Fracture toughness of fibrous membranes

Random fibrous networks exist in both natural biological and engineering materials. While the nonlinear deformation of fibrous networks has been extensively studied, the understanding of their fracture behaviour is still incomplete. To study the fracture toughness of fibrous materials, the near-tip region is crucial because failure mechanisms such as fibril rupture occur in this region. The consideration of this region in fracture studies is, however, a difficult task because it involves microscopic mechanical responses at a small length scale. This paper extends our previous finite element analysis by incorporating the microscopic responses into a macroscopic domain by using a submodeling technique. The detailed study of microstructures at crack tips show a stochastic toughness of membranes due to the random nature of fibrous networks. Further, the sizes of crack tip region, which are sufficient to provide a reasonable prediction of fracture behaviour in a specific type of fibrous network, were presented. Future work includes improving the current linear assumption in the macroscopic models to become nonlinear.

The efficiency of compensation grouting in sands

Physical model experiments on compensation grouting in sands were performed in two different setups (Cambridge and Delft). The effect of water-cement (w/c) ratio, bentonite content (b.c.) and injection rate on compensation efficiency was investigated. Results show a considerable drop in compensation efficiency resulted from reducing the soil density. Injection in dense sand (R.D. = 93%) resulted in efficiencies between 40-90%, whereas injection in medium-dense sand (R.D. = 60-75%) yielded in reduced efficiencies between 10-40%. When the w/c ratio increased from 0.5 to 1.5 for a given density (R.D. = 93%) and the b.c. of 4%, the compensation efficiency value decreased. Typical efficiencies were between 60% and 40-50% for w/c ratios of 0.5 and 1.5, respectively. The values of compensation and grout efficiencies were almost equal, suggesting that pressure filtration happens mainly during injection. Increasing the b.c. improved the compensation efficiency. When a higher b.c. of 12% to 14% was used, typical compensation efficiencies in dense sand were 78 and 90% for w/c ratios of 1.5 and 1.8 respectively. © 2012 Taylor & Francis Group.

Increasing allowable deformation criteria through application of level II LTSM approach

In the central part of the Delft railway tunnel project, an underground railway station is being built at very close distance to the existing station building, which is still in operation. Although elaborate sensitivity analyses were made, some unforeseen deformations were encountered during the first phases of the execution process. Especially the installation of temporary sheet pile walls as well as the installation of a huge amount of grout anchor piles resulted in deformations exceeding the predicted final deformations as well as the boundary values defined by a level I limiting tensile strain method (LTSM) approach. In order to ensure the execution process, supplementary analyses were made to predict future deformations, and this for multiple cross sections. These deformations were implemented into a finite element model of the masonry of the building in order to define probable crack formation. This Level II LTSM approach made it possible to increase the initially foreseen deformation criteria and the continuation of the works. Design steps, design models and monitoring results will be explained within this paper.

Coupled analysis of building damage due to tunnelling

Excavation works in urban areas require a preliminary risk damage assessment. In historical cities, the prediction of building response to settlements is necessary to reduce the risk of damage of the architectural heritage. The current method used to predict the building damage due to ground deformations is the Limiting Tensile Strain Method (LTSM). This method is based on an uncoupled soil-structure analysis, in which the building is modelled as an elastic beam subject to imposed greenfield settlements and the induced tensile strains are compared with a limit value for the material. This approach neglects many factors which play an important rule in the response of the structure to tunneling induced settlements. In this paper, the possibility to apply a settlement risk assessment derived from the seismic vulnerability approach is considered. The parameters that influence the structural response to settlements can be defined through numerical coupled analyses which take into account the nonlinear behaviour of masonry and the soil-structure interaction.