Zetica - e-news
September 2011 Special edition

Zetica Monitoring Services

Large scale construction projects such as Cross Rail and Heathrow Terminal 2A underline the important role of displacement measuring systems such as total stations, inclinometers, piezometers, crack meters, strain gauges and inertial systems to monitor the impact on structures of close construction activity. These point datasets of varying density, are coordinated and centralised for efficient comparison against agreed action thresholds and disseminated using web portals and wireless communication technologies.

A broad assortment of infrastructure monitoring technologies based on geophysical principles of measuring material properties are set to make their mark. Take for example landslips and embankments. Displacement measurement systems are useful but can give just-too-late warning of slope creep or worse. Geophysical measurement of changes in shear modulus, moisture level , and fluid flow achieved using seismic, electrical resistivity and self potential methods respectively can provide a unique relatively early warning system. Permanently installed resistivity systems can remotely monitor groundwater conductivity changes related to leachate egress from a landfill or salt water ingress to a fresh water aquifer in a coastal plain.

Zetica manages such a permanent system in the UK to highlight any geomembrane tears beneath an active landfill. Results are posted to a web portal for change mapping and dissemination. This is the subject of the following case history.

 

Case history - landfill leak detection

A permanently installed leak detection installation provides a cost effective long term monitoring solution for new landfill sites. The system is sensitive, detecting narrow tears allowing a high degree of confidence to be placed in the continuing integrity of the installed liner.

An array of electrodes and cables are buried in a conducting sand layer for protection and to ensure good electrical contact with the ground. The geo-membrane liner is then installed on top of the electrodes and protected on the upper side by another layer of sand or soil. The array of electrodes, now permanently buried below the liner, are connected to a central processing unit. When an electrical current is applied to the material above the liner, it will flow through any liner defects, appearing as a high potential on the nearest electrode (Figure 1). Figure 2 shows the installation under an active landfill.

Figure 1: A below liner leak detection system consists of a permanently buried array of electrodes that detect electrical current flowing through defects before a leachate plume occurs.

Figure 1: A below liner leak detection system consists of a permanently buried array of electrodes that detect electrical current flowing through defects before a leachate plume occurs.

Figure 2: A permanent below liner leak detection installation (left) identifies liner defects beneath an active landfill (right).

Figure 2: A permanent below liner leak detection installation (left) identifies liner defects beneath an active landfill (right).

The permanently installed, below liner leak detection and monitoring system has 340 electrodes buried beneath the geo-membrane liner with a spacing that varies between 10 and 20m. Figure 3 shows the electrode layout and an example dataset with the response from a liner defect.

Figure 3: Electrode layout plan (left). Results (right) from a below liner leak detection test from a permanently installed monitoring system revealed a new anomaly (A) associated with a liner defect. Anomaly B represents a known liner defect that is above leachate level and therefore not in need of repair. Anomaly C is a response to electrical current leakage at the site entrance

Figure 3: Electrode layout plan (left). Results (right) from a below liner leak detection test from a permanently installed monitoring system revealed a new anomaly (A) associated with a liner defect. Anomaly B represents a known liner defect that is above leachate level and therefore not in need of repair. Anomaly C is a response to electrical current leakage at the site entrance

Measurements can be automatically taken according to a predefined program, which identifies when the measured potential of any electrode increases and alerts the user to a potential liner defect.

A mobile surface-based leak detection survey can be undertaken to precisely locate the defect in the area of the identified anomaly to within 0.2m (Figures 4 & 5).

Figure 4: An unmistakable anomaly (arrowed) from a surface-based leak detection survey precisely located the defect shown in Figure 3. Figure 5: Exposed! A liner defect was identified no more than 1hour after the detection survey, within 20cm of the identified anomaly position.

Figure 4: An unmistakable anomaly (arrowed) from a surface-based leak detection survey precisely located the defect shown in Figure 3.

 

Figure 5: Exposed! A liner defect was identified no more than 1hour after the detection survey, within 20cm of the identified anomaly position.

The area was exposed and the defect confirmed on the same day as the survey. Following the repair, a repeat of the below liner test verified the integrity of the liner in this area (Figure 6).

Figure 6: Example of electrode potential dataset collected before the liner repair (left) and after liner repair (right). The anomaly A caused by the defect has clearly disappeared confirming the quality of the repair.

Figure 6: Example of electrode potential dataset collected before the liner repair (left) and after liner repair (right). The anomaly A caused by the defect has clearly disappeared confirming the quality of the repair.

If required, an electrical resistivity section can also be derived to depths of >10m below the base of the containment structure (depending on material types) to monitor the movement of contaminants should a leakage occur.