High energy impact compaction and continuous impact response offer ground improvement and ground investigation in a single package. Max Soudain reports.
High energy impact compaction was launched more than a decade ago in South Africa by developer and ground improvement contractor Landpac, and has since been used extensively there and in Australia, China and the Middle East. But it was only introduced to the UK last year by the company's British arm, Landpac Ground Engineering.
UK managing director Dermot Kelly took a low key approach for the method's introduction, allowing him to prove its benefits to British engineers. He says that after several trials, including one on the Channel Tunnel Rail Link, interest has grown dramatically.
Keller Ground Engineering, for example, used HEIC to improve the ground for a large distribution centre at Beddington Lane in Croydon (GE November 2001).
HEIC uses a pair of linked three- or five-sided drums, pulled by a tracked machine, to compact the ground. As they are pulled along, the drums rotate, their flat sides repeatedly striking the ground to create a compacted layer with higher bearing capacity and stiffness.
The drums are hollow, weighing 10t or 12t and have a drop height of between 150mm and 230mm. Typical depth of influence is between 2m and 4m. This depends on a number of factors, including soil type and variability, water content, the contact pressure of the drum face and how the energy is applied (ie the drum configuration).
While this is not as great as deep dynamic compaction using large weights dropped from greater heights (up to 10m) from cranes, the benefits of HEIC include a unique self-monitoring system provided by another Landpac innovation, continuous impact response (CIR).
Kelly says: 'The advantage of HEIC over traditional dynamic compaction methods is that every square metre of a site is covered. ' Deep dynamic compaction is carried out on a grid, forming 'columns' of compacted material, he adds.
The method is also much faster.
'HEIC can cover an area of 10,000m 2in one hour, ' Kelly claims. 'And because treatment is not concentrated in one area at a time, pore pressure is able to recover slightly between passes, improving compaction. ' The system also causes much lower vibrations - less than 5mm/s2ppv - which allows it to be used closer to buildings and, perhaps, more importantly, occupied buildings. On the Croydon project, HEIC was used after residents were disturbed by deep dynamic compaction.
Typical ground improvement treatment is about 40 to 60 coverages, Kelly says. Induced settlement is typically 5% to 20% of thickness of the material being treated and induced settlements of up to 500mm have been recorded, he adds. 'Strength and compressibility are typically improved by a factor of between two and six. ' Level surveys are carried out after every 10 passes and, at the same time, Landpac uses its CIR system. This recent innovation has seen the market for HEIC expand from ground treatment to become a useful ground investigation and 'proof-rolling' tool for exploring sites with variable fill.
CIR can be used as a quality control not only for HEIC but for other ground improvement processes.
'CIR records real-time ground response before, during and after HEIC, or indeed for any other form of ground improvement, ' Kelly says.
The system was introduced a couple of years ago, when technology had developed sufficiently to make it viable, he adds.
Ground response is measured using an accelerometer mounted on the impact drums' common axle. Deceleration of the drums is measured at each location, pinpointed using a GPS system mounted in the tractor cab. A small deceleration indicates looser material (ie the drums tend to 'sink' into the ground), a higher deceleration indicates denser material (the drums 'bounce').
Colour plots of the testing area showing variations in ground response are then produced in terms of deceleration due to HEIC drum impact. A higher value of deceleration shows denser material, giving engineers a clear indication of potential problem areas of poor ground that may need further testing or treatment.
At Croydon, HEIC aimed to improve areas of weak and loosely compacted fill between 4m and 6m deep on a 4ha site. The CIR plots taken after one coverage and 40 coverages clearly show the relative improvement in densification.
During the first surface coverage, 55% of the area was responding to less than 2Gs (red), 35% to between 4G and 8G and only 10% was responding to greater than 8G (green). After 40 coverages, only 5% was responding to less than 2Gs, 50% between 4G and 8G and up to 45% was responding to over 8Gs.
An average induced settlement of 146mm was achieved by the five-sided roller after 40 coverages. Maximum settlement induced at any one point was 340mm.
Importantly, the system gives 100% site coverage, rather than the very low percentages normally achieved in standard compaction quality control and site investigation boreholes and trial pits. Deceleration data can be correlated to relative variations in soil strength, stiffness and density obtained using conventional ground investigation methods such as cone penetration tests, dynamic probing, plate tests and laboratory testing.
Kelly says the understanding of soil response during HEIC has increased significantly over recent years. 'The relationship between the depth of influence and improvement in insitu soil strength has become more predictable from proven data gathered from a number of projects worldwide. ' As a site investigation tool, HEIC and CIR work together to produce maximum coverage across a treated area, in a similar way to a geophysical technique, but based on physical measurement, with the added bonus that 'we are improving the ground as well as proving it', says Kelly.
He believes this is one of the key growth areas for Landpac systems, as it should open doors for full use of the HEIC system in large-scale treatment projects after clients are shown the benefits.