It is my pleasure to introduce this November 2012 themed issue of Ground Improvement. The eight papers in this issue were selected from the Proceedings of the International Symposium: Challenges and Opportunities held in Bangkok in December 2010. The selected papers were reworked by the authors before being peer-reviewed for publication. Camera-ready papers were resubmitted after extensive revisions following the comments of the reviewers.
The first paper by Chu et al. (2012) is a review of methods for the treatment of soft and high water content fill materials or sewage sludge. For thick deposit, ultra-soft ground improvement – such as prefabricated vertical drains with vacuum preloading and deep cement mixing – the major difficulty lies in forming a working platform at the ground surface so that ultra-soft ground improvement works can be undertaken. The paper details five different approaches that could be utilised to construct this working platform. The improvement of soil sludge is particularly difficult as it contains both organic matter and heavy metals. The authors propose a combined chemical and mechanical method in which the sludge is mixed with binders and waste materials before disposal and is subsequently consolidated at the site.
Horpibulsuk et al. (2012) present the state of the art in strength development of soil–cement admixtures in the second paper of this issue. The clay-water/cement ratio (wc/C) is the prime parameter for consideration in a laboratory mix design but curing time is also an important factor. The strength equation was subsequently modified by incorporating curing time with clay-water content, cement content and ash content. The field-to-laboratory strength ratio was 0·7 and 1·0 for dry and wet mixing methods, respectively. The paper fails however to mention another approach to strength development of cement-admixed clays. Lorenzo and Bergado (2006) and Bergado et al. (2006) proposed that the ratio of after-curing void ratio and cement content are fundamental parameters of cement-admixed clay. Lorenzo et al. (2006) recommended optimum moisture content of clays for most efficient mixing with cement. Lai et al. (2006) back-calculated the properties of cement-admixed clay in full scale embankment tests on deep cement mixing (DCM) piles. Finally, Jamsawang et al. (2011) and Voottripruex et al. (2011) provided confirmation of field strength data compared with laboratory strength data obtained from full scale tests of DCM piles.
The next paper by Liu et al. (2012) presents the improvement of soil strength by lime treatment which is mainly dependent on three factors: lime content, curing time and curing temperature. By quantifying the variations of the unconfined compression tests independently with these three factors, empirical equations were proposed. The validity of the empirical equation was confirmed with respect to the three factors. The paper by Heerten and Werth (2012) discusses the mitigation of flooding by improved levees and dykes. Piping and erosion-resistant flood-control dykes can be designed to incorporate filter design with needle-punched nonwoven geotextiles as well as geosynthetic clay liners. The correct application of geosynthetics products can provide significant improvement in stability and flood protection performance of levees and dykes.
Next, Kongkitkul et al. (2012) present the results of laboratory model tests on geosynthetic-reinforced pile-supported embankments with different embankment heights. Arching effects are mobilised only when sufficient relative settlements on piles and on soils between piles were allowed. Assuming triangular-shaped arching across adjacent piles, the arching slopes of 44° and 59° with the horizontal were back-calculated. The use of stiffer reinforcement resulted in larger vertical loads on piles. In their paper Youwai et al. (2012) describe the results of geosynthetic-reinforced flexible asphaltic pavement under cyclic loading. The permanent deformation of flexible pavement reinforced with geotextiles was significantly lower than the corresponding sections reinforced with geogrids. Moreover, the shear strain at the base of reinforced flexible pavement was lower compared to the corresponding unreinforced pavement.
The next paper by Robles-Austriaco et al. (2012) discusses bio-methods for soil stabilisation. Two of the bio-methods described in the paper are vetiver grass and hydroseeding. The roots of vetiver grass is recognised as a tool for slope stabilisation and erosion control. Voottripruex et al. (2008) presented a case study using vetiver grass for slope stabilisation through root reinforcement of the underlying surface soil. Vetiver grass can easily prevent rill and sheet erosion due to its strong and massive root system. Moreover, it has the ability to survive in extreme weather conditions (-15°C to 55°C) and regrow very quickly. A vetiver network has been established for effective promotion and technology transfer. Hydroseeding is a method to re-vegetate bare soil surfaces and prevent soil erosion. Hydroseeding is commonly combined with either polymer mats or natural fibre nets made of coconut coir, jute, kenaf or water hyacinth fibres.
The final paper by Hazarika et al. (2012) studies the uses of scrap tyre-derived lightweight geomaterials, such as tyre chips and tyre shreds, for geotechnical engineering applications. The shear behaviour was investigated through X-ray computed tomography and particle image velocimetry techniques. A tyre chips–sand mixture does not undergo liquefaction and helps to reduce earthquake-induced permanent displacement. In addition, the dilatancy effect is very minimal for a tyre chips–sand mixture where the volume change shifts from dilative to compressive. Thus, such material is suitable for retaining wall backfill. Tanchaisawat et al. (2008) also studied the performance of a reinforced tyre chips–sand mixture with geogrids in a full-scale test embankment.
