Here, also, the developments in rig capacity make it possible to install augercast piles exceeding 40 m (131 ft) in depth and 1,200 mm (4 ft) in diameter with very high loads in some areas. Retaining Walls In the area of retaining walls, a large variety of technical solutions were observed from country to country, and according to the type of problem. Some of the techniques have largely increased in understanding and/or technical developments. Diaphragm Walls For slurry walls and cutoff walls, strategic projects have driven extreme expectations and design depths while retained height has pushed the limits of “standard execution” challenges. The most recent technological evolution or trends in this field include: • Hydraulic grabs with guiding and correction systems during excavation which help guarantee excavation tolerances of 0.5 % • Hydromills or cutters for excavating in difficult soil conditions, including limited headroom, or in tight working spaces • Monitoring of the execution, where key parameters are registered, as depth, inclination, deviation of the grab in the two directions • The use of improved systems for watertight joints These technological improvements allow tighter tolerances in the execution of the walls, and hence a higher quality of the finished product. Trevi hydromill breaks depth record Following a series of accidents on large infrastructure projects in Europe, recent research has resulted in stricter con- struction practices. The recent Dutch CUR Handbook Diaphragm Walls T114/C174 publication gathers, in the author’s opinion, the most advanced guidelines for good execution practice, certainly in very soft saturated alluvial soils. As a result of these enhanced practices and equipment, expectations in terms of water-tightness and construction toler- ances have reached unprecedented levels in most ground conditions. There is an increasing demand for deeper diaphragm walls in harder soil conditions. Major dam construction and rehabilitation metro projects in Copenhagen, Qatar or Paris, drive the development of cutters and tools for both extreme conditions and tight tolerances. The Trevi Group recently broke the depth record in this field and developed a hydromill capable of building slurry walls down to an unprecedented depth of 250 m (820 ft). Cased Secant Piles As diaphragm walls reach new records, smaller projects in urban areas require the construction of two to three underground parking levels. Two recent developments tend to gain a larger share of the market here: cased secant piling (CSP) and soil mixing. The use of this combined simultaneous drilling of a casing and an auger is perfectly suited for the execution of secant piles. Originally limited to small diameters (<530 mm [1.7 ft]) and limited depths (<15 m [49 ft]) in soft soils, the evolution of rigs makes it possible today to install piles that are 800 mm (2.6 ft) in diameter at depths close to 20 m (66 ft) in harder soils and weathered rock. In this application, CSP have the major advantage to drill through masonry and most man-made obstacles and to guarantee a good level of construction tolerance. This method can also be used in combination with pre- drilling with down-the-hole hammers for use in difficult rock conditions. CSP wall in weathered rock - Belgium Soil Mixing The Deep Mixing Method (DMM) was introduced in the 1960s in Japan and Scandinavian countries. The method had to wait until the 1980s to enter the American market as a ground improve- ment technique. In Europe, initially considered an alternative to the jet grouting application, the DMM made its entrance in the late 1980s with the emergence of various deep soil mixing systems. Recently, we observe the emergence of several methods: mass stabilization, trench mixing (in the beginning of the 1990s) and Cutter Soil Mixing (CSM) in 2003. The DMM system has gained acceptance and application worldwide, in excavation support, seepage control or bottom seals. In parallel with the technological evolution, a better understanding of the characterization of the soil mix material, the design process and its relationship with construction issues, and the elaboration on QA/QC procedures have made tremendous progress. Recent research in Belgium has brought interesting results with respect to design and analysis of the main questions related to the mechanical characteristics of the material, its permeability and its adher- ence with steel for temporary applications. The durability of the soil mix material remains a complex topic while permanent applications are utilized more. The BBRI (in Belgium) and SBRCURnet (in the Netherlands) have worked together to DEEP FOUNDATIONS • NOV/DEC 2015 • 85
