Geotechnical Engineering Online Library

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.

Rainfall-induced slope failures commonly occur within residual soil slopes. One of the possible systems for slope preventive measure is capillary barrier system. A capillary barrier is a two-layer system of distinct hydraulic properties that is used to prevent water infiltration into the soil below the capillary barrier system by utilizing unsaturated soil mechanics principles. This paper presents the numerical simulation of the capillary barrier system as a slope preventive measure against rainfall-induced slope failures. The capillary barrier was constructed on a slope which experienced several shallow failures due to rainfall. In this study, the capillary barrier system was designed to repair the slope and at the same time to provide preventive measures for further failures due to heavy rainfall conditions of the tropics. The capillary barrier system was constructed using fine sand as the fine-grained layer and granite chips as the coarse-grained layer. Both layers were contained in geocells. The slope was instrumented with tensiometers and piezometers. The tensiometers were installed at different depths from about 0.5 m to 2.0 m below the slope surface. In addition, the adjacent original slope without the capillary barrier system was also instrumented using tensiometers in order to investigate the performance and effectiveness of the capillary barrier system in reducing rainwater infiltration and maintaining negative pore-water pressure in the slope. Results of field measurements from one-year monitoring period and numerical analyses of slope with and without capillary barrier system are presented in the paper. The results of numerical analyses of the slopes with and without capillary barrier system indicated that the capillary barrier system performed well in minimizing rainwater infiltration into the underlying soil layer. In addition, the numerical results showed that the factor of safety of a slope with a capillary barrier system was significantly higher than that of an original slope without the capillary barrier system. The field measurement and numerical analyses results were in good agreement, demonstrating the successful application of unsaturated soil mechanics principles in the design and construction of a capillary barrier system.

A numerical method, PEI (Pile behavior in Expansive soil upon Infiltration) developed at the University of Ottawa can be used as a tool for the prediction and interpretation of the mechanical behavior of piles in expansive soils taking account of the influence of matric suction changes associated with water infiltration. This program is employed in back analyses of the in-situ single pile test results for a case study at the Colorado State University (CSU) field test site, which has expansive soil deposits. Four single piles were drilled into the expansive shale of the Pierre Formation west of Fort Collins, Colorado. The piles were 350 mm in diameter and were installed to a depth of 7.6 m. During installation, slope indicator-type VS embedment strain gauges were embedded on the piers at a depth of 1.8 m, 3.1 m, 4.3 m and 5.5 m below the ground surface. Water content variations of the surrounding soil along the pile depth were measured periodically from 1995 to 2004, in addition to other measurements, which include pile axial force distribution and pile head displacement. Extensive analysis regarding this case study with respect to the mechanical behavior of pile associated with changes in water content is available in the literature. In this paper, this case study is illustrated in an innovative way to understand the influence of matric suction on the mechanical behavior of the pile using the program PEI. In addition, in-situ measurements of both pile head displacements and pile axial force distribution are compared with the results of numerical simulations using the PEI. Analyses of the results show that measured in-situ and numerical results are in good agreement. The results of this study are of significant interest for the practicing engineers who routinely design pile foundations in expansive soils.

The construction of the steel arched Tsakona Bridge, designed to overpass an active landslide, required several temporary projects. Auxiliary steel towers were used for the erection and welding of the arch segments. Most of them had to be founded in the body of the sliding mass. An automated instrumentation and monitoring system was installed on site providing early warning to the engineers in charge in case large displacements were measured. The real-time recording data provided valuable information about the landslide movements. These monitoring results defined the geotechnical and structural design criteria for the deep foundation of the temporary towers. Several 2D and 3D geotechnical and structural models were created to evaluate the sliding effect on the foundation components, finalize their dimensioning and calculate the required reinforcement. The procedure followed for the design of the temporary deep foundation is described based on the criteria devised using the monitoring system, emphasizing on the continuous feedback between the geotechnical and structural design teams. Several key structural design issues required during construction are also presented.

Wave energy harnessing is associated with high cost, compared to established renewables such as wind and solar. In order to make the technology commercially attractive, electricity production could be coupled with secondary functions, such as coastal defence. An innovative concept is the integration of wave energy converters (WECs) in caisson breakwaters, offsetting the initial high cost of WECs with coastal defence. Here, the functionality of Chania’s Venetian harbour offshore breakwater was assessed under typical wave conditions. We used measurements from a Nortek AWAC ADCP, deployed in the nearshore, to numerically simulate the wave conditions induced by a typical low energy storm (Mdir=360o, Hm0=1 m and Tp=5.5 s) inside the Venetian harbour. We employed the Boussinesq-type wave model MIKE 21 BW and simulated cases with and without the breakwater. In both cases, Hm0 reached 0.4 m, just inside the harbour’s entrance and, in general, similar wave conditions were observed. Therefore, results indicate that the existing offshore breakwater provides little protection to the entrance and the south part of the harbour from waves coming from the north, which are the vast majority of the winter waves according to the field measurements. Thus, an extension or other modifications are required, so as to provide adequate protection to the entrance and the south part of the harbour. We also used the ADCP measured data for a preliminary analysis of the local wave power potential. During winter 2011-2012, the maximum significant wave height (Hm0) recorded by the AWAC was 3.85 m, whilst peak periods (Tp) higher than 10 s were observed. These wave characteristics yielded mean (Pmean) and maximum (Pmax) wave power values close to 4.8 kW/m and 72 kW/m, respectively. Therefore, integrating a WEC in future breakwater designs might be a feasible alternative, given also the minimal tidal range of a few cm. Apart from offsetting the WEC’s high initial capital expenditure to coastal defence, the electricity from the waves could power the harbour’s lighthouse. Coupled with interpretive displays of the wave energy technology, this could also stimulate additional (eco)tourism opportunities.

During the winter of 2001-2002, a landslide of 80.000 m3 was activated and moved downslope at the Northeast outskirts of the Pissouri village in Cyprus (Limassol prefecture), displacing a section of a country road that was crossing the upper part of the sliding mass and destroying three newly built residences. This paper presents the geologic and hydrogeologic regime of the landslide and its implication on the landslide’s mobilization. Geotechnical site investigation and post failure monitoring, in conjunction with slope stability back analysis formed the basis for the study of the slide and subsequently guided the remediation strategy. Various possible interventions to stabilize the slope and allow the safe reconstruction of the displaced road have been investigated and compared considering both economic and environmental criteria. The performance of the remediation measures has been studied in terms of stability safety, using numerical models previously calibrated via back analyses. Observations made during the execution of the stabilization works provided additional data verifying and supplementing the original design.

St Isaac’s Cathedral in St. Petersburg was completed in 1858 after 40 years of construction; it is today the fourth largest domed Cathedral in Europe. The soil is a relatively soft saturated sediment and carries this 3155 MN structure that is 100 meters high with an imprint of 92 m by 100 m. It is founded on a 7.5 m thick mat of granite and limestone blocks resting on relatively short timber piles of different lengths. The Cathedral has progressively experienced significant deformation including differential settlement causing cracks in the pillars and tilting of the porticoes. The paper summarizes the available geotechnical engineering and engineering geology aspects of the soil on which the Cathedral is built as well as classical issues such as foundation ultimate capacity and settlement analysis through simple calculations and numerical simulations. It also includes some less classical issues such as the influence of microbial activity on the behavior of the Cathedral through the changes the microbes and their activity create in the engineering properties of the soil and in the groundwater composition. The paper concludes with the results of a numerical simulation of the soil and the foundation under load and a comparison with the limited measurements that have been collected.

The growing demand for public and sustainable transportation in heavily urbanized areas like Singapore requires construction of an increasing number of Metro Lines. Complicated technical challenges are associated with such an activity and settlement prediction models play a key role in assessing the tunnelling-related risk assessments and planning mitigation techniques. Although a significant amount of research has been performed to study the settlements induced by side-by-side twin tunnels, the settlement prediction of stacked tunnelling has been, relatively, limited. In this paper, stacked tunnelling induced settlements are predicted using 2D numerical simulation. The extensive data from the instrumentation measurements collected during the construction of a stacked configuration tunnel in Singapore Downtown Line MRT tunnels have been used to study the validity of the principle of superposition for stacked tunnels. Key observations are presented by comparing empirical, numerical and actual field settlement data. Using the Monte-Carlo simulation technique, the empirical trough parameter is back-calculated to fit the actual settlements observed in Kallang formation. This could be used for future settlement predictions in similar ground conditions.