TUNNELLING IMPACTS | TECHNICAL
Vertical, z α=90°
δT+ δz+ δN+ δy+
α+ α=180° α=0° Lateral, y
Left, figure 16:
Notation for a transverse strain analysis of a circular masonry sewer
α=270°
movements are calculated around the periphery of the brickwork at appropriate intervals. Figure 16 provides the nomenclature used in the
calculations of transverse strain of a circular masonry sewer. The vertical and horizontal movements derived from the methods described in Section 6.2 are resolved into movement in the radial (δN) and hoop (δT) (circumferential) directions: δN = δy cos α + δz sin α δT = -δy sin α + δz cos α
The sewer is divided into a number of segments at appropriate intervals anticlockwise around the circumference as shown. The axial strain in each segment is calculated from the change in length of each segment indicated by δT and flexure is derived by analysis of a triplet based on δN (see Figure 8). A similar assessment approach can be applied to
non-circular masonry sewers. The key to masonry sewer stability is to prevent
excessive tensile strains in the haunches and crown so as to prevent bricks falling out giving rise to arch instability. Depending on the sewer position relative to the proposed construction works, the sewer may be in either egging (i.e. increase in vertical and decrease in horizontal dimensions of the sewer) or squatting
mode (i.e. decrease in vertical and increase in horizontal dimensions of the sewer) and both have to be considered during the analyses of the transverse strains so as to ensure that the worst case is detected. Example charts showing the predicted distortion,
displacement and tensile strain profiles of a circular masonry sewer, which runs normal to the tunnel direction including assumed parameters, are shown in Figure 17. Note that the maximum transverse tensile strains are found to be in the crown/invert section and springings, with the sewer under ‘squatting’ and ‘egging’ modes respectively. The hoop compressive strain in the sewer due to the existing ground load is to be considered and this reduces the maximum transverse tensile strains currently shown on the charts.
6.9 Assessment guidance and criteria Early consultation between the developers and the relevant utilities (especially during the initial project planning stage) is highly recommended. This will allow the developers to get a better understanding of the acceptable procedures and criteria which can be applied for the impact assessment. This can reduce the risk of abortive work and the associated delays and unexpected costs to projects.
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Internal radius
External radius
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