Download - Engineering Surveyor

Tunnelling: gradients, profiles & lengthControlling gradientsGradients may be controlled by: theodolite/Total Station set to required gradient profiles fixed to roof (moderately convenient for excavation by drilling and blasting or hand digging) laser aligned to gradient, established by conventional methods and convenient for all methods ofexcavation now commonly used.Checking levels on shallow gradientCheck gradient and reduced levels at regular intervals: establish underground TBM and level points in roof level from underground TBM using inverted staff.Note: TBM and level pins may have to be offset from centre-line to avoid ventilation ducts, etc.Checking levels on steep gradientStandard tunnel levelling procedure may be used but note that short sight lines tend to reduce precision and anyerrors will accumulate rapidly, eg if back-sight and fore-sight distances are not kept equal. Alternatively, check withtheodolite/Total Station: set vertical circle on instrument to gradient using inverted staff, check offset of known roof level compare offsets of pins further forward.Profile of cross-sectionThe site engineer may have to check selected cross-sections for measurement purposes or because groundmovement is suspected. A suitable technique is: align instrument on offset centre-line and to gradient (for convenience, directly below a proven level) measure vertical offset from centre-line set up tripod and target at selected cross-section align target with offset centre-line adjust target height by offset distance to establish centre-point of section check profile dimensions by taping or using a trammel.Note: A horizontal cross bar may be necessary if the cross-section is non-circular (see illustration).Where the tunnel is controlled by lasers, the profile may be checked by direct measurement of offsets from the laserbeam, hold the tape on the beam and move it back and forth to obtain the smallest reading on the tape.Controlling lengthControlling length may be critical if bends or other features must be located precisely. An invar band, rather than asteel tape, may be necessary together with catenary taping techniques, using firm supports with well-defined datum.When using any form of electronic distance measurement in a tunnel, the greatest care needs to be taken to avoidintroducing temperature and refraction errors (refraction will cause the light beam to bend and hence give a distanceover great).CIRIA C709115