When a retaining wall failed, 41,000 cubic yards of rock, soil and trees slumped onto a major urban highway. The failure closed the highway and endangered nearby apartment buildings. Following an emergency response to stabilize the slope and reopen the highway, it was decided to replace the failed section with a bin wall but leave the remaining sections of the retaining wall in place.
Construction would necessarily involve risk to the remaining sections of the wall, abutting properties, and the busy highway below. Safe execution of the project would require continuous monitoring of the remaining sections of wall and implementation of an early warning system.
GEO-Instruments worked with the geotechnical engineer of the project to develop a suitable monitoring system. The system would operate 24/7, provide updated readings for settlement, rotation, and displacement calculations every 15 minutes. Should any movement exceed alarm thresholds set by the project engineers, alarm notifications would be sent out via email and text messaging.
- 41 horizontal electrolevel beam sensors sensors
- 10 vertical electrolevel beam sensors
- 6 liquid level sensors for monitoring differential movements (settlement)
- 34 vertical tiltmeters for monitoring displacement and rotation
- 18 vibrating wire crackmeters for monitoring opening or closing of cracks.
Beam sensors were used to monitor vertical and horizontal deformation of the remaining sections of wall. Self-lubricating washers and spring washers at anchor points allowed free expansion or contraction of the beams, thus eliminating temperature-induced bending of the beams which would have affected readings. A 3-foot horizontal terrace mid-way up the wall interrupted vertical arrays of beam sensors. GEO-Instruments developed a innovative solution for this problem, allowing true calculations of cumulative movement relative to the bottom of the wall.
Prominent cracks along the face of the wall were instrumented with vibrating wire crackmeters to monitor any changes. A liquid settlement system monitored for differential displacements.
Data Collection and Alarms
Sensor readings were collected continuously by on-site data loggers and then retrieved every 15 minutes by a GEO-Instruments server. The retrieved readings were checked for alarm conditions and then stored for access by users. Using just their web browsers, users could see overall maps of the site with an overlay of sensor locations. The color of the sensor (green, yellow, red) indicated its alarm status. By clicking on any sensor, users could see trend graphs for the past month.
GEO-Instruments’ solution earned the praise of the design and construction engineers for giving them tremendous peace of mind during a very difficult project.
This photo shows the scale of the failure. Beam sensors have been installed and are visible (white lines) on the wall to the right.
Northbound lanes were closed by debris.
Installing beam sensors on undamaged sections of the wall.
In some cases, horizontal and vertical beams shared the same anchors.