ICE - Institution of Civil Engineers - Virtual Library

• 10.1. Introduction

• 10.2. General repairs, restoration and cleaning

• 10.3. Rubble-filled walls and columns

• 10.4. Cornices and gutters

• 10.5. Stone decay

• 10.6. Mortar decay and cracked walls

As part of achieving environmental sustainability in ground treatment, there is an increasing desire to use recycled materials for vibro stone column techniques. Materials used in stone column construction are required to be free-draining, hard, inert and comply with acceptable criteria in terms of material type, grading, hardness and chemical stability. Spent railway track ballast and crushed concrete probably have the greatest potential for this application in the UK. New European standards for aggregates allow the use of environmentally sustainable materials in a wider range of applications than under the British Standards they replace. However, where such materials are considered for use, as an alternative to natural primary aggregates, it is important that these materials are fit for purpose. Technical requirements for vibro stone column aggregate are discussed. Of particular importance is the fines (clay/silt) content of the aggregate, as this can have a significant effect on the angle of internal friction of the stone column material (and drainage characteristics). This in turn influences both the carrying capacity of the stone column and the settlement characteristics of the stone column-soil composite. Some applications of recycled aggregate in the context of vibro stone columns are presented, including a case history for spent railway ballast and crushed concrete. Also presented is an innovative technique for avoiding pollutant linkages via stone columns using ‘vibro concrete plug’ technology.

Contrary to public belief, engineers and architects have limited data on the structural behaviour of stone paving that is subject to the movement of modernday traffic. This paper presents a brief history of stone paving and the reasons for its current revival. Recent research studies aimed at both improving understanding of stone paving and obtaining more cost-effective design and construction are described. The current availability of design guidance and engineering standards is reviewed. Lessons from recently completed schemes are used to complement existing guidance in preparing designs, developing specifications and improving construction. The key issues covered include: the need to marry visual and structural requirements; novel designs; factors that increase the risk of failure; and training and aftercare. The paper concludes that successful engineering of stone paving requires careful planning. The risk of failure can be considerable if certain key factors are not considered. In particular, the basic principles of constructing stone paving need to be understood by stone layers. Traditional laying techniques are not appropriate for schemes that accommodate heavy loads. In terms of aftercare, the paper highlights the need for regular inspections to identify early-life defects, and describes a technique that can be used to improve levels of skid and slip resistance without affecting the visual appearance of the stone surface.

This paper investigates a number of issues significant to the evaluation of the sustainability of vibro stone columns and, in so doing, discusses how a carbon dioxide emissions estimation tool could be developed for this area of ground improvement. It involves consideration of the use of recycled/secondary aggregates and how this choice affects the column design process, in terms of the durability of constructed works and the amounts of aggregates required. The paper concludes that assessing the sustainability of vibro stone column construction, in carbon dioxide emissions terms, is a complex process not based simply on whether recycled/secondary aggregates themselves are more ‘environmentally friendly' than primary aggregates. Instead, it requires care on the part of ground improvement engineers, who must balance geographical variation of aggregate sources against the amount of aggregate used in the vibro stone column construction. However, it is noted that the use of carbon dioxide provides a limited view of sustainable performance, and that the continued development of emerging assessment tools could help capture some of the true complexity of the decision-making process.