foundation system of burj al arab

Monitoring of excavation induced building settlement

Theories developed for excavation analysis in the past have many limitations which is treated by limiting excavation depths, adopting conservative design, with these precautions some construction failures were observed. Now with increase in demand, deeper excavation depths are required; again construction sites are located in densely populated urban areas and observed more problems to adjacent properties.

Through modern theories consider many variables related to excavation operations, but not enough to cope with change in site conditions during excavation operation. For this reason a monitoring system is important to monitor site condition during excavation and a well-arranged system can ensure safety and necessary corrections of excavation operation.

Monitoring ground settlement:

Settlement nails are very common to monitor ground settlement, settlement nails are steel nails driven into ground to take foresight of a fixed point followed by back-sight of the installed nails measuring from reference level to determine ground settlement of that point. The fixed point mentioned is datum located at safe distance from influence of excavation. A reference point of building supported on deep foundation (like pile foundation) can be taken as fixed point. If there have not deep-seated building near excavation site a marking object outside the range of settlement can be chosen; if required permanent benchmark is set to treat as fixed point.

Positioning of settlement nails: The settlement of ground to which we are concern may be of soil, concrete pavement or asphalt pavement; concrete pavement and asphalt pavement are of relatively high rigidity. Rigid pavement may not show any signs of underneath settlement, even though supporting soil gets settled. Thus a settlement nail penetrating only depth of pavement may not provide mark of settlement; in this case settlement nails should be driven to reach soil underneath as shown in figure below.

Where buildings exist near excavation site, settlement nails have to be set along direction normal to retaining wall and in central section of excavation zone to determined settlement profile that represent the actual situation. The behavior of soil near central area and retaining wall are similar to plain strain. So installing settlement nails in these regions will show large displacement (settlement for ground and lateral deformation for wall) than that of any other regions.

When a settlement profile of central area at initial level of excavation is recorded, it will be more convenient to determine the settlement of ground at the final excavation level and also in other sections. When settlement of buildings is concern, the measurement methods is the same as discussed above, except the settlement nails have to be on building elements like on wall or on columns etc.