ABSTRACT

Ground improvement as a means for allowing the replacement of piled foundations with shallow footings is systematically being used by engineers for many decades. Some ground improvement techniques that are installed by piling rigs and include cementituous columnar rigid inclusions makes it difficult to distinguish where ground improvement ends and deep piled foundations commence. This paper assists the geotechnical engineer by reviewing the concepts of rigid inclusions, how they differ with piles, and presenting of case studies of very deep applications of rigid inclusions.

The best ground conditions for a project can be envisaged to be when the ground is competent and loads can be applied to shallow footings. In this scenario construction time and costs are both minimal. Terzaghi et al. (1996) define a shallow footing as a footing that has a width equal to or greater than the foundation depth, which is the distance from the level of the ground surface to the base of the footing, and a pile as a very slender pier that transfers a load through its lower end onto a firm stratum or else through side friction onto the surrounding soil. Bowles (1996) defines shallow foundations as bases, footings, spread footings or mats with the ratio of depth of footing to its width being equal to or less than 1, and deep foundations as piles, drilled piers or caisson with ratio of length to width (or diameter) being equal to or greater than 4.

On the other hand, Das (2009) states that studies show that the ratio of footing depth to width of shallow footings can be as large as 3 or 4.
While it could be advantageous to have concise definitions for various concepts and behaviours to avoid confusion and to allow clear communication, it is not possible to simply set an integer as the boundary between two foundation systems. The concept of shallow versus deep foundation is only a simplification for explaining the mechanisms of load transfer to the ground. Bearing of shallow foundations are generally expressed by shear theories originally developed by Prandtl (1920), Terzaghi (1943), Meyerhof (1951) and Hansen (1971). Skin resistance (Tomlinson, 1971; Vijayvergiya and Focht, 1972; Burland, 1973) may become a major contributor as the ratio of footing height to width begins to increase. At the same time, while a large based footing may be categorised as a shallow foundation system due to its depth to width ratio, the depth of soil within the system may be very deep indeed.

To further complicate this simplification, it is possible to convert deep loose or soft soils to adequately competent ground by soil improvement, and to safely dissipate the loads without engaging piles for transferring loads to firm ground. Chu et al. (2009) have classified and described the various ground improvement techniques that are commonly practiced. Some of these techniques are developed to improve the physical and mechanical properties of in-situ soils without the introduction of imported material, and the outcome will remain as what is classically referred to as a shallow foundation. On the other hand, in other ground improvement techniques higher quality materials are added to the ground as inclusions. Some inclusions may be very long, and can form deep foundations that are composed of classical shallow footings and deep soil masses that are improved by the inclusions. These types of foundations cannot be expressed by the classical shallow foundation approaches and require further understanding, analysis and design of the improved ground as part of the foundation system.

The complication in the categorisation can turn into confusion when piling rigs are used to install cementitious columnar inclusions. One very efficient, beneficial and affordable type of such rigid inclusions is the controlled modulus column (CMC).

3.   CONCLUSION

Deep rigid inclusions have complicated the classical definitions of shallow and deep foundation systems. CMCs are a type of cementitious rigid inclusion that resemble piles, but their design concept is very different. They have many advantages, including independence of column strength from in-situ soil parameters, non-reliance of column lateral stability on in-situ soil parameters, significant reduction of the magnitude of settlements, vibration-less installation process, negligible amounts of spoil, and high production rates.

This technology utilises a load transfer platform to distribute the loads between the columns and the in-situ ground. Column loads can be determined by numerous methods. IREX proposes the analyses of ground behaviour and load distribution for grounds improved with columnar inclusions in a variety of cases.
Numerical methods are commonly used for the analysis of CMCs, but a deep insight is required for modelling and assigning input parameters in these methods. Research suggests that classical input parameters may over estimate settlements, and more accurate results may be possible if input parameters are calibrated. Racinais (2015) has proposed a calibration method that is based on the pressuremeter test and matching the curves of Frank and Zhao (1982).
CMCs have been successfully used for improvement of ground to depths of 42 m for highly strategic structures such as oil tanks.

See the main text of the article next to it.

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