Tunnel stability and arching effects during tunneling in soft clayey soil

A series of centrifuge model tests and numerical simulations of these tests were carried out to investigate the surface settlement troughs, excess pore water pressure generation, tunnel stability and arching effects that develop during tunneling in soft clayey soil. The two methods were found to provide consistent results of the surface settlement troughs, excess pore water generation, and the overload factors at collapse for both single and parallel tunneling. The arching ratio describes the evolution of the arching effects on the soil mass surrounding tunnels and can be derived from the numerical analysis. The boundaries of the arching zones for both single tunneling and parallel tunneling were determined. In addition, the boundaries of the positive and negative arching zones were also proposed.     

Structural health monitoring of innovative bridge decks

For more than a century, important civil engineering structures such as bridges, high-rise buildings, dams and marine platforms have contained iron or steel as the reinforcement for concrete or wood. The useful lives of such structures have often been severely limited by the corrosion of this ferrous component. Much thought has been given in recent years to constructing structures that are lighter, stronger and non-corrosive. These innovative structures are new and for these to be accepted by the engineering community monitoring is mandatory. ISIS Canada has been developing such structures and monitoring them. In this paper, innovative bridge decks that have been implemented are described.     

层状破裂顶板的变形分析和计算

受采动裂隙和节理切割的层状破裂顶板不能视为弹性连续梁，其力学结构应力岩块相互挤压而成的压力拱。本建立一种用于这类顶板变形计算的无张纯压拱力学模型，并据计算实例讨论这类顶板桡曲变形特性，分析了不产生屈曲，压溃，剪切滑移及岩梁形成稳定挤压拱的条件。     

Arch-Free flow in aerated silo discharge of cohesive powders

Arching can occur during silo discharge of cohesive powders. In general this happens when the outlet size is not wide enough. Flow aid devices, such as aeration pads, are commonly used in the industry to achieve proper flow of cohesive materials. However, no design criteria are presently available for such kind of devices and, in particular, for the intensity of aeration to be used to avoid arching. Aim of this paper is the evaluation of the limiting aeration condition to produce the collapse of established arches and the minimum aeration rate necessary for no arching discharge flow. Experimental tests are carried out in an aerated flat bottom silo. The measured quantities are the aeration rate at arch collapse and the arch size. Powder permeability is characterized by fluidization experiments. A simplified model is proposed to assess on the prevailing physical phenomena and predictively evaluate the minimum aeration rate to determine no arching discharge flow.     

A simple design approach to analyse the piled embankment including tensile reinforcement and subsoil contributions

There is not one generally accepted approach for the design of geogrid-reinforced pile-supported (GRPS) embankments. Relevant mechanisms include arching of the embankment material, but also the effect of geogrid reinforcement and potentially a contribution from the underlying subsoil. This paper presents a simple design approach to identify the contribution of all three mechanisms, in which the contribution of multi-layered geogrid reinforcement is also presented. To validate the theoretical predictions for the effect of geogrid reinforcement and the potential contribution of underlying subsoil, a series of three-dimensional finite element analyses are conducted. It is found that a point of ‘maximum arching’ is increased with the height of embankment. This study also presents that the reinforcement could reduce the ultimate stress on the subsoil. However, this requires significant sag of the reinforcement. It is found that the sag of reinforcement is very sensitive to the span of the reinforcement between piles, but relatively insensitive to the stiffness of the reinforcement. For a case with three layers of geogrid, the upper two grids carry relatively little tension compared to the bottom layer. This in turn leads to an approximate but simple equation of vertical equilibrium which may be of use in design.     

Cavern roof stability—mechanism of arching and stabilization by rockbolting

For stabilizing of rock cavern roof arches, tensioned rockbolts should be used with caution since they may have a negative influence on the stability of the arch. A proper design of rockbolts should therefore be based on a clear understanding of both the features of rockbolt reinforcement and the mechanism of the rock roof arch structure. For this purpose the mechanism of a rock cavern roof arch is studied and effects of different types of rockbolts are evaluated. Numerical modeling of rockbolt support of the Xiaolangdi powerhouse cavern as well as application of arching theory are carried out with the objective to study the effects of fully grouted rockbolts and tensioned cable anchors in reinforcing the cavern roof and walls, and on the forming of the roof arch.     

Numerical analysis of a geosynthetic-reinforced piled load transfer platform – Validation on centrifuge test

Soft soil improvement techniques using a network of rigid inclusions and geosynthetic reinforcement are investigated to improve our understanding of load transfer mechanisms towards piles. The physical modelling of the system consists in simulating fictional soft soil settlement through downward displacement of a perforated tray above a network of rigid piles placed in the centrifuge swinging basket. Tests are used to validate the results of the numerical study.Elasto-plastic and hypoplastic constitutive models have been used to predict the behaviour of the granular mattress, which simulates a Load Platform Transfer (LPT). A two-dimensional, axisymmetrical model has been adopted, which fulfils the validation on the experimental test and the time needed for calculation.The results of the parametric studies show that load transfer increases with mattress thickness and closer pile spacing. Geosynthetic deflection is reduced when load transfer is high.     

A DEM Analysis of Flow Characteristics of Noncohesive Particles in Hopper

Flow characteristics of material in hoppers, silos, and bins are critical issues for operational stability as well as structural integrity of these units. In this work, flow of noncohesive particles in hopper is studied using the discrete element method (DEM) where each particle is tracked for its position, velocity, and acceleration. Material properties tend to alter during hopper flow due to compaction, expansion, and segregation. These features are difficult to model with a continuum approach. In the first part, material flow patterns are correlated with hopper angle and hopper opening, the two main design parameters. The typical shift from mass flow to funnel flow depending on the hopper angle was successfully simulated. In the second part, the discharge rate of material was quantitatively analyzed as function of hopper design parameters. Beverloo model 1 was tested on these simulated flow rates and it was shown that the simulated flow rates follow the model for this specific granular system. However, the DEM analysis was also able to demonstrate the failure of the traditional Beverloo model in the restricted flow regime. Simulated flow rates also follow the empirical correlations with hopper angle as stated in literature. DEM simulations were validated with experimental data for both material flow pattern and discharge rates.     

Comparison and evaluation of analytical models for the design of geosynthetic-reinforced and pile-supported embankments

Geosynthetic-reinforced and pile-supported (GRPS) embankments are becoming more and more popular as this technique showed good performances in practice. Various design methods were introduced to analyze GRPS embankments. However, the applicability of these design methods was not always fully validated. This paper focuses on the review of projects containing field observations of GRPS embankments. The comparison results showed that the assumptions related to the subsoil support, geosynthetic, arching shape, and its evolution are not consistent in the analytical methods. Comparison results with twenty-five full-scale cases and six series of experiments emphasize that these available design methods produce significantly different results in predicting loads transfer mechanism. The analytical models predict arching for cohesionless fill better that for cohesive fill soils. Besides, the analytical methods which consider subsoil support such as the CUR226 and EBGEO methods give results that are in a better agreement with experimental data as compared to other methods which do not consider the subsoil support. The CUR226 (2016) analytical model seems to be able to give the best performance with measured data when compared to other design methods. Finally, the results pointed out that the limit equilibrium model is adequate and has good performance.     

Analysis of lateral earth pressure on a vertical circular shaft considering the 3D arching effect

The lateral earth pressure on a vertical circular shaft is investigated using both experiments and numerical analyses. The study focused on quantifying the magnitude and distribution of the lateral earth pressure, which was measured by considering the three-dimensional arching effect. A framework for determining distribution of the earth pressure based on centrifuge model tests and 2D FE analysis is introduced. The FE modelling techniques and the constitutive relationships of the soil are presented in detail. Parametric analyses showed that the arching effect on the lateral earth pressure is highly dependent on the diameter and height of the shaft, the internal friction angle and the cohesion value of the soil, the end-bearing conditions and the flexural modulus of the shaft. The study found that when the arching effect is considered, the lateral earth pressure on a vertical circular shaft is approximately 80% less than that calculated using Rankine’s theory. The study also found that the arching effect of the soil is more significant for flexible vertical shafts than for rigid vertical shafts embedded in weathered soil.