Design of Shallow Foundations on Soils and Rocks on Basis of Settlement Considerations

In modern foundation engineering, emphasis is given to settlement, rather than to bearing capacity, Décourt (1999) and Fellenius (1999). In Brazil, as in many other countries, the standard penetration test, SPT is, by far, the most used in situ test. The efficiency of the Brazilian SPT is about 72% and its penetration resistance is designated, as N₇₂. A very important improvement of this test was introduced by Décourt and Quaresma Filho (1991, 1994), the SPT-T. After the sampler penetration into the ground, the torque required to overcome the friction between the sampler and the soil, T, is measured and the concept of equivalent N (N_eq) is introduced, where N_eq = T(kgf.m/1.2 or N.m/12), Décourt (1991). This test provides two quite different and very useful measurements at the same point. The settlement of footings is computed as proposed by Décourt (1994b, 1999), the input data being given by routine penetration tests, SPT, SPT-T, and CPT. The main objective of any good foundation design is to provide equal settlements for all footings. The program ZDSM (zero differential settlement method) computes, for a stipulated settlement value, the stresses, and the dimensions of all footings (assumed square). Saprolitic soils can be very easily and precisely analyzed via the SPT-T instead of the SPT, using N_eq instead of the N_SPT, as the latter is not reliable for such soils, Décourt and Niyama (1994). However, for lateritic soils (LS), up to now, there is no straightforward test that can be used. The use of the shear modulus for low strains, G₀, also as an index test, like it is now being proposed, seems to be very promising. For shallow foundations on rock, it is important to identify its material weaknesses, weathering and fracturing, and to improve its mass properties, by reinforcing it. The main goal is to turn the natural rock into a material, at least as stiff as the high resistance reinforced concrete that might be used for the structure (f_ck≈50MPa). A new method for designing safe, fast, and economical foundations on rocks is now being proposed.