Download presentation
Presentation is loading. Please wait.
Published byBryan Noel O’Neal’ Modified over 7 years ago
1
Chapter 9: Deformation Monitoring and Analysis: Geodetic Techniques
Dr. John Ogundare BCIT, Burnaby
2
Chapter Objectives (1/2)
Be able to do the following: Discuss the role of geodetic deformation monitoring and analysis. Discuss the characteristics of geodetic deformation monitoring techniques in contrast with other similar techniques. Discuss the important differences between absolute and relative geodetic networks and the importance of datum definition. Discuss the differences between deformation monitoring and control surveys. Use the design elements of deformation monitoring schemes to carry out deformation monitoring surveys. Describe the various monumentation and targeting requirements for deformation monitoring networks. Perform geodetic deformation monitoring surveys for hydro-electric dam structures and for subsidence areas. Reduce deformation monitoring data for input into least squares network adjustment software package for further processing.
3
Chapter Objectives (2/2)
Be able to do the following: Explain the importance of single-point movement in absolute geodetic deformation monitoring networks. Explain the concept of the iterative weighted similarity transformation (IWST) and use it to solve the problem of datum instability. Discuss the differences between the observation-difference and coordinate-difference approaches in deformation analysis. Perform statistical and graphical trend analyses of deformations. Discuss the new developments in the automation of geodetic deformation monitoring of slope walls in open pit mining. Discuss the geodetic techniques for deformation monitoring of tunnels during their construction. Discuss the use of geodetic levelling procedure in deducing tilt, strain and curvature resulting from ground subsidence.
4
Overview Deformation – changes in shape, dimension & position of object Vertical deformation of ground surface is ground subsidence Most common parameters of deformable object commonly monitored are deformation, strain, load, stress, ground water pressure, etc. Deformation is of main interest in surveying Deformation monitoring techniques: Produce absolute data Allow geotechnical instrumentation to be connected together Deformation analysis – detect, localize and model point movements based on deformation measurements
5
Role of Deformation Monitoring
Provide safety assurance against possible failure of monitored object- comparing measured deformations with known tolerances Gaining better understanding of the mechanism of rock deformation through experiment and research Verify behaviors of rock masses against their predicted patterns to refine prediction models or validate design assumptions Deriving information in order to resolve dispute on effects on mining impact on surface infrastructure and to help protect the structure Derive information in order to identify and separate various causes of deformation
6
Causes of Deformation Objects requiring monitoring: dams, tunnels, high-rise buildings, bridges, industrial complexes, slopes, glaciers, and areas of landslide, subsidence, recent crustal motion, etc. Possible causes of deformation – tidal effect, changing ground water level, mining activities, tectonic, landslide, alkaline aggregate reaction expansion of concrete, changeable water load on dam structures, seasonal thermal-induced deformations, etc.
7
Characteristics of Geodetic Monitoring Techniques
Characteristics of geodetic techniques compared with high-definition surveying and remote sensing and geotechnical instrumentation techniques They measure ground surface deformations using ground surface network of interconnected points They provide redundant measurements so that they are more reliable They provide overall picture of deformation trend of the whole object monitored with reference to stable points They are labor-intensive and not done frequently, except in automated mode Automation may be very expensive They require intervisibility between observing stations and are affected by the environment
8
Geodetic Monitoring Networks
Two classes of geodetic monitoring networks: Absolute geodetic network – stable points (reference network or reference datum) and unstable points being monitored (object points) Will allow both strain components and absolute movements of object points Relative geodetic network – all points are assumed unstable (object points); no stable points as reference Will allow detection of strain components
9
Advanced Geodetic Technologies
Some of the advanced geodetic technologies for deformation monitoring are robotic total stations, precise level, GNSS with or without pseudolites (Table 9.1) They are all affected by atmospheric refraction and/or tropospheric delay With regard to GNSS, better solution is obtained with 12 h of observations with error in the vertical being twice that of the horizontal Not all are suitable for fully automated and continuous monitoring
10
Deformation Monitoring and Control Surveys
Deformation Monitoring Surveys Control Surveys 1. Absolute positions of points are of interest Changes in positions between epochs are of interest 2. Common systematic errors (constant refraction, calibration error, scale error, configuration defects) are expected to cancel out if the same in all epochs Must be physically removed from measurements or randomized to minimize the effects 3. Absolute scale of network is not required but ability to detect and control change in scale between epochs Absolute scale of network is required 4. Configuration defects (eccentricities of instruments/targets, triangular misclosures are permitted Not permitted 5. Correlation between repeated observations of the same observable is encouraged: same environment and observation conditions, etc. Not encouraged; but randomizes effects of sources of errors
11
Deformation Monitoring Measurements: Error Sources
Geodetic measurements are contaminated with effects of: Observation random errors (due to reading, pointing, centering and leveling of instruments) Systematic errors due to instrument construction errors, atmospheric effects Seasonal (thermal) cyclic expansion of measured objects Other systematic errors due to lack of calibration of instruments
12
Deformation Monitoring Schemes
Monitoring Scheme – total plan of action, including: Choices of types and locations of observables, Timing of measurement campaign Stability of reference points Selecting monitoring techniques and suitable instrumentation Type of monumentation and targeting Determining data processing and analysis techniques Determining actual deformations Criteria for Design of monitoring scheme include Accuracy and reliability Temporal and spatial continuity dictating frequency of monitoring and adequacy of distribution of reference and object points Stability of reference points must be confirmed Cost-effectiveness Choice of monitoring technology
13
Specifications for Dam Monitoring Schemes
No universally accepted standards and specifications for choice of monitoring schemes Monitoring schemes are designed by dam owners or individuals Accuracy of monitoring both horizontal and vertical displacements in concrete dams is 1-2 mm up to 10 mm for horizontal displacements for embankment dams Initial design of monitoring schemes must be revisited time to time with regard to: Configuration (or geometry) of reference and object points – if network points are damaged or lost between epochs, new stations are needed to strengthen the network or new instruments are available Types of observables Timing of campaigns – at intervals needed to detect deformation Accuracy of measurements Economy – if schemes must be reduced to reduce cost
14
Typical Elements of Dam Monitoring Schemes
Typical deformation monitoring scheme of a dam consists of: Horizontal angle or horizontal direction measurements Distance measurements reduced to mark-to-mark and corrected for possible systematic errors Zenith angle measurements for reducing slope distances Orthometric height difference measurements from leveling Forced-centering monuments with only few tripod setups Choice of reference points in suitable locations Ability to confirm stability of well-protected reference points Good spread of object points with suitable target designs Settlement monitoring with leveling rune Huge refraction problems on the crest prevents use of optical alignment on the crest X, Y plane coordinates are based on local coordinate system Map projection coordinate system can also be used
15
Typical Observables in Dam Monitoring
Two-way distance measurements between two stations Measured 15 times each way with standard deviation of the distance computed from the measurements Measurements with discrepancy greater than twice the standard deviation is eliminated as outlier Outlier-free measurements are corrected for atmospheric effects and reduced to mark to mark values Computed standard deviations are only used to eliminate inconsistent measurements; the manufacturer-specified standard deviation of the equipment is used for each observation in least squares adjustment Angles or directions are measured in several sets (not less than 3 sets) with re-leveling of instrument between sets Mean of the sets is used as the angle at the point Current trend in dam monitoring is to integrate various geotechnical/structural and geodetic surveys techniques
16
Monumentation and Targeting
Types of monuments and targets depend on the level of accuracy of monitoring survey and location of monitored object Two monument design philosophies: Reference network points must be durable stable Object points located to be able to reveal local movements Monument design requires knowing durability and stability of rock and soil where reference points will be located – requires expertise of soil and geotechnical specialists before constructing the monuments Centering devices needed on monuments and targets –forced centering types (with typical centering error of ± 0.1 mm) Reference and object points can allow instrument setup, target setup or both Points are usually pillars installed into bedrock below the surface Typical reference control pillar for geodetic dam monitoring is shown in Fig. 9.1
17
Typical Reference Control Pillar
Fig. 9.1 Extensometer anchor point is for stability test of the pillar Fig. 9.2 Instrument pillar design At least 2 of the reference pillars must be stable to be safe
18
Checking Local Pillar Movements
Monitoring pillar with survey marker Fig. 9.3(a) Fig. 9.3(b) Extensometer anchor point on the control pillar and the survey markers on the monitoring pillars (Monitor 1 and Monitor 2) are measured with tape extensometer and the measurements used to perform stability test of control pillar Monitor 1 and Monitor 2 are to be flush with the ground in practice Zenith and horizontal angles and distances are measured from reference pillar to the two monitoring monuments Perform measurement every month for 1 year and determine relative movements by using least squares adjustment method Unstable reference point is isolated from the other reference points
19
Dam Slope and Crest Monuments and Targets
Survey requirement is to monitor the surface and near-surface movements (1.5 – 1.8 m depth) of downstream slope of dams Monument design must be able to accommodate standard targets and prism holders Accurate repeatability of centering in (X, Y, Z) coordinates must be ensured for targets (using specially designed circular spirit leveling device) Crest monument design is similar to that of slope monument (but monuments are flush with the ground surface) – Fig. 9.4 is a typical crest monument
20
Typical Dam Crest Monument
The monument is a brass disk embedded in concrete It can serve as reference and object points Suitable target plates are to be used for slope and crest monuments Concentric circle insert target can be used for the ball and socket centering device installed in the instrument pillars Or specially designed omnidirectional types
21
Monuments for Subsidence Monitoring
Some of the monuments used in mining subsidence monitoring are drilled to between 1.5 and 3.4 m depth depending on the nature of the soil (Fig. 9.5) Typical monuments are made of drilled-in 4“ pipes with survey markers welded to the inner surfaces of the pipes Fig. 9.5
22
Horizontal Monitoring and Analysis
Traditional geodetic techniques use Terrestrial positioning with total station, theodolites, EDM Space-borne GPS survey augmented with GLONASS Typical GPS survey of a mining area may require: Simultaneous use of up to 6 or more geodetic grade receivers/antennas in static relative positioning mode Data rate set at 10 s A high-precision GPS control point is fixed for the network adjustment Horizontal coordinates are commonly provided in map projection grid coordinate system, such as UTM Typical GPS setups are shown in Fig. 9.6
23
GPS Unit Setup for Mining Subsidence Monitoring
GPS survey procedure is a repetitive one: Tripod/tribrach and GPS antenna are centered on a monitoring point Slant antenna height from survey point to the antenna center is measured Fig. 9.6 If the visibility to GPS satellites is poor in the area, total station traverse survey subnet connecting to the main project network can be created using three-baseline surveys (Fig. 9.7)
24
GPS Three-Baseline Survey
This is an example of total station sub-network traverse controlled by GPS control points C1, C2 and C3: Dotted lines are measured GPS baselines TPD3 and TPD4 are local GPS points whose positions are determined in relation to GPS control points Perform open traverse to connect points D8 and D9 using RTS Forced-centering procedure followed Location of TP3 and TP4 are such that GPS control points, GPS satellites and unknown points D8 and D9 are inter-visible from TP3 and TP4 Series of distance, vertical, horizontal angles and meteorological values are measured from TP3 and TP4
25
Adjustment of GPS Three-Baseline and Traverse Surveys
Determine the adjusted coordinates of points D8 and D9 by combined least squares adjustment of the following: GPS determined coordinates of TPD3 and TPD4 with their covariance matrices and Total station measurements and their estimated standard deviations
26
Data Preprocessing (1/2)
Common types of measurements to be pre-processed before use in network adjustment: slope distances, horizontal angles, directions, zenith (or vertical) angles and height differences With regard to EDM distance observables Calibrate the EDM near the structure being monitored Measure about 15 measurements each way in two-way measurement of a distance observable Compute the mean and the standard deviation of the 15 distance measurements, prescreening each measurement using twice the standard deviation as tolerance limit between any two measurements Perform slope reduction using height differences based on differential leveling Correct reduced distances for atmospheric conditions and for the effects of elevation differences between stations, producing corrected mark-to-mark distances Reduce mark-to-mark distances to mapping plane if required Assign variance to the reduced distance using manufacturer specified accuracy
27
Data Preprocessing (2/2)
With regard to angle and direction observables Each angle or round of directions must be measured in at least 3 sets with re-leveling between sets and sampling circle readings at various positions of the horizontal circle Determine discrepancies between reduced directions and the average values In correcting for the effects of refractions, the effect on the vertical is at least one order of magnitude larger than the effect on the horizontal direction Angular observations must be made quickly since coefficient of refraction may change rapidly within a short time In trigonometric leveling or 3D triangulation networks, it may be necessary to reduce zenith angles to their mark-to-mark equivalent (or applying eye-to-object correction) due to differences in heights of instruments and targets To account for non-parallelism of local verticals at the observing stations, the zenith angles must be corrected appropriately
28
Data Processing Techniques (1/3)
Measurements must first be transformed to displacements between epochs at network points in two approaches: Two-epoch (or coordinate differencing) approach Observation differencing approach Concepts of least squares parametric model adjustment method are applied in each approach Two-epoch approach consists of least squares adjustment of single-epoch measurements in two separate epochs and their results are compared to determine deformations between the epochs In least squares adjustment, each epoch measurements are expressed using parametric model: (9.9)
29
Data Processing Techniques (2/3)
The following are needed to solve Eqn. (9.9): Geodetic datum (origin of coordinate system, orientation of axes and the scale of the coordinate system) must be defined Approximate coordinates (x0) of network points – determined from first epoch of measurements and used in subsequent epochs The adjusted coordinates can be determined: (9.13) (9.12) w is a vector of misclosures between approximate measurements determined from approximate coordinates and the corresponding actual measurements; P is a weight matrix of observations
30
Data Processing Techniques (3/3)
Cofactor matrix of adjusted parameters for a given epoch: Variance factor of unit weight is calculated as: (9.15) Note: solution of Eqn. (9.12) will not be possible without proper Definition of datum, solution is possible when there is no datum defect
31
Datum Definition Datum is defined as follows: Note:
For 2D geodetic networks: fix 2 coordinates of a point, one scale (or distance measurements) and one orientation (azimuth of a line) For 3D geodetic networks: fix 3 coordinates of a point, three rotations (one azimuth and angle measurements) and one scale (or distance measurements) Note: Distance measurements in a network provide scale for the datum Gyrotheodolite aziumth provides orientation Network adjustment that has a minimal amount of information just to define a datum is called minimal constraint or free network adjustment
32
Free Network Adjustment (1/2)
If the coordinates of network points are fixed in defining the datum, the network is externally constrained A minimal constraint adjustment in which the center of gravity of the network is fixed is known as inner constraint adjustment or free network adjustment Free network adjustment have the following constraints: No change in the coordinates of the centroid or center of gravity (fixed) after adjustment Average bearing from the centroid to each other point remains unchanged after adjustment (no differential rotation of network) Average distance from the centroid to each other point remains unchanged after adjustment (no scale change) Generally, initial coordinate values assigned to each of the network points at the start of least squares adjustment define the datum
33
Free Network Adjustment (2/2)
Two types of models are adjusted; Parametric model relating observations to unknown coordinates Equations relating all the constraints on the unknown coordinates (forming constraint model) – the number of constraint equations is equal to the number of datum defects Constraint model ensures that shape and size remains fixed after adjustment – internal geometry is fixed Constrain model can be given mathematically as GT = (9.21)
34
Precision Surveying /Chapter 9
G-Matrix Defined For 2-D network constraints: no fixed point, no azimuth and no distance: shape of network is fixed; G matrix can be given as (9.22) (9.24) Row 1: constrains translations in x Row 2: constrains translations in y Row 3: constrains orientation with respect to center of gravity Row 4: Constrains the scale (for distances) 24/10/2017 Precision Surveying /Chapter 9
35
Free Network Adjustment – Steps
Define parametric and constraint models: Linearized parametric model: (9.11) Constraints: (9.21) v is a vector of residual corrections to original observations Solution of free network adjustment: (9.27) (9.28)
36
Free Network Adjustment –Nuisance Parameter Elimination (1/2)
Consider coordinate corrections (1) and nuisance (2) with corresponding design matrices A1 for coordinates, A2 for nuisance parameters: Modified Solution of free network adjusted coordinates: (9.32) (9.29) (9.30) (9.31)
37
Free Network Adjustment –Nuisance Parameter Elimination (2/2)
Residual vector of the observations can be given as: (9.34) Covariance matrix of free network adjusted coordinates: (9.35) A posteriori variance factor of unit weight: (9.36) n is the number of observations, d is the number of parameters to fix to define the datum, u is the number of unknown parameters in the network adjustment
38
Statistical Analysis of Single-Epoch Measurements
Statistical analysis involves Assessment of observation quality to decide whether to include the observation or not in adjustment (through least squares blunder or outlier detection) Assignment of relative weights, ensuring that variance factors computed for pairs of epochs are compatible Outlier detection for each single-epoch adjustment is important Any undetected outlier in one epoch may be assessed as deformation in later analysis Outlier detection in least squares is based on global and local tests based on minimal constraint For outlier detection, the number of observations most be approximately twice the number of unknown coordinates
39
Global Test
40
Local Test n as the number of observations, df is the number of
n as the number of observations, df is the number of degrees of freedom , 0 = /n for in-context test, is the Significance level used in global test The associated observation is an outlier if the above condition is not satisfied
41
Deformation Estimation: Two-Epoch Measurements
Prerequisites for using the approach: Same geodetic datum (fixed points, network scale, orientation of network) used in the two epochs of measurements Appropriate standard deviations for observations are available for weighting the observations Same approximate coordinates for the common stations have been used for linearization purpose Advantages of the approach: No need of measuring the same observable in each epoch Each epoch of measurements can be statistically assessed for blunders Disadvantages: Problem of datum definition and stability of reference datum between epochs May not be able to handle contamination of observations due to systematic errors, which impact variance factor of unit weight
42
Deformation Estimation: Two-Epoch Analysis Steps (1/3)
(9.40) Subscripts 1 and 2 are for epochs 1 and 2 respectively
43
Deformation Estimation: Two-Epoch Analysis Steps (2/3)
(9.41) with referring to upper-tail area of F-distribution (9.42) (9.44) (9.43)
44
Deformation Estimation: Two-Epoch Analysis Steps (3/3)
45
Iterative Weighted Similarity Transformation (1/2)
Use Helmert (Similarity) Transformation matrix (S) will transform from one datum to another datum The transformation process will preserve network geometry by translating, rotating and scaling the given network from one datum (i) into other (j) using well-chosen transformation matrix Sj: (9.45) (9.46) with (9.47) 24/10/2017 Precision Surveying /Chapter 9
46
Iterative weighted Similarity Transformation (IWST) (2/2)
I = identity matrix with principal diagonal elements as one and off diagonal elements as zero Pj = special identity matrix – unity for coordinates defining datum with other elements as zero Pj is an identity matrix for inner constraint adjustment Using Eqns. (9.45) – (9.47) requires iteration until the results converge 24/10/2017 Precision Surveying /Chapter 9
47
IWST: Steps
48
Observation-Differencing Adjustment Approach
Corresponding observations are differenced and adjusted by least squares method Adjusted coordinate differences & their covariance matrices are determined Appropriate datum must exist for the adjustment Conditions satisfied before adjustment: Same approximate coordinates (same geometry) for the 2 epochs Same observables, observers, same conditions
49
Observation-Differencing: Advantages and Disadvantages
Advantages of using observation-differencing approach: Common systematic errors due to instrumentation, observer and atmosphere will be removed Strain analysis can be done if datum is unstable between epochs It can easily accommodate geotechnical data, such as tiltmeter and extensometer measurements Geometric defects (due to eccentric targets, etc.) are permitted Disadvantages: Need to measure the same observables, same instrumentation, observers, etc. which may be impossible practically Statistical analysis of each epoch for blunders is not possible
50
Geometrical Analysis of Deformation Measurements
Deformation is a consequence of dynamic process or dynamic system, which is composed of 3 elements: Factors causing deformation Physical properties of the monitored object Response of the object in form of deformation or other form Dynamic processes are completely described and explained by dynamic models They help to study the 3 elements of dynamic system and to make predictions The model is called deterministic if the factor causing the deformation and the physical properties of the monitored object are known and the deformations of the object are only to be predicted The model is called integrated model if the factor causing the deformation and the physical properties of the monitored object are known with deformation measurements available from geodetic monitoring
51
Dynamic Models Dynamic models can be broken into 3 types: kinematic, static and geometrical Kinematic models consider Monitored object points are moving continuously as function of time No acting forces or loads are involved in the modeling Deformation is described in form of velocity and acceleration of object points using time function and no regard for factors causing the deformation Static models consider No time is involved Monitored objects are not in continuous motion (at least not moving during monitoring time), but are at equilibrium under acting forces (loads) Deformation is a function of only acting forces (loads), not of time Physical and geometric structures of the object, material properties, etc. of the object must be known Response of the object in form of deformation or other form
52
Geometric Models Geometric models consider Geometric models
Object as a set of discrete points in space The object points move only within certain intervals in time and not continuously , but the object is considered as being in equilibrium under the acting forces (loads) Time is not explicitly considered Acting forces (loads) are not considered Geometric models Models monitoring network points or changes in geometry of the monitored object in space and time The models are used in geometrical deformation analysis Geometrical deformation analysis is about detecting, localizing, and modeling monitoring network point movements based on deformation monitoring
53
Geometrical Deformation Analysis (1/2)
Problem of geometrical deformation analysis with regard to absolute monitoring networks Confirm the stability of the reference points and to identify the possible single-point movement that may be due to local phenomena or wrong monumentation of survey markers If unstable reference points are not identified, the object points and other reference points (that are stable) will show movement even when they are truly stable. Geometrical deformation analysis is not easy with regard to relative monitoring networks There may be single-point movement as well as relative movements (due to strains in the materials of the object) of all the network points If there is a discontinuity (due to tectonic faults) in the object, there may be relative rigid translations and rotations of a block of the object with respect to other possible blocks Main problem will be how to identify the deformations caused by strains, relative rigid body translations, and single-point movements
54
Geometrical Deformation Analysis (2/2)
Analysis of relative monitoring network First establish whether any group of points in the network is stable between epochs using IWST If stable group of points exists, treat the network as absolute network with the stable points as datum otherwise, analyze the network using IWST Overall task of deformation analysis is to obtain a displacement function (deformation model) in space and time Once the displacement function is determined, all the basic deformation parameters such as strain components, rotations, and rigid body movements can be calculated at any desired point of the monitored object
55
Statistical Trend Analysis of Deformation (1/3)
After adjustments and IWST transformation, deformation must be detected using two-epoch analysis Using single-point statistical test Using trend analysis Statistical trend analysis of deformation steps: Perform minimal constraint least squares estimation of the coordinates of points and the covariance matrices Determine datum-dependent displacements from the estimated coordinates Perform IWST of the displacements to obtain datum-independent relative displacements and identify the stable reference points
56
Statistical Trend Analysis of Deformation (2/3)
(9.57)
57
Statistical Trend Analysis of Deformation (3/3)
In the case where the a priori variance factor is considered known, Eqn. (9.57) can be replaced by the equivalent Chi-squares value Any point with satisfying the following Chi-squares expression is flagged as unstable: Model the stable points as fixed reference block and determine the displacements of the object points The whole process in step 4 is statistical analysis of deformation trend or the localization of deformation Trend analysis is an intermediate link between deformation measurements and the deformation modeling
58
Graphical Trend Analysis of Deformation
Graphical trend analysis consists of: Plotting network points displacement vectors with their corresponding error ellipses The plot shows the spatial trend over time interval between the two epochs If a displacement vector extends outside the error ellipse, the movement is considered significant at the given significance level and the associated point is considered unstable The point displacement error ellipse is constructed from sub-matrix of displacement cofactor for the given point
59
Displacement Error Ellipse (1/2)
(9.59) The semi-major axis (a), semi-minor axis (b) and orientation () of the displacement error ellipse can be calculated as follows (9.60) (9.61) (9.62)
60
Displacement Error Ellipse (2/2)
(9.66) (9.67) (9.68)
61
Main Features of a Typical Hydroelectric Dam
Sources of deformation of hydro dams: Alkaline aggregate reaction of concrete Instability of bedrock Changeable water load on the dam Seasonal thermal- induced deformations Possible seismic events Fig. 9.8
62
Simulated Dam deformation
Points A, B, C constitute the reference network Point P on the crest of the dam is the object point to be monitored displacement of Angles and distances are measured External minimal constraints and IWST are applied Statistical analysis and graphical trend analyses detected the movement of object point P
63
External Minimally Constrained Displacements
Displacement at point P is outside the error ellipse (95% confidence) Point C is barely stable which is not supposed to be Fig. 9.10
64
Displacement Field After IWST
Displacement at point P is outside the error ellipse (95% confidence) Point C is stable as expected Fig. 9.11
65
Typical Trilateration Network on a Dam
Monitoring is based on precise differential leveling and horizontal control networks Reference and object Networks are well- chosen Fig. 9.12
66
GPS Surveys Points GPS unit installation as part of traditional
monitoring system of an hydroelectric dam Fig. 9.13
67
Deformation Monitoring of Slope Walls
Current trend in monitoring and deformation surveys of slope walls in open-pit mines includes Creating fully automated monitoring scheme based on RTSs and active GPS, and assortment of geotechnical instrumentations Integrating a number of monitoring techniques, including GPS, total stations, reflectorless EDM, and differential leveling In open-pit mines, slope walls can be a few hundred meters deep and 1 or 2 km long and wide Steep mine slopes are designed to reduce cost of mining The steepness of slopes increases the frequency of slope failures, requiring the need for monitoring the slopes Geotechnical instruments are commonly used since they are easily automated Conventional surveying procedures are now common
68
Advantages and Disadvantages of Automatic Monitoring System
Reduction of manpower More frequent data and Fewer errors Disadvantages: Large volume of data to be managed Initial cost; wrong use of collected data; needs for special personnel
69
Automatic Monitoring with Robotic Total Station
Robotic total stations (RTS) are the primary measurement sensors in the automated monitoring system in open-pit mines Target prisms are located strategically throughout the pit on the pit walls Distances, horizontal directions and zenith angles are measured to the targets continuously to determine 3D positions of prisms RTSs can be programmed for sequential self-pointing to a set of target prisms at predetermined time intervals and measurements can be transmitted to remote stations via a telemetry link Important sources of error: refraction and random pointing errors and instability of RTSs
70
Minimizing Errors in RTS Automatic Monitoring
Minimizing effects of both refraction and random pointing errors Maintain short distances from the RTSs to target prisms Take observations in several steps and spread the observations over long periods to randomize refraction effects; measurements can be corrected for meteorological effects Keep lines of sight away from strong sources of heat radiation Instability of RTSs will cause displacements determined to be biased Use RTS with GPS antennas to take care of instability of RTSs
71
Collocating GPS Antenna with RTS (1/2)
Two ways to collocate GPS antenna with RTS to correct instability of the RTS in an open-pit mine monitoring: Collocate GPS antenna and RTS in the unstable region of the mine and do the following Use two other GPS antennas collocated with 360º prisms: Collocated Antenna /Prism A is located on a stable point S (outside the mining region); the other Collocated Antenna/Prism B is located within the unstable mining region U Stable point S must be within m for best results with ATR in the RTS Use Antenna/Prism A to provide orientation for the main RTS in the shelter; GPS will update the position of RTS Updated Position of RTS and the orientation to Antenna/Prism A are used to correct measurements to targets on the unstable object points
72
Collocating GPS Antenna with RTS (2/2)
Collocate GPS antennas with 360º prisms and position them on at least 3 stable points (forming reference points), probably outside the mining region (but within m of the main RTS for best ATR results) and do the following Sheltered main RTS (in an unstable region) will measure to 3 or more reference points in free station computation to determine its position and orientation before making measurements to the targets on the unstable object points The reference points are to be positioned so as to form a strong geometry in order to ensure that free station calculates with high accuracy
73
Main Challenges in Using GPS in Open-Pit Mines
Steep walls – results in poor geometry Large height differences (700 m) between master and rover stations – affects accuracy of positioning Needs for fully automated GPS processor for continuous update in real-time Multipath problem resulting in accurate measurements Power supply to GPS units (difficult in inaccessible or not frequently accessed areas) Sheltering instrument in harsh conditions & against vandalism - Loss of RTS preferred to loss of GPS units
74
Concerns in Using RTS in Open-Pit Mines
Refraction & pointing errors (for pit’s diameter > 1 km) since long sights are involved (>1 km) Complex behaviour of pit (stability problem) as it responds to changes in environment (e.g., excavation, increase in water saturation, etc.) – causes instability of instrument also
75
Deformation Monitoring of Tunnels
Interest: Movement of tunnel walls (inward, settlement, heave and 3-D displacement) Deformation around and ahead of tunnel excavation face Deformation (settlement, tilt, lateral displacement & 3-D displacement) at or near ground surface (structures and utilities) – ensure structures at the ground surface are not harmed by tunneling operations Causes of tunnel deformation: Adverse ground and groundwater regimes Large overburden pressures – due to sensitive or utilities in urban tunnels Intense tectonic activities Concerns: Mountain region – caving in; Urban – damages to utilities
76
Tunnel Monitoring Instruments
Instruments on the ground surface or in the tunnel Ground deformation is likely at the place ahead or close to tunnel face install instrument so as not to interfere with support system (sprayed concrete, steel sets, etc.) Geodetic and geotechnical instruments to complement each other Geodetic provides absolute 3D coordinates – e.g., total stations on brackets and reflectors Geotechnical provides relative displacements in one direction – e.g., extensometers
77
Vertical Deformation Monitoring and Analysis
Special-order or first-order geodetic leveling procedures can be used in monitoring object in order to determine: Tilts based on height difference measurements in bases of virtually limitless lengths between pairs of benchmarks Vertical expansion (settlement, uplift, or subsidence) Absolute height changes with respect to stable points Major problem of differential leveling: Vertical atmospheric refraction is a major source of systematic errors Usual increase in random errors due to rod scale error and settlement of instruments and rods when a large number of setups are involved
78
Vertical Deformation Monitoring with GPS
For reasonable results, three-baseline GPS surveys can be used This method economizes GPS surveys since one can carry out the field operation unassisted Three-baseline method is illustrated in Fig with points C1, C2, C3 representing control points with continuously operating GPS receivers and point P as the point whose position is to be determined
79
Three-Baseline GPS Approach
Set up GPS receiver on the monitoring point P and determine its position by measuring three GPS baselines B1, B2 and B3 The duration of observation sessions must be up to 12 h in order to achieve sub-centimeter accuracy in vertical positioning at 95% confidence level Fig. 9.14
80
Tilt, Strain and Curvature Determination from Leveling
Tilt – deviation () of a surface relative to horizontal reference surface (Fig. 9.15a) Inclination – deviation () of a surface relative to vertical plane (Fig. 9.15b) Advantage of using geodetic approach over geotechnical approach in tilt determination limitless length of base is possible with geodetic approach unlike in geotechnical approach where short bases are usually involved Accuracy of 0.1“ in tilt is possible over 1 km with geodetic method
81
Tilt and Inclination Based on Fig (a), the tilt angle (in radians) can be determined by geodetic leveling as Leveling between points P1 and P2 can be done along any route Fig. 9.15
82
Horizontal Strain and Curvature
Deformation tolerances for assessing impact of ground subsidence on infrastructure are usually based on criteria: tilt (vertical displacement) horizontal strain (or horizontal displacements Curvature of subsidence trough Subsidence has not significantly impacted surface structures if the following are satisfied: tilt and horizontal strain around the structures have not exceeded deformation tolerance of 2.5 and 1.5 mm/m, respectively radius of ground curvature around the structures is larger than 20 km Consider points P1, P2 and P3 located on a subsidence bowl (Fig. 9.16)
83
Tilt, Strain and Curvature of Subsidence Bowl
Tilt (T), Strain () and curvature of subsidence bowl (K) can be given as (9.70) (9.71) (9.72)
84
Integrated Leveling Surveys (1/2)
Geodetic leveling surveys in a gallery of a dam can be used to determine tilt and absolute height changes of the dam structures The setup of level instruments in Fig can be used to determine elevations of unknown points and the borehole extensometer collar location The points (e.g., Pi and Pj) are monitored with the setup after to determine relative movements of the points Control is taken from known benchmark (BM) Two scales (one at upper floor and the other at lower floor) attached to suspended plumbline are used
85
Integrated Leveling Surveys (2/2)
(9.73) (9.74) For k = 1,…n: Fig. 9.17
86
Tilt and Strain Rate Determination
A plot of yk against time xk can be fitted with a sinusoid to derive vertical displacement rate (mm/year) When the displacement rate is divided by the horizontal separation between Pi and Pj tilt rate (mm/m/year) is obtained When the displacement rate is divided by the vertical separation between Pi and Pj extension or strain rate (mm/m/year) is obtained Equation (9.74) can be expressed from Fig (given that Δhab will always be constant) as (9.79)
Similar presentations
© 2025 SlidePlayer.com Inc.
All rights reserved.