Optical Surveys for Geotechnical Engineering Projects
Optical surveys in geotechnical monitoring are used to monitor ground, guide wall, and slurry wall movements. Surface and building settlements are most commonly monitored by optical surveys, while wall deflections calculated from surveys are generally used as a back-up reference. The measured movements are, in most cases, referenced to preconstruction surveys that determine the initial conditions of adjacent buildings. Unfortunately, preconstruction survey data was not archived for most projects in this study.
Building reference points typically comprise steel bolts socketed into an existing building or surface (Fig. 3.1). Surface reference points are steel nails or markers inserted into the desired surface. Slurry wall offset surveys can be made using an arrangement like the one shown in Figure 3.2. Occasionally, wall movements are measured at the top of wall reinforcement (Case study B-6: One Memorial Drive).
Measurements are made with the aid of an optical or laser gun and an optical reflector mounted on a steel rod. Optical surveys can give very accurate results if they are performed carefully. However, the precision can be affected by inadequate setting up of the tripod construction details and by the care that is taken during measurements. Also, measurements should be taken against steady reference points that are not influenced by the excavation.
The problem with optical surveys is that reference points can easily be destroyed or influenced by construction equipment that passes nearby. For example, a heavy crane may pass on top of a surface settlement marker and thus the resulting settlement will not be associated with the excavation. In addition, street markers can show a lot of variations since traffic conditions vary. For these reasons, the scatter in the offset data can be larger than the actual settlement when the magnitude of the settlements is small. On the other hand, advances in electronics, computers, and software can make measurements, data reduction, and presentation more efficient and economical.
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