Crest Settlement Research Articles

A regression-based approach to predict crest settlement of embankment dams under earthquake shaking

Settlement and deformation of the embankment dams are among the major damages caused by earthquakes, eventually leading to dam instability. Therefore, accurate assessment of the seismic settlement of embankment dams is of particular concern. This study aims to evaluate the settlement of embankment dams subjected to earthquake loads using regression-based methods. A wide-ranging real data on crest settlement of embankment dams caused by earthquakes was collected and analyzed. Yield acceleration of dam, maximum horizontal earthquake acceleration, fundamental period of dam body, predominant period of earthquake, and earthquake magnitude were considered as the most influential parameters affecting the seismic crest settlement of embankment dams. Using support vector regression method as well as multiple linear regression method, two models were developed to estimate earthquake induced crest settlement of embankment dams. Subsequently, sensitivity analysis was conducted in order to assess the behavior of the proposed models under different conditions. Finally, the accuracy of the proposed models was compared with the existing relationship for estimation of earthquake induced crest settlement of embankment dams. Although both MLR- and SVR-based models have an acceptable accuracy in estimation of the crest settlement of embankment dams under earthquake loading, the SVR-based model has a higher accuracy.

A statistical analysis on concrete cut-off wall behaviour

Concrete cut-off walls are a well-established technique for forming seepage cut-off in pervious and compressible foundations in newly constructed dams. This study investigated the behaviour of cut-off walls on the basis of statistical analysis. A set of monitoring records and construction details of 43 case histories of in-service concrete cut-off wall constructed in the past 50 years were gathered. Detailed statistical analyses of horizontal displacement, crest settlement, cracking behaviour and stress of the walls were performed based on the case histories. Loading characteristics and relative settlement between the wall and the alluvium foundation were analysed. The statistical analyses also attempted to identify quantitatively the major factors affecting the wall behaviour; these factors include wall location, wall material, foundation deformation characteristics, wall depth and valley shape. The wall location was found to be the deterministic factor that affects the wall behaviour. The findings and results of this study provide a significant insight into concrete cut-off wall behaviour and provide a valuable reference for future wall design and construction.

Investigating the effect of geogrid on stabilization of high railway embankments

This paper investigates the effect of geogrid on controlling the stability and settlement of high railway embankments using laboratory testing and finite element modeling. To do this, five series of embankments with 50 cm height were constructed, at a scale of 1:20 and then were uniformly loaded on the crest in a loading chamber in dimensions of 240 × 235 × 220 cm. In this regard, the embankments of the first series were constructed without geogrid reinforcing layers. Following to preliminary numerical simulations for determining the appropriate level of geogrid layers installation, the second to fifth series of embankments were constructed. These embankments were reinforced with one to four layers of geogrid respectively and finally, the results of their load in terms of settlements were compared. In these studies, the reinforced embankments with a single geogrid layer had 7.14% raise in bearing capacity and 11.24% reduction in settlement respectively, in comparison with the unreinforced embankment. The obtained results for the third to fifth series of embankments were respectively in order of (19, 36.14), (26.3, 52.8) and (28.9, 53.42)%. In the next stage, by modeling the embankments in the PLAXIS 2D software, the results were validated by the values obtained through laboratory models. In continuation of the study, real embankments with heights of 5, 10, 15, and 20 m were simulated and placed under LM71 loading pattern (Eurocode, 2003). In this respect, the impact of important effective parameters such as number of geogrid layer, soil characteristics, embankment dimensions, interface coefficient between soil and geogrid and tensile strength of geogrid on bearing capacity and settlement have been studied. The numerical results like the experimental ones, confirmed the increase in bearing capacity and settlement diminishing with definite increase in the geogrid layers, so that more geogrid layers do not affect these parameters. With respect to improving the soil characteristics and reducing the height of embankments, the FEM models showed decreasing effect of geogrid tensile strength on embankment crest settlement. On the other side, the value of geogrid-soil interface coefficient has minor effect on both settlement and sliding safety factor.

Liquefaction analysis and damage evaluation of embankment-type structures

The increasing importance of performance-based earthquake engineering analysis points out the necessity to assess quantitatively the risk of liquefaction of embankment-type structures. In this extreme scenario of soil liquefaction, devastating consequences are observed, e.g., excessive settlements, lateral spreading and slope instability. The present work discusses the global dynamic response and interaction of an earth structure-foundation system, so as to determine quantitatively the collapse mechanism due to foundation’s soil liquefaction. A levee-foundation system is simulated, and the influence of characteristics of input ground motion, as well as of the position of liquefied layer on the liquefaction-induced failure, is evaluated. For the current levee model, its induced damage level (i.e., induced crest settlements) is strongly related to both liquefaction apparition and dissipation of excess pore water pressure on the foundation. The respective role of input ground motion characteristics is a key component for soil liquefaction apparition, as long duration of mainshock can lead to important nonlinearity and extended soil liquefaction. A circular collapse surface is generated inside the liquefied region and extends toward the crest in both sides of the levee. Even so, when the liquefied layer is situated in depth, no significant effect on the levee response is found. This research work provides a reference case study for seismic assessment of embankment-type structures subjected to earthquake and proposes a high-performance computational framework accessible to engineers.

A statistical review of the behaviour of concrete-face rockfill dams based on case histories

The concrete-face rockfill dam (CFRD) has become a preferred dam type, but its design remains largely based on past experiences. Understanding the behaviour of CFRDs is critical to the design and safety assessments of these dams. The main objective of this study is to review the behaviour of CFRDs from the perspective of statistical analysis. A set of monitoring records and construction details of 87 case histories of in-service CFRDs constructed in the past 50 years is collected. The analysed data that are presented are based on a review of the literature. Detailed statistical analyses of post-construction crest settlement and face slab behaviour, settlement of the dam body during construction and leakage after reservoir filling are conducted on the basis of the above-mentioned case histories. Several suggested empirical relationships are obtained by regression analysis to estimate the deformation and seepage behaviour of CFRDs. Findings offer reliable insights into dam behaviour and provide an engineering analogy and reference for the planning, design and construction of CFRDs in the future. The statistical analyses also attempt to identify quantitatively the major influencing factors of the deformation of CFRDs, such as intact rockfill strength, foundation characteristics, valley shape and seepage flow. Intact rockfill strength and foundation characteristics are found to be the main influencing factors of dam deformation behaviour. The results from the statistical analysis of the case histories and the analyses performed are also presented herein.

Seismic Response of WasteTyre Reinforced Embankment Slopes Based on Shaking Table Test

The seismic behavior of unreinforced and waste tyre-reinforced embankment slopes was investigated through 1-g shaking table tests. This study examined the effects of various factors on the stability of embankment slopes. The factors include the vertical waste tyre reinforcement spacing, seismic waves, peak ground accelerations. The influence of reinforcement methods on acceleration amplification factor, crest settlement and internal failure mechanism are also investigated. The results suggest that the beneficial effects of tyre-reinforcement are more pronounced in embankment slopes with smaller vertical reinforcement spacing and larger peak ground acceleration. Tyre-reinforced slopes can decrease the crest acceleration amplification factor by more than 32% compared to unreinforced slopes when subjected to a 0.4g Wenchuan earthquake input motion. In addition, with Wenchuan earthquake input motion, the average acceleration amplification factor reduction of slopes reinforced by triaxial geogrids is roughly 1.4 to 2 times greater than that of slopes reinforced by a composite of waste tyres and triaxial geogrids, or by a composite of waste tyres and tyre shreds. Along the surface of the slopes, from about one-third of the slope height, the acceleration amplification factor of all embankment slopes increases gradually with elevation, and the maximum accelerations were observed on the top of the slope. The unreinforced slope displaced more significantly compared to the reinforced slope when subjected to the same peak ground acceleration. The relative displacement of an unreinforced slope is almost 1.25 times greater than that of a tyre-reinforced slope, for a 0.1g Wenchuan earthquake input motion. The corresponding multiples are 1.4 and 1.58 for 0.2g and 0.4g accelerations, respectively. The captured failure pattern on embankment slopes further indicates that a tyre-reinforced slope has smaller cracks with shallow depth and more uniform deformation than those in an unreinforced slope, when subjected to a 0.4g Wenchuan earthquake input motion. Therefore, the tyre-reinforced soil, which has the potential to be more widely used and the benefit of reducing waste disposal costs, could improve some engineering properties of slopes much like the triaxial geogrids.

Long-term reinforcement strains for column supported embankments with viscous reinforcement by FEM

The time-dependent performance of deep-mixing-method column supported embankments reinforced by viscous reinforcement is investigated for different long-term (at 99% degree of consolidation) reinforcement strains using fully coupled three-dimensional finite element method. The influence of long-term reinforcement strains on long-term net embankment height, maximum crest settlement, maximum differential settlement at the crest and horizontal toe movement of embankments numerically constructed over two soft foundations is explored with the consideration of viscosity of two reinforcement products. Based on a series of numerical simulations, an approach to controlling the deformations of column supported embankments to modest levels while maximizing their long-term service heights is proposed. Also, a correlation between long-term reinforcement strain and end-of-consolidation reinforcement strain is suggested for the studied cases.

Stabilising railway embankments using an integrated tied back-to-back strengthening system

This study investigates the role of an integrated tied back-to-back strengthening system in increasing the failure load and decreasing the crest settlement of railway embankments. The strengthening system consists of bars or tendons installed through the embankment and tied together with continuous plates on each side. Numerous three-dimensional numerical finite-element models of railway embankments with and without the strengthening system were analysed to obtain the optimum level for installation of the system and the mechanical specifications required to satisfy embankment stability requirements. Six 10 m-high embankments with 1:1 side-slopes, three with the strengthening system and three without, were constructed at a 1:20 scale in a loading chamber and uniformly loaded on the crest. The results revealed that the unstrengthened embankments failed under an average stress level of 123 kN/m2 with an average crest settlement of 38·5 mm. The strengthened embankments failed under an average stress level of 186·67 kN/m2 with a crest settlement of 29·0 mm, representing a 51% increase in failure stress and a 25% decrease in crest settlement.

Seismic Resistant Effects of Composite Reinforcement on Rockfill Dams Based on Shaking Table Tests

In this article, the seismic resistant effect of a new reinforcement measure on rockfill dams, the Composite Reinforcement, is investigated based on 1-g large-scale shaking table tests. A 1 m high rockfill dam, which is scaled from the Yele rockfill dam in China, is tested by inputting a series of motions designed from a realistic Wenchuan earthquake record. The model dam is divided into two sections, with one section representing a normal rockfill dam (RD) while the other section employing the Composite Reinforcement (composite reinforced rockfill dam (CRRD)). The seismic-resistant effect of the Composite Reinforcement is investigated by comparing the recorded dynamic response of the two dams, in terms of the peak acceleration amplification factor distribution along the dam core, the crest settlement, and the visualized failure pattern. Results show that the application of the Composite Reinforcement can effectively mitigate the seismically induced deformation of the rockfill dam by reducing the peak crest acceleration and residual crest settlement by 18% and 55%, respectively, when subjected to a 0.5 g input motion. The investigation of the captured failure pattern on the two dams further indicates the effective reinforcing effect of the Composite Reinforcement on preventing the sliding failures of rockfill materials in the top 20% height of the dam body.

Numerical investigation of the response of the Yele rockfill dam during the 2008 Wenchuan earthquake

In this paper the seismic response of a well-documented Chinese rockfill dam, Yele dam, is simulated and investigated employing the dynamic hydro-mechanically (HM) coupled finite element (FE) method. The objective of the study is to firstly validate the numerical model for static and dynamic analyses of rockfill dams against the unique monitoring data on the Yele dam recorded before and during the Wenchuan earthquake. The initial stress state of the dynamic analysis is reproduced by simulating the geological history of the dam foundation, the dam construction and the reservoir impounding. Subsequently, the predicted seismic response of the Yele dam is analysed, in terms of the deformed shape, crest settlements and acceleration distribution pattern, in order to understand its seismic behaviour, assess its seismic safety and provide indication for the application of any potential reinforcement measures. The results show that the predicted seismic deformation of the Yele dam is in agreement with field observations that suggested that the dam operated safely during the Wenchuan earthquake. Finally, parametric studies are conducted to explore the impact of two factors on the seismic response of rockfill dams, i.e. the permeability of materials comprising the dam body and the vertical ground motion.

Mechanical behavior of embankments overlying on loose subgrade stabilized by deep mixed columns

Deep mixed column (DMC) is known as one of the effective methods for stabilizing the natural earth beneath road or railway embankments to control stability and settlements under traffic loads. The load distribution mechanism of embankment overlying on loose subgrades stabilized with DMCs considerably depends on the columns' mechanical and geometrical specifications. The present study uses the laboratory investigation to understand the behavior of embankments lying on loose sandy subgrade in three different conditions: (1) subgrade without reinforcement, (2) subgrade reinforced with DMCs in a triangular pattern and horizontal plan, and (3) subgrade reinforced with DMCs in a square pattern and horizontal plan. For this purpose, by adopting the scale factor of 1:10, a reference embankment with 20 cm height, 250 cm length, and 93% maximum dry density achieved in standard Proctor compaction test was constructed over a 70 cm thick loose sandy bed with the relative density of 50% in a loading chamber, and its load-displacement behavior was evaluated until the failure occurred. In the next two tests, DMCs (with 10 cm diameter, 40 cm length, and 25 cm center-to-center spacing) were placed in groups in two different patterns (square and triangular) in the same sandy bed beneath the embankment and, consequently, the embankments were constructed over the reinforced subgrades and gradually loaded until the failure happened. In all the three tests, the load-displacement behaviors of the embankment and the selected DMCs were instrumented for monitoring purpose. The obtained results implied 64% increase in failure load and 40% decrease in embankment crest settlement when using the square pattern of DMCs compared with those of the reference embankment, while these values were 63% and 12%, respectively, for DMCs in triangular pattern. This confirmed generally better performance of DMCs with a triangular pattern.

Hydromechanical constraints on piping failure of landslide dams: an experimental investigation

Understanding the internal structure and material properties of landslide dams is essential for evaluating their potential failure mechanisms, especially by seepage and piping. Recent research has shown that the behaviour of landslide dams depends on the internal composition of the impoundment. We here present an experimental investigation of the hydromechanical constraints of landslide dam failure by piping. Experiments were conducted in a 2 m × 0.45 × 0.45 m flume, with a flume bed slope of 5°. Uniform dams of height 0.25 m were built with either mixed or homogeneous silica sands. Uniform-sized pebbles encased in a plastic mesh were used to initiate internal erosion. Two laser displacement sensors were used to monitor the behaviour of the dams during the internal erosion process while a linear displacement transducer and a water-level probe were deployed to monitor the onset of internal erosion and the hydrological trend of the upstream lake. Five major phases of the breach evolution process were observed: pipe evolution, pipe enlargement, crest settlement, hydraulic fracturing and progressive sloughing. Two major failure modes were observed: seepage and piping-induced collapse. Majority of the dams composed of homogeneous material failed by seepage and downstream slope saturation, whereas dams built with mixed material failed by piping. We found that an increase in soil density and homogeneity of the dam materials reduced the potential to form a continuous piping hole through the dams. Furthermore, the potential for piping and progression of the piping hole through the dams increased with an increase in the percentage of fines and a decrease in hydraulic conductivity. The rate of pipe enlargement is related to the erodibility of the soil, which itself is inversely proportional to the soil density. This study provides new insights into the governing conditions and breach evolution mechanisms of landslide dams, as triggered by seepage and piping.

Finite element simulation of an excavation-triggered landslide using large deformation theory

Numerical simulation of an excavation induced landslide in a strain softening material is presented and the results are compared with field measurements. The simulation is based on the methodology proposed to estimate the post-failure deformation of slopes in strain softening materials. The method includes: a) the Updated Lagrangian formulation which is essential in capturing the changing geometry and configuration of the slope during failure, b) a strain softening constitutive model which enables simulation of the progressive failure mechanisms, c) a stable solution scheme to prevent problems associated with numerical convergence in strain softening materials, and d) the h-adaptive mesh refinement technique to prevent excessive distortion of the finite element mesh due to large deformation and to increase the accuracy of the numerical solution.For the slope considered here, it is shown that failure initiated due to the excavation at the toe of the slope and propagated upward due to the strain softening behavior of the geomaterials which eventually led to the progressive failure of this slope. The failure surface is mainly within a thin layer of soil with substantial strain softening behavior but propagates to the surrounding soil as the excavation proceeds. The predicted crest settlement, toe movement, and deformed shape of the slope are lower than the observed behavior of the slope. However, the numerical analysis clearly predicts the triggering factor and the failure mechanism of the landslide and the impact of the large deformation of the soil mass on the houses at the toe of the slope.

Performance of reinforced, DMM column-supported embankment considering reinforcement viscosity and subsoil’s decreasing hydraulic conductivity

It has been widely accepted that reinforcement made of polyethylene and polypropylene is susceptible to creep and soil’s hydraulic conductivity varies with its void ratio. However, unfortunately there is no available sensitivity analysis on time-dependent embankment behaviour taking either reinforcement viscosity or time varying hydraulic conductivity of subsoil into consideration. The influence of geosynthetic reinforcement viscosity and decreasing hydraulic conductivity with consolidation on the time-dependent performance of embankments with floating columns is investigated using a fully 3D coupled model. For an embankment at the working height corresponding to a post-consolidation polypropylene geotextile strain of about 5%, it is shown that the assumption of constant hydraulic conductivity and the failure to consider the viscous behaviour of geosynthetic reinforcement can underestimate time-dependent embankment deformations (including differential crest settlement and horizontal toe movement). The effects of factors including the foundation soil, reinforcement stiffness, column stiffness, column spacing, column type (floating and fully penetrating), and construction rate, on the time-dependent behaviour of column supported embankments are explored.

Numerical modelling of prefabricated vertical drains and surcharge on reinforced floating column-supported embankment behaviour

The short-term and long-term performance of reinforced, deep mixing method (DMM) floating-column supported embankments with and without prefabricated vertical drains (PVDs) are investigated using a fully 3D coupled model. The concurrent use of PVDs with the other two forms of soft ground improvement significantly shortens time for subsoil consolidation and also substantially reduces post-consolidation lateral displacements and post-construction crest settlements. The combined use of PVDs and surcharge preloading to meet a requirement of 100 mm post-construction crest settlement for a bridge approach embankment is examined. Compared to a comparable case with surcharge preloading but without PVDs, the synergistic effects of PVDs and 0.5 m fill surcharge preloading diminish the calculated post-construction crest settlement from 0.228 m to 0.036 m and reduce the post-consolidation horizontal movement below the toe at the ground surface level from 0.115 m to 0.067 m even though embankment is constructed 5 times faster in the case with PVDs.

Overtopping failure analysis of coastal flood defences affected by climate change

Sea defence structures are expected to protect coasts for a long period, hence requiring reliable performance assessment strategies, in order to ensure their integrity and functionality. It has been demonstrated that rising sea level together with changing wave height can lead to increase risks of the failure to coastal defence structures. This paper presents a method for assessing the risk of wave overtopping failure, analysing the joint probability of sea water level and significant wave height under future hydraulic conditions due to climate change. Monte Carlo simulations are utilised to analyse the time-dependant overtopping failure probability of a seawall in the UK subjected to sea level rise. The numerical results for the flood defence example show that the seawall subjected to the sea level rise with high emission scenario could face to a significant increase of the frequency and the rate of overtopping discharge in comparison with the present date conditions without consideration of seawall crest settlement.

Seismic response of reduced-scale modular block and rigid faced reinforced walls through shaking table tests

This paper focuses on understanding the seismic response of geosynthetic reinforced retaining walls through shaking table tests on models of modular block and rigid faced reinforced retaining walls. Reduced-scale models of retaining walls reinforced with geogrid layers were constructed in a laminar box mounted on a uniaxial shaking table and subjected to various levels of sinusoidal base shaking. Models were instrumented with ultrasonic displacement sensors, earth pressure sensors and accelerometers. Effects of backfill density, number of reinforcement layers and reinforcement type on the performance of rigid faced and modular block walls were studied through different series of model tests. Performances of the walls were assessed in terms of face deformations, crest settlement and acceleration amplification at different elevations and compared. Modular block walls performed better than the rigid faced walls for the same level of base shaking because of the additional support derived by stacking the blocks with an offset. Type and quantity of reinforcement has significant effect on the seismic performance of both the types of walls. Displacements are more sensitive to relative density of the backfill and decrease with increasing relative density, the effect being more pronounced in case of unreinforced walls compared to the reinforced ones. Acceleration amplifications are not affected by the wall facing and inclusion of reinforcement.

The feedback of rheological deformation of concrete face rockfill dam based on engineering analogy method

In view of relationship for axial strain, volumetric strain and the generalized shear strain are different in different documents when the rheology deformation of rockfill dam is analyzed. Volume rheology and shear rheology are assumed different, three-dimensional rheology rate of rockfill material is deduced and the reasonable relationship for axial strain, volumetric strain and the generalized shear strain is also deduced. Since the difference between indoor test parameters and rockfill dams actual parameters is bigger, the measured deformation for the rockfill dam to be built cant timely obtain and the rockfill dams actual parameters also cant timely feedack. The engineering analogical method is presented for the feedback of rockfill dams actual parameters. The measued deformation for already built rockfill dam, with similar dam height, upstream and downstream slope rate, valley shape factor and rock behaviour of rockfill material, is selected, and the parameters for rockfill dam to be built is feedback with the rheological numerical calculation. Example analysis shows that, analogy the measured deformation for the already built rockfill dam, the specific value between actual modulus factor of rockfill dam to be built and indoor test modulus factor is about 0.7; the specific value between the largest settlement and the dam height is 0.667%-0.720% when considering rheological deformation; the specific value between concrete faces deflection and the largest settlement is 0.252-0.272, and the incremental settlement of dam crest increses when the rheological parameter c decreases, but the value is small, the incremental settlement of dam crest is 3.537 cm after running 3 years.

계측자료를 이용한 중심코어형 석괴댐의 장기침하량 예측

In this study, the prediction methods for the crest settlement after impounding and the maximum internal settlement during dam construction were proposed through the analysis on settlement data at 46 monitored points of 37 Center-Cored Rockfill Dams (CCRDs). Results from this analysis provided that the crest settlement increases with elapsed time, and from the relationship between the dam height and the maximum internal settlement during dam construction, it is confirmed that the internal settlement was largely evaluated when the coarse-grained material was used as the dam core. This internal settlement increased in proportion to the dam height. In addition, the crest settlement of the CCRD with the core compacted with fine-grained material was relatively large. It is expected that the results of this study would provide the practical tool for the design, construction and management of CCRDs.

PERFORMANCE OF SLOPE UNDER SEISMIC CONDITIONS

The seismic stability of earth slopes has been a topic of considerable interest in geotechnical engineering practice for the past few decades. The failure of slopes during earthquakes can lead to the significant losses. The seismic stability of slopes is influenced by many factors and complete slope stability evaluation must consider the effects of each like geological, hydrological, topographical, geometrical, strong ground motion parameters during an earthquake and soil characteristics all influence the stability of a slope. Hence, the seismic performance of these structures requires appropriate evaluation during their design. Such performance is best evaluated through an assessment of the potential for seismically induce permanent displacements. This paper presents numerical studies on response of unreinforced slopes due to dynamic loading based on finite element analysis. Model embankments of silty sand and clayey sand have been considered. The parameters selected for the dynamic analysis includes embankment of height 5m and 12m with gentle slope 300 and steep slope 500. The sub base depth (D) is varied with height of the slope (H) with D/H ratio from 0.5 to 2.0.The various past earthquake records are considered with different amplitudes ranging from 0.1g to 0.4g.The results are presented in the form of factor of safety against slope failure under static and seismic conditions and settlement of crest of slopes. It is observed that as the height of the slope increases the maximum crest displacement increases for both type of soil and sub soil conditions.