Barry Slocombe believes many engineers underestimate the critical importance of good design and sound construction technique in ground improvement work.
Ground improvement techniques using vibro stone column methods are in widespread use worldwide, yet how many engineers can honestly say they understand the system? Many seem to think it is simply making a hole in the ground and filling it with stone. This is not the case, and seriously underestimates the critical importance of good design and sound construction technique.
Vibro methods aim to strengthen and stiffen the ground by the installation of stone columns using vibrating pokers. Properly used, the pokers ensure the column materials are densified, and radial stress levels around the columns are raised, thus ensuring the columns and soils interact to provide the necessary enhanced performance. In 'cohesionless' soils a significant improvement is gained from the vibratory densification effect. The main applications have been in providing foundation systems for all kinds of structures, from houses to civil engineering projects such as railway embankments.
There is no British Standard to define a minimum achievement for these techniques in either design or construction. Many engineers rely on the Specification for Ground Treatment published by the Institution of Civil Engineers in 1987, but this does not address many of the fundamental issues.
The art of vibro stone columns involves assessing the prevailing soil and the correct determination of the necessary depth, spacing and degree of treatment to achieve the required bearing capacity, coupled with total and differential settlement performance appropriate to the structure. Within this simple statement are a number of basic considerations fundamental to the success of the scheme.
The first is to gain a clear understanding of the nature of the untreated ground. However, the specialist designer is still being presented with too many inadequate site investigation reports that render such assessment a lottery.
The second is the level of improvement that can be achieved by closer grid centres, densification effects, deeper treatment, different vibrator characteristics, time/drainage effects and the like, to match the design requirements with the vibro scheme. Regrettably, some of the basic information of the proposed loadings, foundation type and specified settlement performance is often not given within the tender documents.
The third requirement is the factor of safety appropriate to the type of structure and state of knowledge of the soils. Often the required settlement performance will be the key factor. About 75% of UK vibro work involves fills. Such soils are inherently variable and this must be taken into account.
Successful application of vibro stone columns begins at the investigation and design stages with a clear understanding of the technical objectives and of the principles underlying improvement to the ground. The design concept must be followed through on site with the use of appropriate equipment and materials for the soil types, together with the correct installation method.
This last aspect is frequently ignored in spite of its critical effect on performance, and must be closely defined, monitored and controlled. Appraisal of the process during installation is necessary to check that the design assumptions are justified. There have been a number of recent technical developments in site methods of monitoring and control - hard-copy computer printouts for bottom feed and standard crane-hung vibro construction methods.
However, both bearing capacity and settlement enhancement are fundamentally dependent upon the stone column diameter, the hardness of the stone and its density within the column. Consistent stone column diameter will be achieved in consistent soils by uniform construction technique. However soils will vary, so the technique must be capable of constructing continuous columns with locally larger diameter in weaker soils. Some weak aggregates, such as certain sandstones and limestones, will break down during construction, particularly below water. This can be recognised by the experienced operator and anticipated by the specialist designer who should opt to use more expensive harder aggregate.
This process of applying greater energy to better quality stone to form larger diameter columns which have higher bearing capacity and settle less, will take longer to construct than simply making a hole and filling it with stone. Risk of subsequent failure is clearly markedly reduced. A further benefit is that by carefully monitoring the response of the ground to its effective pre-loading during proper stone column construction, local anomalies can be identified and dealt with on site. Experience is therefore vital in ensuring that the end product is fit for the purpose.
Barry Slocombe is engineering manager of Keller Ground Engineering's Ground Improvement Division