Hisham Mohamad

Performance monitoring of a secant piled wall using distributed fiber optic strain sensing

An optical fiber strain-sensing technique, on the basis of Brillouin optical time domain reflectometry (BOTDR), was used to monitor the performance of a secant pile wall subjected to multiple props during construction of an adjacent basement in London. Details of the installation of sensors and data processing are described. Distributed strain profiles were obtained by deriving strain measurements from optical fibers installed on opposite sides of the pile to allow monitoring of both axial and lateral movements along the pile. Methods for analyzing the thermal strain and temperature compensation are also presented. Measurements obtained from the BOTDR were found to be in good agreement with inclinometer data from the adjacent piles. The relative merits of the two different techniques are discussed.

Monitoring Twin Tunnel Interaction Using Distributed Optical Fiber Strain Measurements

In this field trial, a new monitoring technique using distributed strain sensing known as Brillouin optical time-domain reflectometry (BOTDR) was introduced to monitor the behavior of bolted-concrete linings of a recently completed tunnel when a second bored tunnel was constructed side by side at a distance less than one tunnel diameter apart. This was done by measuring circumferential strains in 12 rings using optical fiber that was installed using the point-fixing method. The strain distributions around the circumference of the rings show a generally similar profile. Maximum compressive strains measured below the tunnel springline nearest to the excavated tunnel were larger than the maximum tensile strains measured at the tunnel crown, distorting the circular tunnel into an oval/ellipsoid that was about symmetrical to the horizontal axis. Several methods were introduced to compare strain measurements made by BOTDR and diameter changes recorded by tape extensometer. This involves the use of a symmetrical tunnel distortion model and the basic differential equation for a circular arch. The calculated results showed some degree of similarity between the two methods. The analysis highlighted the importance of measuring the axial strain in the tunnel ring and subtracting the axial strain component to calculate the actual deflection of the lining as a result of bending.

Behavior of Weak Soils Reinforced with End-Bearing Soil-Cement Columns Formed by the Deep Mixing Method

This article reports on a series of small-scale, plane strain, 1 g physical model tests designed to investigate the bearing capacity and failure mechanics of end-bearing soil-cement columns formed via Deep Mixing (DM). Pre-formed soil-cement columns, 24 mm in diameter and 200 mm in length, were installed in a soft clay bed using a replacement method; the columns represented improvement area ratios, ap, of 17%, 26%, and 35% beneath a rigid foundation of width 100 mm. Particle Image Velocimetry (PIV) was implemented in conjunction with close-range photogrammetry in order to track soil displacement during loading, from which the failure mechanisms were derived. Bearing capacity performance was verified using Ultimate Limit State numerical analysis, with the results comparing favorably to the analytical static and kinematic solutions proposed by previous researchers. A new equation for bearing capacity was derived from this numerical analysis based on the improvement area ratio and cohesion ratio of the soil column and ground model.

Assessment of Building Damage Induced by Three-Dimensional Ground Movements

AbstractDamage in buildings caused by excavation-induced ground displacement is a major concern in any underground work of an urban area. To predict the building damage, it is generally assumed the onset of visible cracking is associated with the limiting or critical tensile strain of the structure. The present analytical models of calculating tensile strains analyze individual walls of the building as elastic deep beams. Because of the two-dimensional nature of these methods, the walls are assumed to be perpendicular to the excavation axis. To obtain the maximum tensile strain, only in-plane movements are considered, whereas the effects of three-dimensional displacements such as twist are ignored. In this study, the building is represented as a simply supported rectangular elastic thick plate. The relationship between the state of tensile strain and the onset of visible cracking is applied to the plate theory. The building damage criteria based on critical displacement parameters, namely deflection ratio...

Fibre optic installation techniques for pile instrumentation

An innovative technique based on optical fibre sensing that allows continuous strain measurement has recently been introduced in structural health monitoring. Known as Brillouin Optical Time-Domain Reflectometry (BOTDR), this distributed optical fibre sensing technique allows measurement of strain along the full length (up to 10km) of a suitably installed optical fibre. Examples of recent implementations of BOTDR fibre optic sensing in piles are described in this paper. Two examples of distributed optical fibre sensing in piles are demonstrated using different installation techniques. In a load bearing pile, optical cables were attached along the reinforcing bars by equally spaced spot gluing to measure the axial response of pile to ground excavation induced heave and construction loading. Measurement of flexural behaviour of piles is demonstrated in the instrumentation of a secant piled wall where optical fibres were embedded in the concrete by simple endpoint clamping. Both methods have been verified via laboratory works.

Prediction of building damage induced by tunnelling through an optimized artificial neural network

Ground surface movement due to tunnelling in urban areas imposes strains to the adjacent buildings through distortion and rotation, and may consequently cause structural damage. The methods of building damage estimation are generally based on a two-stage procedure in which ground movement in the greenfield condition is estimated empirically, and then, a separate method based on structural mechanic principles is used to assess the damage. This paper predicts the building damage based on a model obtained from artificial neural network and a particle swarm optimization algorithm. To develop the model, the input and output parameters were collected from Line No. 2 of the Karaj Urban Railway Project in Iran. Accordingly, two case studies of damaged buildings were used to assess the ability of this model to predict the damage. Comparison with the measured data indicated that the model achieved the satisfactory results.

STRIP FOOTING SETTLEMENT ON SANDY SOIL DUE TO ECCENTRICTY LOAD

This study investigates the effect of eccentricity load on settlement of shallow foundation rested on sandy soil model. A series of small-scale 1g physical modeling tests was carried out by preparing a model of medium dry sand with 50% of relative density on a rigid testing chamber. A rigid plate was used to replicate a strip footing foundation. Three different locations of loading were created which located at the centre, 0.05 B and 0.1 B from the centre with respect to the width of the footing to investigate the eccentricity effect applied from the footing. Close range photogrammetry and particle image velocimetry (PIV) methods were used to examine the failure mechanisms under the eccentricity design load. In this study, the ultimate, allowable bearing capacity and bearing capacity factor (Nγ) under the eccentricity loading were investigated. It was found that with the increasing of footing eccentricity, the bearing capacity decreased with increasing settlement

Size Dependent Uniaxially Loaded Stability of Triaxial Weave Fabric Composites

This paper explores the size effects on the stability of simply supported triaxial weave fabric (TWF) composites subjected to a uniaxial compressive load. Three material expressions: homogenized, segmentation, and classical lamination theory are subscribed as the constitutive relation for the 2D composite finite element model used in the study. Remarkable dependency of critical buckling load on the material size is exhibited in all models, the least affected being those of homogenized and segmentation.