Detection of Differential Settlement of Man-Made Structures Coupled with Urban Development by Using Persistent Scatterer Interferometry (PSI)

Abstract

Many cities are prone to land subsidence, particularly due to the overuse of ground water. However, because man-made structures are normally built upon foundations that are stiffer than the surrounding ground, such structures react to land subsidence to a lesser extent. This settlement mismatch between ground and buildings, also known as differential settlement (DS), may cause serious problems in urban management, such as foundation overloading due to down-drag forces and damage to underground pipelines. Here, we present a technique for determining DS from multi-temporal satellite synthetic aperture radar (SAR) images. Permanent scatterers originating from ground and man-made structures are extracted using the differential interferometric SAR (DInSAR) technique, whereupon the DS is obtained by subtracting the settlement of the former from that of the latter. For validation purposes, we demonstrate that the estimated DS in Bangkok is consistent with field observations. Keywords: differential settlement; differential interferometric synthetic aperture radar (DInSAR); persistent scatterer interferometry (PSI) IntroductionCities all over the world have developed rapidly in recent decades, and many have suffered land subsidence due to excessive extraction of ground water, e.g., the Mekong Delta in Vietnam [1] and the Konya Plain in Turkey [2]. This has had considerable negative impacts on those cities, such as increased risk of flooding in coastal areas, cracks in buildings and infrastructure, destruction of local groundwater systems, tension cracks on land and the reactivation of existing faults. Among these various negative impacts, we focus especially on the settlement mismatch between the ground and buildings. This can cause serious problems for urban management, such as building instability and damage to underground pipelines. In the field of geotechnical engineering, this phenomenon is generally referred to as differential settlement (DS) [3,4].Land subsidence has been a serious problem in the city of Bangkok in Thailand, particularly in the early 1980s, when ground subsidence rates as high as 120 mm per year were reported [5]. The ground formation in Bangkok is soft clay with a thickness of 12-16 m, underlain by medium clay, followed by the first stiff clay layer. The first sand layer (Upper Bangkok aquifer) is then encountered at 22-24 m in the inner city area [5]. Because the soft clay layer is highly compressible, most buildings in the city are supported by piles. A normal design practice is to set the tips of piles for low-and mid-rise buildings in the first sand layer. The tips of piles for high-rise buildings are normally located in the second sand layer, which lies at a depth of 30-58 m from the ground surface [6]. Based on [6], the compression in the first 50 m of depth contributes to 30-50% of the total settlement observed at the ground surface. Because the piles are stiffer than the soft clay layer, the ground around a building will settle faster than the building. Figure 1 shows examples of DS encoun...

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