PURPOSE OF SETTLEMENT MONITORING
Results of measuring the settlement of the construction are used to evaluate and verify the theory of foundation design. It also serves as the basis for making necessary measures to prevent possible incidents. Results of measuring settlement are also considered in combination with other geotechnical documents and soil mechanics testing documents.
The following kind of constructions must be measured and determined the settlement:
- High-rise construcitons are liable to settlement;
- Constructions are sensitive with irregular settlement;
- Constructions locate on soft ground;
- Other kind of constructions which are required to measure and determine settlement.
The settlement of the construction’s foundation should be systematically measured and promptly informed by cycle, in order to receive specific parameters of settlement and stability of the foundation and to check the estimated data of the settlement of the construction for types of foundation.
The measurement of the settlement should be carried out regularly until stability (the settlement rate of construction is from 1 mm to 2 mm in a year). The measurement of the settlement can also be stopped if in the process of measuring, the cyclic settlement value of the measuring points fluctuates within the permitted accuracy. In case if the construction shows signs of sudden changes (subsidence, cracking, tilting, sliding), it is necessary to measure in time to determine the settlement parameters, to find out the cause and extent of danger for the construction. On that basis, making recommendations and necessary solutions.
METHOD OF SETTLEMENT MONITORING
Reference benchmark is altitude control mark, is the basis for determining the settlement of the construction. Benchmarks must be satisfied requirements below:
- Maintaining a stable altitude during monitoring period.
- Allowing to reliably check the stability of other marks.
- Allowing to lead conveniently altitude to settlement marks.
Positions of reference benchmarks should be placed in a good and stable soil layer (sand, gravel or hard clay with a large thickness), far away from the source causing vibration greater than the depth of benchmark (for deep benchmark). The distance from benchmarks to construction (civil and industrial construction) is usually from 50m to 100m.
Settlement mark is directly attached to the specific position of the bearing structures on the foundation or body of the construciton, used to observe the settlement of the construction. Settlement marks are divided into the following categories:
- Marks are attached on walls and columns;
- Marks for foundation;
- Deep marks are used to measure settlement by soil layer.
Before monitoring, benchmarks must be measured to test and to evaluate for stability and accuracy.
The method is determining the altitude of settlement mark according to the altitude of benchmarks by the short-sighted geometry method.
The monitoring of settlement carried out in cycles, the settlement value of each mark in each measurement cycle is determined based on the elevation difference between the two measurements (two cycles).
In the process of measuring benchmarks and settlement marks, it is necessary to comply with the process and limits in TCVN 9360: 2012 ” Technical process of settlement monitoring of civil and industrial building by geometrical levelling”.
Settlement monitoring process is carried out in each cycle, each measurement cycle is conducted in 2 steps:
- Step 1: Measuring standard grid. Standard grid is a grid that connects benchmarks together. The purpose of measuring the standard grid is to check the stability of benchmarks (the stability of benchmarks after evaluation must meet the conditions specified in Annex I in TCVN 9360: 2012 “Technical process of settlement monitoring of civil and industrial building by geometrical levelling”).
- Step 2: Leading the altitude from benchmarks to settlement marks (including ground settlement and adjacent building settlement): The purpose of it is to determine the actual altitude of settlement marks in the current cycle.
- TCVN 9360:2012 “Technical process of settlement monitoring of civil and industrial building by geometrical levelling”