Geotechnical Engineering Online Library

Forensic analysis of past failures is valuable to improve our understanding of levee behavior. In this article a new systematic approach of forensic analysis for levee failures is proposed and applied to the Breitenhagen levee breach that occurred along the river Saale in Germany in 2013. The purpose of this study is to identify the cause of the breach based on the proposed approach, even though limited data is available. Based on the information prior, during and after the breach of the levee, a slope stability model is developed for the entire event. First, results from this model are obtained based on the expected values of the uncertain parameters and the best estimates of the situation. Uncertainty of the model is included in the calculation subsequently by defining possible failure scenarios. The most likely failure scenarios are derived from the data and included into the model so that it is possible to eliminate or validate all possible causes by means of a sensitivity calculation. It is concluded that the levee breach is likely caused by locally weak soil conditions, unexpected high water pressures due to a connection between a pond and the aquifer and unexpected saturation of the levee. These conditions are associated with the occurrence of a previous breach at this location.

Slope failures are a common occurrence in tropical regions with a high intensity of rainfall. Tropical areas such as Singapore are normally covered with residual soils whose behaviour does not follow the principles of classical saturated soil mechanics because these soils are often unsaturated in nature. The negative pore-water pressure in unsaturated soil is highly influenced by the changes in the flux boundary conditions, resulting from the variation in climatic conditions. On the other hand, the negative pore-water pressure contributes additional shear strength to the unsaturated soil. As water infiltrates into the slope, pore-water pressure in the slope increases (matric suction decreases), and the additional shear strength due to matric suction will decrease, causing the slope to be more susceptible to failure. Singapore is a land scarce country with a critical need to optimize land utilization. Steepening slopes or cutting back slopes and supporting them using a retaining structure is one way to create new spaces. In this study, a new type of retaining structure, Geobarrier System (GBS) is proposed. A GBS is a man-made three-layer cover system designed as a vegetative layer combined with a two-layer unsaturated system, which harnesses the distinct difference in unsaturated hydraulic properties between a fine-grained layer and a coarse-grained layer. GBS consists of recycled materials and does not use steel or concrete and is hence more cost effective, thereby making it economical for use in urban areas. Geobag for vegetative layer is supported by specially designed pockets for planting different types of sustainable plant species. The paper presents the design, construction procedures, material selection and field performance of a GBS constructed at an inclination angle of 70o in response to rainfall infiltration. In addition, the results of the finite element seepage and slope stability analyses of the GBS subjected to extreme rainfalls are also presented. The results from field instruments and numerical analyses showed that GBS was able to protect the slope from rainfall infiltration; therefore, the stability of the slope retained by GBS was not affected by the rainfall.

The paper describes five different dike failures that occurred in recent decades. The case histories were located on different rivers and involved different loading conditions. The observed failure mechanisms involved erosion and stability problems. The types of erosion observed were both internal erosion due to extreme groundwater flow and external erosion caused by the river flow and wave action. The case involving instability was caused by uplift, i.e., increased hydraulic head led to a sharp reduction in maximum shear strength between subsoil layers. Two cases demonstrate the importance of dike management and maintenance. Despite the variations in the loading conditions and failure mechanisms, all the cases show that the strength of a dike depends not only on the material used to build the dike but also on the strength of the subsoil.

A large road box cut was constructed in hilly terrain of Muscat city, Oman, in an area underlain by the geologically complex Sahtan and Mahil formations comprising of limestone, dolomite, sandstone and siltstone with interbedded soft calcareous mudstone. The rockmass contains a large number of small scale faults, shear zones, minor folds and solution cavities. The maximum height of the cut was 50 m with slope angle of 1V:0.25H, and intermediate benches generally 3 m wide. To control vibrations in the urban neighborhood located nearby and avoid damage to the rockmass, a very cautious and time consuming excavation approach was adopted using rock breakers. The excavation work was completed in 2010. Oman has a hyper-arid environment with scarce rainfall for many months. However, occasionally the area is hit by intense rainstorms. During Cyclone Phet and accompanying rains in June 2010, part of the cut slope failed with a downslope movement of about 800 m3 of massive rock. This necessitated a post-excavation re-evaluation of the cut slope, which culminated in design modifications, and construction of long term slope stabilization measures that included high capacity rock anchors, rock bolts and reinforced shotcrete for stabilizing potential shallow and deep seated failures. This case history describes the initial design of the cut slope, causes of slope failure during the cyclone, field and laboratory testing carried out for determination of rock engineering parameters, analytical techniques used for design modification and construction details of the designed remedial works.

A 30-m long railroad embankment failure that occurred on May 25 2011 in the city of Ann Arbor, Michigan, is presented. Emphasis is given on the field observations of the failure, the characterization of the site conditions and the seepage and slope stability analyses, all of which represent important components of the training and practice of a geotechnical engineer. The failure occurred following a record wet season that resulted in ponding water against the embankment and high enough water pressures and exit gradients that resulted in instability of the railroad embankment. Detailed background material and the methodology for using the case history in geotechnical engineering education are presented.

The objective of the paper is the presentation of a case study developed for geotechnical engineering instruction. The examined case study is suitable for undergraduate instruction and refers to the geotechnical design of a railway embankment focusing on slope stability analyses and settlement calculations. The presented geotechnical data have resulted from the geotechnical study implemented by Edafomichaniki S.A. within the framework of the project “Geotechnical Design of the New Railway Alignment between the Existing Line and Kavala’s Port”. The geotechnical design of a railway embankment is described thoroughly in the current paper and includes the following: a) the evaluation of the available geotechnical data encompassing the determination of soil stratigraphy and geotechnical design parameters of the encountered formations, b) the performance of geotechnical calculations regarding slope stability analyses and settlement calculations with detailed reference to consolidation theory and c) the presentation of the construction methodology of the embankment. The usefulness of the examined case study is highlighted through the achievement of a significant number of learning outcomes. The current paper is accompanied with detailed supplementary material.