of shearing resistance using the concept of stress dilatancy for very dense sands, which then Robertson and Hughes (1986) extended for loose to medium-dense sands. In general, several authors concluded that the shear modulus describes a non-linear stiffness curve during expansion. In terms of the EB, those concepts are helpful for the interpretation of the injection curves because, from the practical point of view, the limit pressure is the value used for the calculation of the toe capacity and the side friction of the EB. Design Methodology An unique advantage of the EB system is that the actual condition at the pile toe can be monitored and evaluated, which pro- vides characteristic information pertaining to the horizontal pressure, in-situ strength and stiffness properties. Any deviations from the design assumptions can be considered by adjusting the design according to the achieved pile toe resistance. t is calcu- The total capacity of the EB (Q ) lated as the sum of the end bearing or toe resistance (Q ) and the side resistance (Q) : Q = Q + Qf p t p p Q is calculated based on the pres- suremetric formula recommended by LCPC-SETRA (1985) , which was developed in accordance with cavity expansion theory: Q /A = [k(P -s )] + s p k lim h v where A = cross-sectional area of the base of the EB; = bearing capacity factor = 1.95 to 2007); 2.24 for local soils of Santa Cruz (Coutand, P = limit pressure; s = total lim h horizontal pressure at the base; and s =v total vertical pressure at the base. P is lim based on geotechnical information and in accordance with load tests performed in similar soils and is used to compute after Q t measuring the actual P lim during expansion. The base area of the EB used for design is the fully expanded area. The area used to evaluate the capacity after inflation is the actual expanded area, which is computed using the calibration curve and the actual injected volume. The side (friction) resistance of the EB is calculated using the following relationship: Q = AP tan f l lim l f where A = surface area of the frictional surface of the EB and tan f = tangent of the friction angle of the soil-EB steel interface. Due to various uncertain- 90 • DEEP FOUNDATIONS • MAY/JUNE 2018 f Post-grouting below the pile toe (EBI) and injection curve ties, the value of f is typically assumed as 10°, which is a highly conservative value based on the work of various researchers. The design process is based on correlations between P lim and SPT N or CPTu q values, and are being improved 60 c (less erratic) as the database is rapidly increasing. For sands in Santa Cruz, Bolivia, the correlation with N is either P lim = 0.08N60 (in MPa) for N < 20 or Plim = 0.10N 60 (in MPa) for N > 20. In the case of 60 60 60 CPTu, the correlation for soils in Santa Cruz, Bolivia is P = 0.13q (in MPa). After lim lim c the expansion process, the typical standard deviation of P ranges from 9 to 13% of the average value of the piles on a project. Post-Grouting Below the Pile Toe During inflation, the diameter of the EB increases while its length is shortened. The risk of soil decompression below the pile toe due to this shortening is usually insignifi- cant. However, to prevent soil decom- pression and to maintain intimate contact between the soil and the bottom of the EB, a new development (EB Incotec [EBI] system) allows the possibility to post-grout the soil below the pile toe after inflation of the EB. As such, the strength and stiffness of the soil below the EB can be ensured. By measuring the grouting pressure, valuable information is obtained regarding the improvement effect below the pile toe. Conclusion The Expander Body (EB) technology has been used successfully to increase the end resistance of bored piles in loose to medium dense soil. An important advantage of the EB system is the ability to monitor the expansion process, which provides important information regarding soil strength and soil stiffness. As such, a high degree of quality control and assurance can be achieved as the expansion process is monitored, recorded and documented. The EB system can now incorporate the possibility of post-grouting below the expanded pile toe (EBI system). Although the EBI system requires a more sophisticated installation process, the increased pile resistance makes this system a cost-efficient deep foundation solution, compared with low-cost, less reliable pile types. Recent field tests demonstrate that pile capacity can be increased at least four times compared to conventional, bored piles of the same geometry. Mario A. Terceros H. received a B.S. degree in civil engineering from Universidad Católica de Córdoba, Argentina, and an M.S. degree in hydrology, geotechnics and foundations from Universidad Politécnica de Madrid, Spain. He is the managing director of Incotec in Bolivia, and has also been a professor at the Universidad Privada de Santa Cruz, Bolivia, and the Monterrey Institute of Technology. Mario Terceros A. received a B.S. degree in civil engineering from Universidad de Monterrey, Mexico and an M.S. degree in geotechnics from Universidad San Francisco Xavier, Sucre, Bolivia. He was the operations manager of Incotec until 2016, when he became chief executive officer.
