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Excavation monitoring

Due to the increasing development in the construction industry, the sensitivity of construction and maintenance of new structures needs more careful consideration. Because just as new structures are useful and instructive in human life, inaccuracies in their design, operation, and maintenance may cause irreparable damage. Therefore, in order to maintain accuracy, it is necessary to use special measurement methods and tools. Geomatics is one of the sciences that is used for accurate measurements. In this science, various methods are used for accurate measurements, and micro geodesy is one of the most important methods available. Microgeodesy (small-scale geodesy) means determining the location of a small structure or area so that the effect of the earth’s sphericity is not discussed.

The position and shape of each structure change over time such as deformation of bridges due to the passage of heavy machinery, deformation of dams due to water pressure, settlement in a vertical direction due to drillings and drainage of underground resources, as well as displacement of walls due to excavation, earth-moving and stabilization of the excavation (earth retention).

Therefore, in short, it can be said a set of measures that are used by instruments to measure the behavior and study the movements of a large structure such as dams, bridges, oil reservoirs, structures adjacent to excavation, etc. with great accuracy and using observations and geodetic calculations are called microgeodesy.

deep excavation monitoring - istasazeh Co
deep excavation monitoring – istasazeh Co

Use of  deep excavation monitoring results in execution

Prediction of displacement is one of the most important issues in the earth retention. This prediction can be done by various numerical software including Plaxis 2D and Plaxis 3D finite element software. In this software, the initial estimate of displacement at each stage of excavation is calculated using soil characteristics, excavation depth, etc. To ensure the accuracy of design assumptions such as adhesion coefficient, friction angle, specific gravity and soil modulus of soil elasticity, the excavation monitoring results at each stage of excavation are compared with the output results of the software. If the results of excavation monitoring and software calculations are consistent, the design assumptions are ensured and there is no need to strengthen the stabilization. If at any stage of excavation, it is determined that the results of excavation monitoring are greater than the result of software displacement, there is a need to review the soil characteristics and modify the design. Therefore, the continuation of the execution should be stopped and negotiations should be held with the stabilization designer and new conditions should be presented so that a suitable reinforcement plan can be prepared and notified to the contractor.

The importance of monitoring in earth retention projects
The importance of deep excavation monitoring in earth retention projects

Microgeodesy network

A high-precision local microgeodesic network designed and executed with the aim of calculating displacement has a high accuracy in the coordinates of network points. Design, execution and calculation of microgeodesy networks are important topics in geomatics.

Various projects and plans in this regard have been done or are being executed in the country, which can be referred to the following:

  • Studies to detect the movements of Milad Tower
  • Microgeodesy project of Bushehr power plant nuclear facilities
  • Control and behavior measurement project of Karun Dam 3
  • Mako Dam control and behavior project

 

Microgeodesy network
Microgeodesy network – excavation monitoring

In addition to controlling sensitive and civil structures, geodetic networks are also widely used in determining the deformation of natural phenomena of the earth’s crust, which can be attributed to such phenomena as:

  • Movements of tectonic plates
  • Tidal effects on the Earth’s crust
  • Shell loading
  • Faults
  • Volcanoes

Different methods of excavation monitoring

A) Visual monitoring:

Regular examination to identify any cracks, displacement and settlement on the ground, building walls, etc. is called visual monitoring. Therefore, the areas around the excavation, including the street, building, yard and adjacent walls, are periodically inspected and cracks and changes in the condition of the cracks are reported. Taking photos and videos of the studied areas for visual monitoring can be useful and show the trend of changes.

B) Monitoring using instruments:

In this method, relocation and displacement of tolls adjacent to the excavation are done with the help of equipment and instruments.

Strain gage, inclinometer, etc. can be mentioned among the instruments used in excavation monitoring. These instruments are installed on suitable points of the structure and continuously collect data. The results of this data show the amount of horizontal displacement (towards the excavation) and its vertical displacement (settlement).

Monitoring the excavation using instruments
Monitoring the excavation using instruments

C) Excavation Monitoring using geodetic method: In order to monitoring the excavation and calculate the displacement of the excavation walls, assuming the impossibility of creating permanent surveying stations on the ground, three methods have been proposed. In all methods, a number of target or tape points are installed on the wall of the excavation:

Use of surveying equipment to monitor displacements of the excavation walls
Use of surveying equipment to monitor displacements of the excavation walls

1) Use of reference stations: Selecting a number of reference stations that do not move and then determining the coordinates of ground stations and target points, and in the next stages, first fixed ground stations are identified and then the target points of the walls are harvested by them.

2) Use of fixed points: In this method, having a ground station is not mandatory. At first, all the target points of the excavation walls are harvested without justifying the camera, and in the next stages, the justification of the camera is done by the target points that have not moved, and the rest of the target points are harvested.

3) Use of inner constraints: In this method, there is no need for ground stations and all target points are harvested. In the next stages, the horizontal distance and the difference in height between the target points are done and considering the inner constraints which are assumed to be constant, and the coordinates of the azimuth center of gravity guarantee the target points relative to the center of gravity and the average distance to the center of gravity, identify the fixed target points of the excavation wall and then, with computational justification by the mentioned points, the new coordinates of the other points are calculated.

Different methods of excavation monitoring
Different methods of excavation monitoring

Related contents:

Federal Highway Administration (FHWA)
Geocisa , EXCAVATION WORKS MONITORING

The Constructor , Excavation Monitoring System -Types and Objectives for Safety in Construction

Deep Excavation , Geotechnical instrumentation – monitoring

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