the surface for heave and this gives us confidence that we have not only filled the void but dealt with loose material too, removing the risk for the developer.
“This is probably the fourth site in the UK we have treated in this way in the last 18 months.”
According to Keller, working on sites with voids at depth can present significant health and safety challenges.
“At Keller we have a standard operating procedure developed centrally by harnessing experience from different parts of the globe,” says Hernández. “We look at the depth of the feature and design a working method to ensure both our personnel and plant are not exposed to the risk of catastrophic failure of the ground into a hole.
“We have to ensure the routing of plant in accessing the area does not cross the feature and always work from the outside in. If possible we like to see a geogrid material beneath a working platform.”
With the technique now successfully being applied at the Maidstone site, the developer is now undertaking the above ground construction work.