软基路堤施工期稳定性分析与评价方法

从软土地基路堤施工的实际出发、提出孔隙压力法、有效应力路径法、有效固结应力验算法、侧向位移法等四种路堤施工期稳定性评价方法，以及实例综合分析评价，对于软基路堤既安全又经济的施工程序设计与实施具有重要的指导作用。     

Design of reinforced embankments on soft clay deposits considering the viscosity of both foundation and reinforcement

The combined effects of the viscoplastic nature of foundation soil and viscoelastic behaviour of geosynthetic reinforcement (polyester, polypropylene and polyethylene) are investigated. A new method for defining the critical stage, with respect to embankment stability, and the operational field strain rate for use in assessing the undrained shear strength of rate-sensitive foundation soils similar to those examined is proposed. The effect of construction rate on the reinforcement stiffness at the critical stage is examined. The study shows that the selection of a design stiffness using the data obtained from a creep test provided reasonable and conservative results. The effects of the undrained shear strength profile, reinforcement stiffness and soil viscosity on embankment performance under working stress conditions are explored and a new limit equilibrium based design procedure is proposed. Finite Element analyses are used to examine the potential effectiveness of the proposed simplified design procedure.     

A simplified method for prediction of embankment settlement in clays简

The prediction of embankment settlement is a critically important issue for the serviceability of subgrade projects, especially the post-construction settlement. A number of methods have been proposed to predict embankment settlement; however, all of these methods are based on a parameter, i.e. the initial time point. The difference of the initial time point determined by different designers can definitely induce errors in prediction of embankment settlement. This paper proposed a concept named “potential settlement” and a simplified method based on the in situ data. The key parameter “b” in the proposed method was verified using theoretical method and field data. Finally, an example was used to demonstrate the advantages of the proposed method by comparing with other methods and the observation data.     

Seismic behaviour of reinforced embankments in dynamic centrifuge model tests

A series of dynamic centrifuge model tests was conducted to investigate the effects of reinforcement on the seismic behaviour of hillside embankments consisting of sandy soils and resting on stiff base slopes. In total, three types of seismic reinforcements, namely, large-scale gabions, drainage-reinforcing piles, and ground anchors with pressure plates, were employed in the tests. The test results showed that: (1) the seismic performance of both lower and higher embankments was remarkably improved by installing large-scale gabions at the toe as they restrained the completion of the formation of sliding planes; (2) the installation of drainage-reinforcing piles at the embankment toe was rather effective in reducing the overall earthquake-induced deformation due to their high permeability and restraint effect against sliding displacement at the reinforced region; and (3) the embankments improved by ground anchors with pressure plates were not vulnerable to earthquake-induced damage due to their constraint effects even under high water table conditions. The improvement effects by the above-mentioned three types of reinforcements were presented by evaluating the global safety factors based on the results of a series of triaxial compression tests.     

Experimental study on the mechanical behavior of shored mechanically stabilized earth walls for widening existing reinforced embankments

Shored mechanically stabilized earth (SMSE) walls have been increasingly applied in embankment widening projects because of their good mechanical performance, simple construction, low cost, and low site requirements. In this paper, several large-scale model tests were conducted to explore the mechanical behavior of the composite structures with different connection forms and relative densities, and the wall deformation, earth pressure, reinforcement strain, potential failure surface and the effects of the connection forms behind SMSE walls were also analyzed. The results show that the deformation of SMSE walls is mainly concentrated on the upper middle part, showing a “bulging” failure trend. The deformation of the SMSE walls can be effectively controlled by improving the relative density and adopting a “sandwich” connection behind the walls. The horizontal earth pressure against the SMSE wall facing shows a “K”-shaped distribution, and the vertical earth pressure is large in the upper part and small in the lower part. The potential failure surface originated at the junction of the old and new retaining walls, forming a “double-line” failure surface. For a “sandwich” connection, the failure surface moves forward and occurs where the primary and secondary reinforcements overlap, and this connection form is recommended in engineering practice.     

An approach to shorten the construction period of high embankment on soft soil improved with PVD

Prefabricated Vertical Drains (PVDs) are being used to accelerate the consolidation of subsoil for construction of high embankment on soft ground. The construction is carried out in stages and height of the first stage construction depends on in-situ undrained shear strength. Each subsequent stage construction is carried out after completion of either 90% primary consolidation or percent consolidation at inflection point. The height of subsequent stages depends upon the gain in undrained strength of subsoil. In this paper, the authors have advocated an approach to shorten the construction period for high embankments. In this approach, the first stage construction would be carried out based on the in-situ undrained shear strength of subsoil. Instead of waiting for 90% primary consolidation or percent consolidation at inflection point, the embankment is raised in layers of 0.2 m thickness. Based on the time required to gain strength with the construction of the 0.2 m layer, the waiting period is determined for each subsequent layers. The waiting period depends on soil parameters such as subsoil thickness, C_r/C_v ratio and different PVD factors viz. smear, drain spacing and well resistance, pattern of laying of PVD, etc. Using this approach, there would be increase in the consolidation rate and overall reduction in the construction period. A typical practical example has been solved to demonstrate the usefulness of this approach over the two conventional methods. For a 4.5 m high embankment, it is observed that waiting period is reduced by 77% and 43% as compared to the 90% primary consolidation method and inflection point method respectively.     

Permafrost Studies in the Qinghai–Tibet Plateau for Road Construction

Advances have been made in engineering–geological studies of roadbeds in permafrost areas of the Qinghai–Tibet Plateau. The methodology used in engineering–geologic investigations has been greatly improved and provisions and regulations have been devised to guide engineering activities. Special attention has been paid to ground temperature and ice content because they are two of the most important and unique parameters for permafrost. Based on the data collected from recent studies, recommendations are made to enhance the stability of roadbeds. Under climate warming, a “roadbed cooling” approach is suggested for road constructions in “warm” permafrost. Advances have been also made in the prediction of changes in the permafrost engineering–geological environment, as induced by climate warming.     

Qinghai-Xizang Railroad Construction in Permafrost Regions

Construction of the Qinghai-Xizang Railroad (QXR) in permafrost regions presents a number of significant engineering problems. The engineering properties of permafrost can vary greatly, and climate warming, especially warm permafrost with high ice content, must be considered. Permafrost warming could induce ground ice thaw, producing embankment settlement. Consequently, thermal stability is a key consideration for the QXR construction in permafrost regions. In order to ensure permafrost thermal stability under the background of climate warming, ideas of embankment cooling to preventing permafrost change are proposed. Many methods of embankment cooling have been proposed to prevent the thawing of ground ice. For example, block-stone embankment, block and debris slope protected, thermosyphon, and special bridge designs. The amount of engineering practice and observational data testify that measurements of embankment cooling effectively decrease permafrost temperature and heighten the permafrost table beneath embankments.     

Wetting-Induced Compression of Compacted Oklahoma Soils

The wetting-induced compression of compacted Oklahoma soils was investigated. One-dimensional oedometer tests were conducted on 22 Oklahoma soils that encompassed relative compaction and moisture contents within typical embankment specifications. Results show that factors directly related to the fines composition can be used for preliminary estimation of collapse potential. Statistical analysis of the oedometer test data indicates that variables having the most impact on collapse index were compaction moisture content, dry unit weight, plasticity index, and clay-size fraction. Charts were developed to facilitate the estimation of collapse settlement of fills for different conditions, including fill height, moisture content, and soil type. Three case histories involving embankments that experienced significant settlement are presented for comparison. The comparison shows a reasonable agreement between predictions and field estimates of collapse settlement at the embankment centerlines; the limited evidence suggests that predictions based on one-dimensional assumptions may underestimate actual settlements possibly due to the two-dimensional nature of embankments.     

In Situ Pore Pressure Responses of Native Peat and Soil under Train Load: A Case Study

Settlement and formation of piping holes on surfaces were observed along a rail embankment subject to normal traffic load. Piezometers were installed in the native peat and soil underneath the embankment inside and outside problematic area to measure the pore pressure responses during train traffic. Peculiar pore pressure responses were observed. Cyclic pore pressures were only measured during the first 60–80?s of the 6-min train passage, and thereafter the pressures decayed rapidly to the initial values. The pore pressure changes in the shallow peat layer were lower than those in the deep soil layer. Possible mechanisms causing such peculiar pore pressure responses, surface settlement, and piping holes were explored and identified. It was found that the stiffness contrast between the stiff, upper granular fill and the soft, native peat material could lead to a redistribution of tensile stress in the granular fill layer to the peat layer due to the moving train load. This stress redistribution promotes the propensity of vertical piping in the peat layer.