Embankment Deformations Research Articles

Thermal and deformational repairing effect of crushed rock revetment acting as reinforcement along Qinghai–Tibet railway in permafrost regions

Due to the particularity and complexity of permafrost subgrade, research on its long-term maintenance and reinforcement under climate warming and engineering activities is of great significance. To mitigate subgrade diseases caused by thermal disturbance during the engineering construction and operation in the initial stage, the crushed-rock revetment (CRR) was additionally paved with a thickness of 1.5 m and 1.0 m on some sunny and shady shoulders of the traditional embankments along the Qinghai–Tibet railway, respectively. The improving effects for thermal and deforming stability are evaluated based on observation data of ground temperatures and embankment deformations at two sites from 2002 to 2014. The results show that a larger uplifting magnitude in the artificial permafrost table (APT), greater ground temperature decreasing amplitudes and reduction ranges of settling rate appear under the shady embankment shoulder in warm permafrost region, and both sides in the cold permafrost region when reinforcing with CRR. However, in warm permafrost region, the laying of CRR on the sunny slope of subgrade may cause considerable thermal disturbance to the underlying permafrost foundation, combined with the resulting additional stress, induce the further expansion of differential settlement. Moreover, the thermal stability strengthening effect of the CRR is closely related to the variation of the APT thickness in the earlier stage, convection intensity inside the CRR, ‘cold energy reserve’ in the deeper permafrost, and amount of solar radiation received by the CRR. More effective reinforcements should be implemented to alleviate the potential threaten beneath sunny embankment slope in warm permafrost regions.

Using Conditional Random Fields for a Spatially Variable Liquefiable Foundation Layer in Nonlinear Dynamic Analyses of Embankments

Two-dimensional nonlinear dynamic analyses (NDAs) are performed for a series of hypothetical embankment dams on a spatially variable liquefiable foundation layer to evaluate the utility of representing the foundation layer with random fields conditioned on different levels of site characterization information. A set of two-dimensional parent models (PMs), each representing a true foundation condition, were generated using unconditional random fields of equivalent clean sand, corrected standard penetration test (N1)60cs values. Different levels of site characterization were then represented by combining different numbers of local borings (i.e., columns of data from the PM) with the optional inclusion of constraints on the geostatistical properties that might come from sitewide explorations. NDAs were performed using the same input motions for the PM (which represents perfect knowledge of soil conditions), a set of realizations conditioned on the local borings alone, and a set of realizations conditioned on the local borings with sitewide statistics. Embankment deformations obtained for the conditional realizations are compared to those for the PM to evaluate the potential benefits of increasing levels of site characterization in terms of deformation prediction accuracy. Parametric analyses include varying the embankment size, scales of fluctuation in the foundation stratum, number of conditioning borings, and ground motions. The results of these comparisons illustrate that the beneficial effects of using conditional random fields were generally limited to cases with the horizontal scale of fluctuation approaching the scale of the embankment base width and to cases with a large number of borings (more than three borings per horizontal scale of fluctuation), which may not be practical in many situations. Additional potential benefits and limitations of using conditional random fields for representing spatial variable liquefiable foundation layers in embankment dam NDAs are discussed.

Evaluation of induced trench twin culverts constructed in deep fill

When culverts are installed beneath high embankments, earth loads become excessive and the induced trench construction method is a viable design option to reduce the culvert loads to acceptable levels. However, limited field studies evaluating the performance of induced trench twin culverts are reported in the literature and the practicality and effectiveness of the induced trench construction method (in general) has been subject to recent criticism. This paper describes the performance of twin 3048 mm inside-diameter reinforced concrete culverts constructed with an induced trench beneath 15.3 m of fill. Research instruments and autonomous data acquisition systems were installed during construction to monitor (i) culvert earth pressures, (ii) embankment deformations, and (iii) groundwater elevations in the vicinity of the compressible fill. The experimental observations recorded throughout the construction phase are presented herein; the embankment deformations are indicative of effective positive arching within the induced trench region, and the average earth pressure at the culvert crown was reduced to approximately 48% of the overburden soil pressure. The experimental data are compared with those reported in the literature by others, and the conclusions attained from this study demonstrate the effectiveness of the induced trench construction method.

Simplified modelling approaches for DCM column-supported embankments

Deep cement mixed (DCM) column-supported embankments are widely used to support embankments over soft ground. The main challenge faced by engineers when designing these embankments is to predict the deformations realistically. Although three-dimensional models are realistic, it is not efficient to carry out a large number of three-dimensional finite element analyses at the design stage to investigate the efficiency of different embankment configurations. Therefore, in practice, engineers opt for simplified modelling approaches to predict the embankment deformations. The research presented in this paper examines four different simplified models commonly used by design engineers in practice: (i) two-dimensional plane-strain with equivalent areas (EA), (ii) two-dimensional plane-strain with equivalent properties, (iii) axisymmetric unit cell and (iv) three-dimensional unit cell. Results confirm that out of the four simplified models considered, only the plane-strain EA model has the ability to capture the deformations realistically when there is a likelihood of column failure.

Crack formation of a highway embankment installed with two-phase closed thermosyphons in permafrost regions: Field experiment and geothermal modelling

Two-phase closed thermosyphon (TPCT) is proved to be an effective countermeasure to thaw-induced problems for engineering constructions in permafrost regions. However, cracks often occur on the road surfaces of the embankments installed with TPCTs, which are supposed to be closely associated with the unstable thermal states of the underlying soil layers. The crack is a potential danger to the embankment stability, and thus a better understanding of the crack formation is urgently needed. In this paper, in order to address the problem, 5-year ground temperatures and embankment deformations at different soil layers were monitored at a road section with TPCTs and its neighboring road section without TPCTs. The ground temperatures were mainly employed to verify an air-TPCT-soil coupled model, and then the model was used to reproduce the spatial-temporal geothermal profiles of the two road sections. In combination with the geothermal profiles and the monitored deformations, the causes and the occurrence time of the cracks are discussed. Corresponding suggestions are proposed to allay the problem.

Nonlinear deformation analyses of an embankment dam on a spatially variable liquefiable deposit

Nonlinear deformation analyses of an embankment dam on a spatially variable (stochastic), liquefiable alluvial deposit are presented and compared to results of analyses using uniform properties for the same deposit. The results are used to examine the basis for selecting representative properties (as a percentile of the stochastic distributions), which when used in uniform models, produces embankment deformations consistent with the median response of the stochastic models. Analyses are performed for four different sets of realizations for the stochastic alluvial deposit and a range of shaking intensities. The results suggest that the use of 33rd percentile properties for representing stochastic deposits in uniform models is reasonable to slightly conservative for cases where the length scale of the deformation mechanism is significantly greater than the scale of fluctuation in the deposit.

Cooling performance of two-phase closed thermosyphons installed at a highway embankment in permafrost regions

Two-phase closed thermosyphon (TPCT) is a popular way to prevent permafrost layers from degrading, and consequently ensure the stabilities of engineering constructions in permafrost regions. Although TPCTs have been numerically and experimentally investigated for many years, long-term field monitored data concerning the cooling performance of TPCTs are limited. This paper presents the ground temperatures, embankment deformations and some related meteorological factors for a road section of the Qinghai–Tibet Highway before and after installing the TPCTs in permafrost regions. Based on the monitored data, three main aspects are analyzed: 1) cooling scope and period of the TPCTs; 2) cooling effects for the soil layers, especially for the permafrost layers; and 3) remedying effects with respect to embankment deformation and crack formation. Some corresponding suggestions are proposed for a better design of TPCTs in the construction of roadways in permafrost regions.

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.

Measured Performance and Stability Analysis of Large-Scale Reinforced Model Embankments at Different Moisture Contents

This paper describes construction and instrumentation of two 1.65-m high reinforced embankment models that were tested at two different gravitational water contents (GWC) on the dry and wet sides of the optimum moisture content (OMC); i.e. OMC−2 % and OMC+2 %. The embankment models were constructed using a clayey sand that was reinforced with four layers of instrumented geotextile with a uniform vertical spacing of 300 mm. Each model embankment was subjected to a line surcharge load (strip footing) near its crest until failure. The magnitudes of GWC, earth pressure, geotextile strains, footing settlement and embankment deformations were measured using EC-5 sensors, earth pressure cells, wire potentiometers, linear variable deferential transformers and reference plates, respectively during the construction and loading phases of each test. The GWC values in each model embankment were measured using EC-5 sensors and the oven drying method to ensure that they were close to the target values. Slope stability analysis was carried out to study the equilibrium of embankment models and to validate the predicted bearing capacity and factor of safety results against the experimental data. Results of this study show that the embankment model constructed on the dry side of OMC (i.e. OMC−2 %) had a 10 % greater serviceability failure load than the model built at OMC+2 %. Meanwhile, stability analysis results indicate that the bearing capacity of an unreinforced model embankment at OMC−2 % would be 72 % lower than that of the corresponding reinforced model. It is also found that compaction-induced energy could increase the earth pressure within the embankment by up to 70 % of the vertical stress in the soil mass due to gravity.

Impact of permafrost degradation on embankment deformation of Qinghai-Tibet Highway in permafrost regions

Based on long-term monitoring data, the relationships between permafrost degradation and embankment deformation are analyzed along the Qinghai-Tibet Highway (QTH). Due to heat absorbing effect of asphalt pavement and climate warming, permafrost beneath asphalt pavement experienced significant warming and degradation. During the monitoring period, warming amplitude of the soil at depth of 5 m under asphalt ranged from 0.21 °C at the XD1 site to 0.5 °C at the KL1 site. And at depth of 10 m, the increase amplitude of ground temperature ranged from 0.47 °C at the NA1 site to 0.07 °C at the XD1 site. Along with ground temperature increase, permafrost table beneath asphalt pavement decline considerably. Amplitude of permafrost table decline varied from 0.53 m at the KL1 site to 3.51 m at the NA1 site, with mean amplitude of 1.65 m for 8 monitoring sites during the monitoring period. Due to permafrost warming and degradation, the embankment deformation all performed as settlement at these sites. At present, those settlements still develop quickly and are expected to continue to increase in the future. The embankment deformations can be divided into homogeneous deformation and inhomogeneous deformation. Embankment longitudinal inhomogeneous deformation causes the wave deformations and has adverse effects on driving comfort and safety, while lateral inhomogeneous deformation causes longitudinal cracks and has an adverse effect on stability. Corresponding with permafrost degradation processes, embankment settlement can be divided into four stages. For QTH, embankment settlement is mainly comprised of thawing consolidation of ice-rich permafrost and creep of warming permafrost beneath permafrost table.

Liquefaction-induced deformation of earthen embankments on non-homogeneous soil deposits under sequential ground motions

Damage of embankments during earthquakes is widely attributed to the liquefaction of foundation soil. Previous studies have investigated the dynamic response of embankments by mainly considering uniform sand foundation and a single earthquake event. However, the foundation of an embankment consists of many sublayers of soil from liquefiable sand to relatively impermeable layer, and during earthquakes a mainshock may trigger numerous aftershocks within a short time which may have the potential to cause additional damage to soil structures. Accordingly, the investigation of liquefaction-induced deformation of earthen embankments on various liquefiable foundation conditions under mainshock–aftershock sequential ground motions is carried out by a series of dynamic centrifuge tests in this study. The liquefiable foundation includes uniform sand profile, continuous layered soil profile, and non-homogeneous soil profiles. Effects of various foundation conditions on embankment deformations are compared and analyzed. From the test results, it is found that the embankment resting on non-homogeneous soil deposits suffer more damage compared to the uniform sand foundation of same relative density. The test results also suggest that the sequential ground motions have a significant effect on the accumulated deformation of embankment.

Numerical Analysis of Widened Embankments with Geogrid Reinforcement

The mechanism of geogrid reinforcement in an embankment widening project was investigated in this study using numerical analysis. It was found that the geogrid reinforcement mainly affects the new embankment by decreasing settlement slightly and restraining the horizontal outward displacement effectively. This effect of geogrid reinforcement on embankment deformations is due to the stress redistribution in the new embankment and in the subsoil caused by geogrid reinforcement. The inclusion of geogrid reinforcement produces a small decrease in vertical stress in the new embankment and leads to a relatively larger increase in horizontal stress in the subsoil, thereby decreasing the shear stress of the subsoil. The effect of geogrid reinforcement on the new embankment and subsoil increases with the increasing geogrid layer number.

Remedying embankment thaw settlement in a warm permafrost region with thermosyphons and crushed rock revetment

Due to the special engineering geology characteristics of permafrost, construction in permafrost regions tends to result in serious permafrost-related engineering problems. Thaw settlement induced by permafrost degradation is the principal challenge for railway construction on the Qinghai-Tibetan Plateau. It threatens the stability and safety of the railway system, especially in warm and ice-rich permafrost regions. Thaw settlement in section DK1139+780 along the Qinghai-Tibetan railway is a potential risk to the safety of the railway, and a combination of closed thermosyphons and crushed rock revetment was used to remedy permafrost warming and thaw settlement of the embankment. Based on ground temperatures and embankment deformations observed at this site since 2002, the effects of the remedial measures were evaluated. The results show that the remedial measures lowered the ground temperature and raised the permafrost table. The crushed rock slope protection acted as an insulation layer and reduced heat flux into the embankment. The thermosyphons lowered the permafrost temperature and had a good cooling effect on the underlying permafrost. The results show that the remedial measures using two-phase thermosyphons and crushed rock revetment decreased the settlement of the embankment and improved the stability of the railway system.

Choice of allowable long-term strains for reinforced embankments on a rate-sensitive foundation

ABSTRACT: The time-dependent behavior of reinforced embankments constructed over rate-sensitive soils is examined for different maximum allowable long-term reinforcement strains using finite element analysis. The long-term service height, lateral toe spreading, maximum differential settlement and maximum settlement of reinforced embankments are examined. The viscoelastic nature of geosynthetic reinforcement is considered. Various factors affecting the performance of reinforced embankments on rate-sensitive soil are investigated: soil viscosity, reinforcement type and construction rate. Based on a series of finite element analyses, the ideal allowable long-term reinforcement strains that control the embankment deformations to modest levels while maximizing the long-term service height of reinforced embankments, considering the combined effect of soil and reinforcement viscosity, are suggested.

Characteristics and mechanisms of embankment deformation along the Qinghai–Tibet Railway in permafrost regions

Abstract Based on field monitoring datasets, characteristics of embankment deformation were summarized along the Qinghai–Tibet Railway in four permafrost regions with different mean annual ground temperatures (MAGTs). Then, further analyses were carried out at some typical monitoring profiles to discuss mechanisms of these embankment deformations with consideration of detailed information of thermal and subsurface conditions. The results indicated that in regions with MAGT

Earthquake retrofit of highway/railway embankments by sheet-pile walls

A series of highly instrumented dynamic centrifuge model tests were performed to investigate the potential of using sheet-pile walls for mitigating the adverse effects of foundation liquefaction on overlying highway embankments. The response of a prototype 4.5 m high cohesive highway embankment supported on a 6 m thick loose saturated sand layer was analyzed under dynamic base excitation conditions. In a series of four separate model tests, this embankment-foundation system was studied first without, and then with the following three different liquefaction countermeasure techniques, all involving sheet-piles: (a) sheet-pile extending to the foundation surface, (b) sheet-pile with toe area gravel surcharge berm, and (c) sheet-pile with toe area gravel surcharge berm extending into the foundation. Model response was monitored by numerous accelerometers, pore pressure transducers, and displacement gages. The underlying mechanism and effectiveness of each countermeasure is discussed based on the recorded dynamic response. All of the implemented countermeasures were found to significantly reduce embankment deformations. Particularly in the case of the sheet-pile with toe area gravel berm, cracking and lateral spreading of the embankment were practically eliminated.

Foundation Liquefaction Countermeasures for Earth Embankments

A centrifuge-testing program is conducted to assess the earthquake performance of countermeasure retrofit techniques for a liquefiable foundation under an existing embankment. Currently, such testing results offer a valuable alternative to actual full-scale dynamic response, which is virtually nonexistent for retrofitted embankments. The response of a cohesive highway embankment supported on a loose saturated sand layer is analyzed under dynamic base excitation conditions. In a series of five separate model tests, this embankment-foundation system is studied first without, and then with the following four different liquefaction countermeasure techniques: densification, cement deep-soil-mixing, gravel berms, and sheet-pile enclosure. The underlying mechanism and effectiveness of each countermeasure are discussed based on the recorded dynamic response. Effects of each countermeasure on both foundation excess pore pressures and embankment deformations are analyzed and compared. The implemented countermeasures are found to reduce embankment vertical deformations by a maximum of about 50%. However, in some cases cracking and lateral spreading of the embankment are practically eliminated. Thus, retrofit techniques that constrain lateral flow of the underlying liquefied soil are deemed to be particularly effective in preserving overall embankment integrity.

Analyses of Dam Failures in 1985 Chilean Earthquake

During the Central Chile earthquake of 1985, two small dams suffered substantial embankment deformations indicating liquefaction and major strength loss in the embankment or foundation soils. However, the configuration of the embankments after failure suggested that the soil had retained a small but significant residual strength after initial liquefaction (ru≈100%). Because of the scarcity of field values of residual strength after initial liquefaction, a study of these dams was undertaken. Standard penetration test (SPT) values were determined and normalized for effective confining stress and hammer energy. Silty sand layers of low density were identified near the base of both embankments. Subsequent analysis of the results of the field study gave upper and lower bounds of residual strength in these loose layers. Calculated values of residual strength were found to vary with the inferred pre‐earthquake densities of the materials. These values were compared with those determined from previous case studies and shown to be in reasonable agreement.

El Infiernillo and La Villita Dams: Seismic Behavior

El Infiernillo and La Villita rockfill dams in Southern Mexico underwent an earthquake of magnitude 7.6 (Ricter) on March 14, 1979, with epicenter about 68 mi(110 km) from each dam. As a consequence measurable deformations occurred in both dams, including settlement, slope distortion and cracking. These deformations and the earthquake accelerations on the rock foundations and on the embankments were recorded. The patterns of permanent deformation are analyzed and the relative contributions of compaction and shear distortion are estimated. In both dams the fraction of the observed crest settlement attributable to compaction was larger than that due to shear distortion, and a clear correlation was observed between cracking and settlement patterns. The observed settlements attributable to shear distortion were found to lie between those computed from sliding-block methods (lower estimate) and those from a method of accounting for overall embankment deformations (upper estimate).

Discussion of “Analysis of Embankment Stresses and Deformations”

Discussion of “Analysis of Embankment Stresses and Deformations”