DEFORMATION SURVEYS AND PRECISE LEVEL SURVEYS FOR SETTLEMENT

Deformation survey is the systematic measurement and tracking of the alteration in the shape or dimensions of an object as a result of the application of stress to it. The purpose of a deformation survey is to determine whether or not movement is taking place and subsequently whether the structure is stable and safe. The first method for deformation surveys is to use three-dimensional positions of specific points on an object. Many deformation projects are done with measurements taken at intervals of days, weeks or months. A number of data sets need to be collected for comparison. The differences between these sets of positions are then analyzed to determine if there is any movement. This data can be further analyzed to see if the movement is due to seasonal factors, daily variances etc. and then more importantly use the information to determine future movement of the structure.

Accuracies required for deformation surveys depend on many factors but generally accuracy to the millimeter or better may be required.

RDM Surveying Consultants uses the Trimble S6 Robotic Total Station to perform this type of deformation survey.

Precise level surveys for settlement are done by Differential Leveling. Differential leveling provides height difference measurements between a series of benchmarks. Vertical positions are determined to very high accuracy (±1 mm) over short distances (10-100's of meters) using precision levels. Two major classes of precision levels commonly used for making deformation measurements are automatic levels and digital levels. Standard deviation smaller than 0.1mm per set-up may be achieved in height difference determination when using an automatic level, equipped with parallel plate micrometer and with invar graduated rods.

RDM Surveying Consultants uses a variety of equipment to perform this type of survey. The following is a list of the equipment that we use.

Wild N-3 Precise Level

Topcon AT-G2 Automatic Level

Nikon AS-2 Automatic Level with Optical Plate Micrometer (10mm)

Wild GPLE2 10mm Invar Rod - 2 Meter

Wild 10mm

Deformation Parameters

A. General. The main purpose for monitoring and analysis of structural deformations is:

· To check whether the behavior of the investigated object and its environment follow the

predicted pattern so that any unpredicted deformations could be detected at an early stage.

· In the case of abnormal behavior, to describe as accurately as possible the actual deformation status that could be used for the determination of causative factors which trigger the deformation. Coordinate differencing and observation differencing are the two principal methods used to determine structural displacements from surveying data. Coordinate differencing methods are recommended for most applications that require long-term periodic monitoring. Observation differencing is mainly used for short-term monitoring projects or as a quick field check on the raw data as it is collected.

(1) Coordinate differencing. Monitoring point positions from two independent surveys are required to determine displacements by coordinate differencing

(2) Observation differencing. The method of observation differencing involves tracking changes in measurements between two time epochs. Measurements are compared to previous surveys to reveal any observed change in the position of monitoring points.

(1) Horizontal displacements. Two-dimensional (2D) displacements are measured in a critical direction, usually perpendicular to the

longitudinal axis preferably on a selected grid or by random selected                  Taking Floor Elevation At The Pine Street Elementary School In Ballston Spa

points determined by the consulting engineer or surveyor. Alignment

techniques for alignment-offset measurements are made in relation to a pair

 of control points having well-known coordinates that are not subject to being

compromised by on going site actives. Horizontal movement can also be

determined with respect to plumblines having a stable anchor point.

(2) Vertical displacements. Vertical displacements are measured in relation to stable project benchmarks, such as deeply anchored vertical borehole extensometers, or alternatively, to deep and/or reliable benchmarks located near the subject movement employing differential leveling. Hydrostatic leveling is also sometimes used to determine settlements. Settlement gauges are used to detect settlements of the foundation, or of interior structural parts which are not readily accessible (core, foundation contact).

Settlements of individual layers of embankments should be monitored through settlement gauges.

c. Relative displacements. These measurements are intended to determine small differential movements of points representative of the behavior of the object or structure, its foundation, and/or abutments with respect to other points on the structure, or even on the same structural element.

Frequency of Measurements

a. General. Geodetic monitoring surveys (for periodic inspections) are conducted at regular time intervals rather than by continuous measurements that are more typical of automated structural or geotechnical instrumentation. The time interval between deformation surveys will vary according to the purpose for monitoring, but is generally correlated to condition of the structure. Design factors such as the structure's age, hazard classification, safety regulations, and probability of failure determines an appropriate frequency for surveys, or the need for establishing more frequent survey campaigns.

b. Continuous monitoring. With automatic data acquisition, such as by DGPS or robotic total stations, the frequency of measurements does not impose any problem because the data can be decoded at a pre-programmed time interval without difficulty and at practically no difference in cost of the monitoring process. Continuous monitoring systems with geodetic measurements are not yet commonly used and the frequency of measurements of individual observables must be carefully designed to compromise between the actual need and the cost.

c. Age-based criteria. Guidelines for the frequency for conducting monitoring follow a time table based on the age of the structure.

d. Hazard based criteria. The frequency for conducting monitoring surveys are related to the hazard classification

 (1) Class I: HIGH RISK STRUCTURES. The high risk of Class I structures may warrant

continuous monitoring of the structure.

(a) Type A: Potential Failure Imminent. Gather data as prudent. Data is very valuable for later analysis of why the structure failed. Use any method available to gather data without risk of life or interference in processes ongoing to save the structure and/or alert the population at risk.

(b) Type B: Potential Failure Suspect. Monitor structure continuously. After potential solution to save structure is applied, use continuous monitoring until is determined that structure is stabilized.

(c) Type C: Dams or Reservoirs Undergoing Initial Impoundment. Gather initial data before

impoundment procedures start. Monitor continuously until failure is suspected or until impoundment procedures have halted. Continue monitoring continuously until it is determined that structure has stabilized and will maintain as planned under load.

(2) Class II: MEDIUM RISK STRUCTURES. Such structures are of a category of risk such that monitoring every year to every other year is prudent. Structures of this category are stable, but whose failure would affect a population area, result in a high dollar loss of downstream property, and cause a devastating interruption of the services provided by the structure.

(3) Class III: LOWER RISK STRUCTURES. Such structures are of a category of risk such that monitoring every other year is prudent. Structures of this category are stable, but whose failure would not affect a population area, not result in a high dollar loss of downstream property, not cause a devastating interruption of the services provided by the structure.