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Different Methods of Health Assessment in Embankments and Levees
Health assessment in embankments, levees and civil infrastructure is very important to find the reliability of the structure and to ensure the long-term stability, safety and performance of it. As Geotechnical infrastructure failure become more apparent because of natural disaster rather than the age of the structure as the United States Army Corps of Engineers noted in early 2007 that nearly 150 United States levees pose an unacceptable risk of failing during a major flood (ASCE, 2008). The target is to maximize the functionality and to decrease probability of failure as possible. Different methods have been implemented to find health assessment of embankments and levees with varying degree of accuracy. Several methods and types of instrumentation are available to measure settlement and vertical displacements. The methods vary depending on what type of displacement is to be measured, and what sort of measurement methods is feasible. Every method uses different type of sensors and different sensing techniques. Besides surveying monuments to determine lateral displacements, inclinometers are the primary method to monitor for lateral displacements at levees. Inclinometers can monitor
for lateral displacements or offsets within the body of a levee embankment and/or within its foundation. Also, Remote sensing method and fiber optic method, which are all discussed in this paper.
Keywords:Deformation, Real-Time, Surface Displacement, Monitoring
Health assessment in levees and embankment is very important because failure of the structure may lead to huge loses in human life and properties. The failure of levees during hurricane
Katrina and subsequent catastrophic flooding of New Orleans is a highly illustrative example of the consequences of levee failure. Having a Health assessment in embankments and levees will help to predict any issues and to avoid any risks. There are different methodologies to get health assessment, some of them are MEMS technique (micro-electro-mechanical systems) -based in-place inclinometer system by using of advanced
in situ monitoring devices of soil systems, such as the
Shape Acceleration Array (SAA), autonomous sensor array that is capable of measuring in situ deformations and accelerations up to a depth of 100 m and case histories of SAA applications at a bridge replacement site in New York, and an Urban Flood test site in Boston, UK. The system, called SAA, is manufactured by Measurand, Inc and can measure three-dimensional (3D) ground deformations at 30 cm intervals and 3D acceleration at 2.4 m
intervals to a depth of 100 m, (T. Abdoun • V. Bennett • T. Desrosiers • J. Simm • M. Barendse). Recent technologies and applications are providing new methods the Civil Engineer
assess these structures, and present engineers with a set of monitoring tools. Distributed fiber optic technologies create sensors that are of scale and size to finally match the dam or levee,
and present an interesting, reliable, cost effective way of monitoring these structures everywhere. distributed fiber optic sensing (DFOS) technique distributed sensor offer the unique characteristic of being able to measure physical parameters along their whole length, allowing the measurements of thousands of points using a single transducer (Inaudi and Glisic 2007). The most developed technologies of distributed fiber optic sensors are based on Raman and Brillouin scatterin (Inaudi D. et al. 2012). The technique enables distributed strain measurement along an optical fiber over dozens of kilometers while maintaining relatively high degrees of accuracy and spatial resolution, which shows great potential in the field of geotechnical monitoring (Mohamad et al. 2012, Zhu et al. 2012, Feng et al. 2015). considerable progress has been made in applying DFOS systems to monitor the deformation of a variety of geotechnical structures (Habel and Krebber 2011). A commercially available POF (Mitsubishi’s GH4001 fiber) was tested in the study.
And there are far-range sensors like Interferometric Synthetic Aperture Radar (InSAR) which is a remote sensing-based health assessment with a technique referred to as joint pixels InSAR (JPInSAR). (InSAR) is a remote sensing technique that offers the capability for recovering topographic profiles and measuring possible displacements of radar targets along the line of sight (LOS). Operating in the microwave domain, high spatial resolution, wide-area measurements can be obtained
day or night, in all weather conditions using InSAR.
Fig. 1. Dams and levees safety risk analysis
Fig. 2. Foundation failure in embankment dams
2.1 MEMS Technique
(micro-electro-mechanical systems), is a sensing tool for
simultaneous measurement of 3D soil acceleration and 3D ground deformation up to a depth of one hundred meters, with an accuracy of ±1.5 mm per 30 m.
inclinometer-accelerometer instrumentation measuring angles relative to gravity. Three accelerometers are contained in each 30 cm long rigid segment for measuring x, y, and z components of tilt and vibration. The rigid segments are connected by composite joints that are designed to prevent torsion but allow flexibility in two degrees of freedom. Triaxial MEMS measure deformation and vibrations caused by construction activities or earthquakes and providing this measurement as a signal to sensor node connected to it by wireless to enable real-time monitoring
And by using SAA (Shape Acceleration Array) system to measure deformation in the three dimensions with a specific interval and will be installed in the soil, the system is manufactured by Measurand, Inc and is capable of measuring three-dimensional (3D) ground deformations at 30 cm intervals and 3D acceleration at 2.4 m intervals to a depth of 100 m. this system is now being further developed to include digitally integrated pore pressure measurement in the form of vibrating wire piezometers equipped with microprocessors (called SAAPs). The SAAPs can convert vibrating wire data to digital data downhole, and they integrate easily into the SAA system. The additional insight into subsurface behavior provided by the SAAPs is integral in the development of a comprehensive system for monitoring and management of civil infrastructure. The preliminary testing indicates the suitability of this new multi-parameter system for inclusion in a multi-scale monitoring and health assessment framework, which will be implemented in New Orleans, LA in 2012.The frequency and spatial abundance of data made available by this sensor array enables tools for the continuous health monitoring effort of critical infrastructure under a broad range of static and dynamic loading conditions (Bennett et al. 2007).
each segment of the SAA contains three orthogonal sensors, arrays can be installed vertically or horizontally. The intended array orientation does not need to be specified prior to installation.
Data corrections procedures have been used forseveral decades to remove any long-term drift of the gravity sensing transducers from the calculated deformation data of probe inclinometers.
2.1.1 Previous case study
Sensors were installed at Champlain Canal in upstate New York, showing vertical position of sensors see Fig. 3. a comparison between the horizontal SAA systems and settlement plates. As shown in Fig. 3, SP is settlement plates, SAAH is the horizontal SAA, SAAV is the vertical SAA, and PVDs are prefabricated vertical drains (Barendse 2008).
Fig. 3.soil profile and vertical position of sensors at Champlain Canal , NY site (Barendse 2008)
Figure 4 shows the settlement profile from the horizontal SAA and a row of settlement plates (SP1, SP2 and SP3). This figure includes the horizontal SAA settlement data shown as a contour plot through February 2008, The settlement plates (SP1, SP2 and
SP3) experienced greater total settlement, approximately 280 mm versus 225 mm maximum observed SAA settlement This difference is attributable to the fact that the settlement plates were located approximately 4 m east of the horizontal SAA, a location
bearing more of the surcharge load.
Fig. 4. Settlement profile from horizontal SAA (contour plot)
and nearest set of surface settlement plates (discrete lines)
The SAAs have been extensively tested in the past to validate the reliability of the deformation and acceleration measurements (Bennett 2010) and are still providing stable measurements after 6 years of installation.
2.2. fiber optic sensing (DFOS) technique
Traditional fiber optic cable design aims to the best possible protection of the fiber itself from any external influence. It is necessary to shield the optical fiber form external humidity, side pressures, crushing and longitudinal strain applied to the cable. These designs have proven very effective in guaranteeing the longevity of optical fibers used for communication and can be used as sensing elements.
Unlike electrical and localized fiber optic sensors, distributed sensor offer the unique characteristic of being able to measure physical parameters along their whole length, allowing the measurements of thousands of points using a single transducer (Inaudi and Glisic 2007).Plastic optical fiber sensors are distributed and embedded in the soil that measure deformation of structures fiber over dozens of kilometers while maintaining relatively high degrees of accuracy and spatial resolution. The distributed fiber optic sensing (DFOS) technique enables distributed strain measurement along an optical fiber over dozens of kilometers while maintaining relatively high degrees of accuracy and spatial resolution, which shows great potential in the field of geotechnical monitoring (Mohamad et al. 2012, Zhu et al. 2012, Feng et al. 2015). The distributed sensing fibers have been directly embedded in soil masses and successfully captured the process and pattern of ground settlements (Glisic and Yao 2012, Klar et al. 2014) and slope movements (Naruse et al. 2000, Iten et al. 2008, Olivares et al. 2009, Wang et al. 2009, Zhu et al. 2014, Sun et al. 2014, Zeni et al. 2015, Zhu et al. 2015).
2.2.1 Pervious experiment and application examples
A commercially available POF (Mitsubishi’s GH4001 fiber) was tested. The diameter of the PMMA core was 0.980 mm, and the thicknesses of the fluorinated cladding and polyethylene (PE) jacket were 0.02 mm and 1.2 mm, respectively. The PE jacket can protect the fiber from external physical and chemical influences and add more mechanical strength to the fiber. This fiber has been used for several geotechnical monitoring applications (Liehr et al. 2008, 2009).
Any movement in soil layers will induce elongation of the fiber and the strain transfer from the soil to the fiber and it could be shown as in Fig. 5. below,
Fig. 5. Elongation of an embedded plastic optical fiber (POF) due to lateral deformation of a soil layer under external pressure
The specimen used contained a POF embedded within a cutting ring filled with compacted soil, with both ends passing through the holes drilled in the side walls. The diameter of the cutting ring was changed to 79.8 mm to obtain a longer fiber–soil bonding length. For all the specimens, the soil dry density was kept at 1.80 g/cm3, and the water content remained 10.0%.
the relationship between the pullout force and pullout displacement under each CP was obtained for this fiber.
The iLevees project” Intelligent Flood Protection Monitoring Warning and Response Systems”, in the state of Louisiana, has the goal of providing an alerting and monitoring system capable of preventing early stage failure, both in terms of ground instability and seepage. A number of test sections have been instrumented, including an I-wall and T-wall section instrumented with distributes strain and temperature sensors. SMARTProfile distributed strain and temperature sensing cables were installed in the levee foundations and on top of the I-wall and T-wall section. These sensors allow the detection and localization of events such as levee failure onset, seepage, tunneling, and formation of cracks in wall sections or abnormal joint movements.
2.3. Remote sensing deformation monitoring
A remote sensing-based health assessment of flood-control
infrastructure that can identify weak sections and impending failures can be a key to the sustainability of flood-control systems.
It is a wide-area covering measurement that can obtain displacement of radar targets along the site and the ability of recovering topographic profile. Given the hundreds of miles of levees with potential subsidence issues, the use of a remote sensing-based health assessment can quantitatively identify weak sections and impending failures. Continuous remote sensing technology, such as satellite and airborne monitoring, can serve as an accurate early warning system, as well as a valuable resource for assessing gradual and abrupt ground subsidence. Khalilzad and Gabr (2011) defined earth embankment limit states by correlating shear strain values to the horizontal deformation at the embankment toe location.
2.3.1 Joint Pixel Technique
By increasing the signal-to-noise ratio (SNR) joint pixel InSAR is founded to get time-series of displacement for health assessment of levees. The essence of this technique is based on joint subspace projection (JSP), i.e., the projection of the joint signal subspace onto the corresponding joint noise subspace. JSP as proposed by Li et al. (2006). And it is build by using joint pixel vector and the corresponding covariance matrix,
To validate the effectiveness of JPInSAR applied for health assessment of levees, grass-covered levees at Sites 1 and 2 (subsamples of the 30 km by 50 km images) were selected as testing areas within New Orleans, Twenty-four TerraSAR-X Stripmap descending orbit images of this area were acquired between March 2009 and October 2011, to acquire high-quality and high-resolution (up to 1 meter) SAR images. The data set was processed using both the standard PSI approach and the JPInSAR
algorithm, the JPInSAR data processing chain significantly increases the number of pixels where the deformation-ratecan be accurately estimated as compared to the pixels that can be processed by the classical PSI technique for the grass-covered levee area.
Due to the increase in the density of measurement points, JPInSAR is more suitable to be applied in health assessment.
Fig. 5. Estimated Displacement-Rate using JPInSAR.
2.3.2 Finite Element and Domain Model
The finite element program was used to model the Sherman Island levee section. The section geometry was selected on the basis of the information presented by Jafari et al. (2016), as shown in Figure 1b. Global Navigation Satellite System (GNSS) displacement recording locations.
Predicted displacement between points was compered to the measured one using finite element modeling. The displacement depends on how fine the mesh is, therefore, changing the mesh from coarse to fine will increase the displacement, and the boundaries are defined.
The purpose of health assessment in embankments and levees is to check the stability and to prevent failure. Health assessment of a structure is founded by comparing measurement for the same places at different time, the intervals between data collection depends on the sensitivity and the importance of the structure, as some of them are real-time monitoring. There is a different approach in measurement of deformation method. Each method has advantages and disadvantages, as for health assessment using remote sensing, it can cover big areas but with lower accuracy and it can fail to estimate deformation at some areas due to low coherence and low SNR., for fiber cables, the cable can be used for many objectives as sensing settlement and temperature but the plastic influences the measurement, and then inclinometers, which is the common method that have the best accuracy for deformation measurement
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