Now showing 1 - 10 of 124
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Steps toward a probabilistic framework for tunnelling damage

2015-05-28, Moradabadi, Ehsan, Laefer, Debra F., Clarke, Julie

Globally, the high rates of urbanization over the past century have spurred unprecedented levels of tunnel construction. With each tunnel installation, there is a large affiliated risk for damage to aboveground structures, especially those of unreinforced masonry. Such damage (and the subsequent costs and litigation) occur, despite huge sums committed to construction monitoring and pre-tunnel mitigation. Arguably, damage still happens because the wide range of parameters and the extent of their variability are not sufficiently considered in the risk assessment process. To address these uncertainties, a probabilistic framework for the large-scale risk assessment of existing, unreinforced masonry buildings subjected to bored tunnelling is proposed by the Urban Modelling Group (UMG) at the University College Dublin (UCD). This paper summarizes the initial steps needed to achieve such a framework.

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Unexplained blasting vulnerabilities in a historic town

2008-07, Laefer, Debra F., Loughlin, B., Hickey, S., O'Farrell, S., O'Mahony, G.

Historic residential and commercial structures in an early twentieth century town in the southeastern United States have regularly and repeatedly been damaged by an adjacent quarry, despite the diligent keeping of blasting levels below peak particle acceleration levels previously thought to be safe. This paper explores the possibility of the damage mechanism being blasting-induced partial, repetitive liquefaction. The detonation of high explosives has been used in research to induce liquefaction at designated field sites and in laboratories, but the possibility of this occurring as an unintentional side-effect of commercial blasting has not been explored. This paper evaluates data from an actual community to assess the likelihood that the recorded damage is being caused by the quarrying in the southeastern United States. Although the results are not definitive they are strongly indicative that induced liquefaction is endangering this historic town.

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Fostering development of women faculty in geotechnical engineering

2007-02, Laefer, Debra F., Gassman, Sarah L., Luke, Barbara (Barbara A.)

A one-day workshop was convened to address obstacles to success of American women faculty in Geotechnical Engineering. The workshop, sponsored by the National Science Foundation (NSF), was held in October 2003, in conjunction with another workshop for the United States Universities Council on Geotechnical Education and Research(USUCGER). At that time, the 17 participants represented 40% of all tenure-track women faculty in Geotechnical Engineering at American universities. The workshop provided an opportunity to discuss academic career issues, including diversity, research, teaching and service. Concerns voiced by the participants spanned all of these topics. The leading need that emerged was for childcare during professional activities away from home. For most major concerns, specific action items resulted, and these are presented. The NSF, USUCGER, professional organizations such as the American Society of Civil Engineers, university governance, and the women faculty themselves were seen as potential catalysts for problem-solving. Some concerns such as service loads and recognition for diversity-based contributions still defied the envisioning of specific action items.

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Gender disparity in engineering as a function of physics enrollment and its implications for civil engineering

2009-07, Laefer, Debra F.

Despite tremendous advances by women in the natural and applied sciences, where in selective fields women have surpassed men in the earning of doctoral degrees for more nearly 20 years, female enrollment levels in engineering continue to be a fraction of male enrollment. Gender disparities of more than 60% persist in undergraduate engineering enrollments and have recently worsened. As American female Civil Engineering enrollment has been flat for over 25 years, efforts must be taken to understand this stasis. This paper focuses primarily on secondary education preparation in terms of both attitudes towards and enrollment levels in pre-engineering courses such as calculus, chemistry, and physics. Additional consideration is given to enrollment and achievement in advanced placement courses, as reflected in national examination rates. This paper concludes that secondary school participation and achievement in physics courses is a critical differential factor as one explanatory element of female engineering enrollment levels and provides specific recommendations as to how to increase interest, enrollment, and achievement in physics, including the segregation of entry-level engineering courses based on previous experience.

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Chemical Demolition of Unit Masonry: A Preparatory Study

2016-09-16, Natanzi, Atteyeh S., Laefer, Debra F., Mullane, Susan

Chemical demolition of concrete and rock has been conducted on a commercial basis since the 1970s, although not widely adopted. Furthermore, to date there have been no publications on the application of this class of products to unit masonry. To begin to fill this gap, this paper documents a laboratory study on the insertion of a soundless chemical demolition agent into a series of unit masonry wallettes. The paper introduces initial steps towards developing a procedure for the non-percussive demolition of such a wall and observations on the progressive nature of cracking in such arrangements. Although neither the assembled units nor the mortar in these tests were of historic material, the paper provides critical insight into the application of this technology into a building configuration in which it is not usually seen. The results show the promise this technology holds for highly selective material removal, without endangering the surrounding material for historic brick and stone buildings.

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Predicting Tunneling-Induced Ground Movement

2004-01-01, Laefer, Debra F., Kim, Wan

Cost-effective and permissible tunneling can occur only if ground movement prediction is refined to accommodate changes in both the urban environment and tunneling technology. As cities age, tunnels are being installed closer to existing structures and in increasingly complicated belowground conditions. The reality of stacked tunnels, abandoned facilities, and more extensive use of underground space raises the question of whether relationships derived for single open-shield tunnels in free-field conditions can adequately predict ground movement for modern tunneling techniques with more complicated site conditions. Traditional empirical methods to predict maximum surface settlements and the percentage of lost ground for paired tunnels of the new Austrian tunneling method (NATM) in noncohesive soils are evaluated. Predictive data are compared with field measurements for grouted and nongrouted sections. Results showed that the estimated maximum settlement values of an NATM tunnel were highly similar to those of an open shield tunnel for both the grouted and ungrouted sections, although in some cases the Gaussian shape significantly underestimated the depth of the settlement trough in the outer 30% to 40%. Grouting substantially altered the amount of settlement. The average percentage of volume of lost ground with grouting was 1.6%, whereas the value was 5.2% where no grouting occurred. The empirical methods typically generated a fairly reasonable set of responses for an NATM tunnel.

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Predicted tunnel-induced settlement and damage to Findlaters church with respect to freefield and constructed side considerations

2010-02, Murphy, J., Gaynor, S., Laefer, Debra F.

Dublin, Ireland is scheduled to constructing its first metro in 2010, with a pair of tunnels connecting the city center to the airport. This study presents first-order predictions of the anticipated soil settlement and related building damage for a single structure on the route. The structure is a 19th century, stone church on glacial till situated almost directly above the shallowest portion of one of the tunnel crowns and immediately adjacent to a station box. To assess potential damage, a settlement trough is predicted based on another recent Dublin tunnel and the anticipated settlement is applied to the church. Damage predictions based on these freefield predictions are made and then revised as to the anticipated impact of the structure itself based on building stiffness, which significantly reduces the maximum differential settlement from 63mm to 36mm resulting in minimal predicted damage.

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Pioneering Real-time Computational Models for Building Damage Prediction During Adjacent Tunnel Excavation

2013-04-19, Clarke, Julie, Laefer, Debra F.

Numerical modelling is commonly employed prior to tunnel excavation to estimate surface settlements and to predict the response of adjacent structures. Unfortunately, geotechnical and building parameters are difficult to determine for the large geographical extent of a tunnelling project. As such, parametric values for modelling purposes are frequently assumed and are rarely revised to provide updated predictions as field data becomes available. Given advances in 'real time' data availability from subsurface- and surface-based monitoring systems, the question arises of how to better fully exploit this data for improved adjacent building protection. To achieve this, integration of numerical models into the monitoring process to provide updated 'real time' building response predictions is explored. This paper extends existing frameworks which utilize geotechnical field data to provide 'real time' predictions to also include building considerations.

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Flying Voxel Method with Delaunay Triangulation Criterion for Façade/Feature Detection for Computation

2012-11, Truong-Hong, Linh, Laefer, Debra F., Hinks, Tommy, et al.

A new algorithm is introduced to directly reconstruct geometric models of building façades from terrestrial laser scanning data without using either manual intervention or a third-party, computer-aided design (CAD) package. The algorithm detects building boundaries and features and converts the point cloud data into a solid model appropriate for computational modeling. The algorithm combines a voxel-based technique with a Delaunay triangulation–based criterion. In the first phase, the algorithm detects boundary points of the façade and its features from the raw data. Subsequently, the algorithm determines whether holes are actual openings or data deficits caused by occlusions and then fills unrealistic openings. The algorithm’s second phase creates a solid model using voxels in an octree representation. The algorithm was applied to the façades of three masonry buildings, successfully detected all openings, and correctly reconstructed the façade boundaries. Geometric validation of the models against measured drawings showed overall dimensions correct to 1.2%, most opening areas to 3%, and simulation results within 5% of those predicted by CAD-based models.

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A semi-random field finite element method to predict the maximum eccentric compressive load for masonry prisms

2015-02, Moradabadi, Ehsan, Laefer, Debra F., Clarke, Julie, Lourenço, Paulo B.

An accurate prediction of the compressive strength of masonry is essential both for the analysis of existing structures and the construction of new masonry buildings. Since experimental material testing of individual masonry components (e.g., masonry unit and mortar joints) often produces highly variable results, this paper presents a numerical modelling based approach to address the associated uncertainty for the prediction of the maximum compressive load of masonry prisms. The method considers a numerical model to be semi-random for a masonry prism by adopting a Latin Hypercube simulation method used in conjunction with a parametric finite element model of the individual masonry prism. The proposed method is applied to two types of masonry prisms (hollow blocks and solid clay bricks), for which experimental testing was conducted as part of the 9th International Masonry Conference held at Guimarães in July 2014. A Class A prediction (presented before the tests were conducted) was generated for the two masonry prisms according to the proposed methodology, and the results were compared to the final experimental testing results. The root mean square deviation of the method for prediction of eccentric compressive strength of both types of prisms differed by only 2.2 KN, thereby demonstrating the potential for this probabilistic approach.