Strengthening foundations of seismically weak buildings on sandy soils in Denizli, Turkey

Abstract

The Marmara earthquake in 1999 was a turning point for Turkey in terms of the emphasis to construct earthquake resistant buildings and to control the performance of existing buildings. To this end, a large number of seismically weak buildings have been inspected. Inherently, a majority of them required some remediation in their foundations. Grouting studies around the foundations of five stories, reinforced concrete buildings with brick walls and strip foundations on three different sites have been conducted. The buildings are built on SC (silty sands) and SM (clayey sands) soils with high ground water level. A grout with a 1:3 cement–water ratio was injected near the columns, and the grouting holes were as much as 7 m deep around the basement level. The injection density was about 10–12 m2/borehole. The soils had low standard penetration test (SPT) (number of blows N) values before grouting. After grouting, the 54.7 mm (NX size) core samples that were obtained from verification boreholes demonstrated that their uniaxial compressive strength and Young’s modulus were remarkably advanced, reaching up to 35.4 and 51.2 MPa, respectively. Meanwhile, grouting caused a slight increase in the unit weight of the soils in the examined cases. Similarly, a remarkable relationship was not observed between the operational factors, like grouting pressure and the physico-mechanical properties of grouted soil. Thin sections of the core samples were visualised under polarized light from crossed nicol prisms, which illustrated that small voids that are about 10–30 μm in diameter were homogeneously distributed; additionally, individual voids may reach up to 300 μm in size. This also demonstrated that soil has been replaced by cement and that the amount of voids is highly noticeable, which may explain why the unit weight was not considerably increased. In these cases, a relationship between soil densification and an improvement of compressive strength and Young’s modulus was thus not observed. The study showed that grouting caused soil replacement rather than soil densification in the examined cases. It has dramatically increased the bearing capacity of the tested soils. This practice can be employed to form tough soils when foundations of existing buildings need strengthening.