Abstract
Due to the increase in the number of mass movements on hillslopes during the rainy season in the city of Recife, the importance of analyzing and understanding the unsaturated behavior of the soil is verified. Therefore, this paper covers the stability analysis of an unsaturated soil belonging to the Barreiras Formation located at an important hillslope of Recife. This area has been affected by landslides in 2000, and about 100 families were left homeless after losing their homes. The slope is approximately 56 m high and 414 m long (lined up with Chagas Ferreira Avenue). The results of physical characterization and parameters from shear resistance of soil obtained in laboratory tests are presented. The modeling work was divided in two parts: (1) transient unsaturated seepage analysis; (2) stability analyses. To achieve this, computational solutions were implemented applying the SLOPE / W and SEEP / W programs, which analyzes the stability and flow of the slope. For analyzes, a rainfall of July / 2019 and pontual leaks was considered in the values of of 0.5, 1.0 and 1.5 m3/day. It was possible to observe that, as the intensity of the leaks increased, the pore water pressure values increased considerably at the deepest levels. The water table was also compatible with the increase in the intensity of water infiltration. In the slope stability analyzes conducted was observed that the increase in moisture content and the decrease in suction during intense precipitation periods combined with leak infiltration reflect in the decrease in the safety factor to values around 1.
Highlights
The Barreiras Formation is one of the most extensive units observed along the Brazilian coast, extending from the State of Rio de Janeiro to the State of Amapá (Suguio and Nogueira 1999 [1]), covering Mesozoic sedimentary deposits from several coastal basins.In the Metropolitan Region of Recife, the deposits of the Barreiras Formation are unevenly distributed over the crystalline basement and the Cretaceous basins (SubBasin Cabo and Sub-Basin Olinda) reaching up to 150 m in thickness in the northern area of Recife
Quantities such as shear strength, hydraulic permeability and water storage capacity of unsaturated soil depend on the matric suction value, which varies with the water content of soil
The field investigation consisted of five Standard Penetration Tests (SPTs) with NSPT obtained in each meter
Summary
The Barreiras Formation is one of the most extensive units observed along the Brazilian coast, extending from the State of Rio de Janeiro to the State of Amapá (Suguio and Nogueira 1999 [1]), covering Mesozoic sedimentary deposits from several coastal basins. Quartz sands, clays and silts are used as sediments from the Barreiras Formation (Silva 2007 [3]) In this context, the predominantly clayey or sandy soils make the slopes susceptible to instability processes, the importance of studying the geotechnical characteristics of these materials. The water retention, unsaturated shear strength, and hydraulic conductivity function are very important unsaturated soil property functions (USPFs) These have to be determined when analyzing the stability of unsaturated slope exposed to transient rainfall infiltration process (e.g., Peranick et al 2019 [4]). This article presents the field and laboratory activities with typical soil of Barreiras Formation and the flow and stability analyzes conducted to investigate the importance of the unsaturated zone for the process of rainfall infiltration and activation of slopes
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