WEB-BASED REAL-TIME MONITORING AT PERRIS DAM USING IN-PLACE INCLINOMETERS AND PIEZOMETERS WITH AN AUTOMATIC NOTIFICATION

SYSTEM

John Lemke, PE, GE1 Mike Driller, PE, GE2

Dan Wilson, PhD3

ABSTRACT A real-time monitoring system was used to monitor subsurface movements and groundwater levels at Perris Dam during the construction of a liquefaction remediation test section. The test section considered herein included dewatering, driving sheet piles, excavation and replacement of soil, and deep soil cement mixing. The California Department of Water Resources (DWR) implemented a geotechnical instrumentation program to automatically and continuously monitor changes in subsurface movements and groundwater levels using in-place inclinometers and electronic piezometers, respectively. Each remote station at the project site automatically transmitted readings to a server at selected time intervals using wireless Internet modems. Plots of in-place inclinometer displacement profiles and water elevation were monitored by DWR project team members via a secure web page during and after construction. Water levels and displacement profiles were calculated and compared automatically, in real-time, to pre-established threshold levels. The server computer was programmed to initiate phone calls to a list of designated project members whenever threshold levels were exceeded. Each automated message included the project name, remote station number, and relevant reading from the in-place inclinometer or piezometer instrumentation. The real-time monitoring system used at Perris Dam provided continuous feedback during remediation construction work and provided a cost effective way to achieve continuous field observation with an automatic notification system. This paper will discuss the specification, set up, and performance of the geotechnical monitoring system during the test section monitoring.

INTRODUCTION Perris Dam, located in western Riverside County, California, is a 125-foot high earth dam extending 2.2 miles in length (Figure 1). A residential neighborhood is located immediately downstream of the dam. A 2005 California Department of Water Resources (DWR) study of the Perris Dam foundation indicated that thin sandy layers in the dam foundation are potentially susceptible to liquefaction and severe loss of strength during a large earthquake event. A test program was initiated to evaluate planned repair work scheduled for 2013. This paper describes a real-time monitoring system used to measure ground water levels and lateral subsurface displacements in an effort to ensure public safety during the test program construction.

BACKGROUND While the foundation conditions at Perris Dam were considered adequate by the standards of practice during its design phase in the late 1960s and early 1970s, significant advances in soil liquefaction engineering, including soil sampling, testing, and computer modeling methods, have

resulted iperformeindicatedto liquefafoundatiorelease fr

In 2007, to remedand replacement dconstructsections idesign. Tdewaterinequipmenshows thdownstreplace incexcavatioduring, avaries buexcavatio

in new interped a detailed d that the preaction and seon soils coulrom Perris L

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acement of thdeep soil mixtion of the plimmediatelyThe test sectng of the fount, procedure layout of t

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and predicten in 2005 (DWm foundationearthquake

f the dam and

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s decided to wn in Figureest excavatioest section tohe required sThe vegetati

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In the event any data reduced from the instrumentation readings exceeded the established threshold levels, a computer generated phone message would automatically be sent with a description of what station exceeded the threshold criteria along with the measured value. A detailed plan was added to the project specifications that described what action would be required for a given notification. The required action varied with each threshold level. For example, a level 1 notification would be sent only to the Instrumentation Engineer and DWR Project Manager for review. In contrast, exceeding the level 3 threshold required the construction foreman to be on site within 30 minutes and key staff from the contractor and DWR to be on site within 60 minutes of notification. The automatic notification system was deactivated for some instrumentation at various times during construction based on the nature of the construction activity and the anticipated measurement changes. For example, the notification system was disabled for two of the in-place inclinometers when sheet piles were installed adjacent to the inclinometers. Similarly, the notification system was disabled for the piezometers during several periods during construction when it was anticipated that dewatering activity would initiate false notifications. The majority of construction work requiring automated monitoring occurred during the period of time beginning in January and extending through the middle of March 2010. Figure 11 shows the maximum cumulative displacement results for in-place inclinometer IPI-4. The displacement threshold levels are also included on Figure 11 for comparison with the measured results. The results indicate that IPI-4 never exceeded the lowest displacement threshold criteria of 0.1 inches over a period of 24 hours. Cumulative displacements shown in Figure 11 and identified as “24-Hr Average” were calculated by averaging 12 hours of readings and comparing the resulting averaged profile with a set of readings (averaged over a 12 hour period) occurring 24 hours prior to the first set. In addition to the 12 hour averaging method, maximum cumulative displacements were also calculated using a single inclinometer reading and one reading 24-hours prior to the first. Figure 11 illustrates that using single reading events results in increased displacement calculations as expected without averaging, but both calculation methods produced displacements below the established level 1 criteria. None of the inclinometer instrumentation readings exceeded the level 1 threshold for this project.

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number of measurement points captured the physical behavior being monitored and provided an enhanced measurement criteria for identification of potential shear plane development based on closely spaced inclinometer sensors, 2) frequent sampling adequately characterized the performance of the dam during construction and provided continuous observation to increase safety, 3) real-time evaluation of instrumentation measurements provided a robust automatic notification system, and 4) browser based software with interactive charting and analysis tools moved real-time data delivery beyond static data tables and plots delivered as gif images in standard HTML pages, and 5) significant cost savings associated with automation of data collection, data reduction, plotting, and evaluation of data to determine if immediate corrective action is needed. In this project there was no automated alarm triggers and thus no automated notifications or response. The continuous monitoring program was considered a success, however, in that even developing the multi-tiered response strategy contributed to a more thorough communication of risks and concerns to all stakeholders. The test construction phase concluded in March 2010. The data collected during the construction activities is being analyzed by the design teams in evaluating the proposed techniques toward determining the final remediation strategies to be used at Perris Dam.

REFERENCES California Department of Water Resources (2005). “Perris Dam Foundation Study.”