The River Supply Conduit project, a steel water main to the City of Los Angeles

Company: Geocon West, Inc.
Author: Jelisa Thomas, Project Engineer
Location: Los Angeles (USA)

About Geocon

Geocon is a comprehensive geotechnical, geologic, construction inspection, materials testing, and environmental consulting firm. Geocon is experienced performing geotechnical instrumentation services for a variety of projects including water, transportation, and utility infrastructure as well as building structures. Geotechnical instrumentation services have included the installation and monitoring of slope inclinometers, multi-point borehole extensometers, piezometers, utility monitoring points, strain gauges, crack gauges, and tiltmeters.

The project purpose

The River Supply Conduit project is a new 78-inch diameter welded steel water main that is a major part of the water delivery system to the City of Los Angeles. The purpose of the project is to replace a significant portion of the near 100 year old system. The new water main is being installed with a combination of cut and cover techniques, as well as tunneling. The alignment is located in an densly developed urban environment.

44 multi-point extensometers were installed to monitor ground movement during construction of nearly 9,000 feet of trenchless pipe installation (tunneling) in unconsolidated sandy soils.

Project Participants

  • Los Angeles Department of Water & Power (LADWP) – project owner
  • W.A. Rasic – General Contractor
  • SAK – Tunneling Subcontractor
  • Geocon – Geotechical Instrumentation Specialists

Hardware

  • 44 multi-point extensometers installed with Double Action Borros Anchors and 150mm vibrating wire (VW) displacement transducers.
  • A-5 head assembly (Geokon Model No. 1250-1-2-1 and 1250-1-2-2)
  • Modified Wireless Datalogger (Geokon Model No. 8026-1X)

Hardware location, communication & parameters

The extensometers and dataloggers were fully contained in an 18-inch diameter flush-mount well box equipped with a cellular antenna. Each datalogger is equipped with a dedicated modem, which communicated directly with VDV through LoggerNet. Parameters measured were vertical deflection, i.e. settlement and heave, due to tunneling activities.

Purpose of VDV

VDV is being used to display the TBM data in real time, as well as the extensometers. VDV’s alarm system is being used to monitor if ground movement measured by the extensometers exceeds the project threshold limits. We are using the alarm system within VDV to notify the project team if settlement thresholds are exceeded. VDV is accessed by the Contractor, Subcontractor, Owner, and Instrumentation Specialist (Geocon).

Monitoring Period

The extensometer data is being continuously monitored. The frequency of the data collection and transmittal needs to change based on the location of the TBM relative to the instrument. VDV has worked with us to give us the ability to make changes to the data collection as-needed without delay. This is a benefit in that the data collection and upload to VDV is seamless, which is what project owners expect to see.

Most useful VDV features

    • The ability to sync up the TBM station to compare it with the extensionmeter movement
    • The ability to create different contact groups for system maintenance versus settlement monitoring.
    • The ability to manually enter data.
    • Alerts when data collection has not occurred (important for urban environment where lots of interference can happen).
    • Alerts for when battery voltages drop (important for not missing data.

Project Outcome

We are very happy with VDV. So far, on this project, the tunneling contractor has had a high degree of success with little to no ground movement. On a prior project where we also used VDV, the tunneling did not go as smoothly. Though the use of VDV, we were able to notify the tunneling contractor that movement in excess of the threshold values was occurring, which allowed them to stop tunneling and adjust their excavation methods before excessive settlement affected a major roadway in Los Angeles.