Ground improvement or pressure grouting, also termed ‘permeation’ or ‘compaction’ grouting by CIRIA C514 (2000) and other research papers, is applied in low risk buildings in areas of loose permeable or very soft soils. Unlike pile and beam construction, this solution can be difficult to prove by design and/or testing.
The process involves drilling boreholes through foundations and into the subsoil along the area in distress. This is usually completed by hand held rock drills or micro piling rigs to depths of about 2 m, depending on the ground investigation. Following this, a neat cementitious grout is pumped or filled into the subsoils to fill voids, increase strength and reduce compressibility of the underlying soils to stabilise the structure. The grout is very mobile and whilst following the path of least resistance, travels through the voids providing both compaction and densification, therefore increasing the bearing capacity of the soil. The borehole is finally strengthened with a reinforcement bar along its length.
Pressure grouting may in fact be described as an art based on natural and scientific laws, however, it requires experience and engineering judgement. Rigid rules for the exercise of this art cannot be established, and only general procedures and guidelines can be applied on the ground. It goes without saying that nobody can tell where the grout is travelling beneath the ground; however, we address this by controlling grout pressures, grout volumes, flow rates and consistent site monitoring.