Temporary earth restraining structure (Singapore

Wireless Sensor Networks for Civil Infrastructure Monitoring ISBN 978-0-7277-6151-4 ICE Publishing: All rights reserved http://dx.doi.org/10.1680/wsncim.61514.153

Chapter 21

Temporary earth restraining structure (Singapore) This case study describes the deployment of a prototype wireless sensor network (WSN) on the temporary earth restraining structure (TERS) of a mass rapid transit (MRT) construction site in Singapore, monitoring strain and environmental conditions. The primary goal of this deployment is to validate the WSN design (i.e. wireless communication and validity of measurements) and to provide an archive of high-resolution strain data (15 min intervals) for offline analysis.

21.1. Description 21.1.1 Asset The asset comprises several I-beam struts on a TERS at an MRT excavation site in Singapore.

21.1.2 Background The larger goals of this project are to apply wireless sensing to TERS monitoring in order to reduce monitoring costs but maintain safety through dense, cheap measurements and integration of data into an evolving model of the construction (Wilkins et al., 2015). These initial deployments are to evaluate the performance of wireless communication and data measurements within the construction over a period of a number of months. For sensors, there are several challenges and constraints that complicate their deployment: n n n n n

There is limited deployment access time, and a low priority for the installation of sensor nodes. Deployments are ongoing with the progress of the construction. There is limited space available to deploy on the strut, and thus no capability for power harvesting on sensor nodes. It is impractical to retrieve previously deployed instrumentation after excavation. The environment is harsh in terms of performing sensor calibration and deployment validation.

21.1.3 Monitoring objectives The prototype network has several goals, as follows: n n n

Strain measurements are to be made on-strut. Data are to be transmitted wirelessly at 15 min intervals, to target a 1 year lifetime (or the length of the construction). The data must be able to integrate into a TERS model. 153

Downloaded by [ Coventry University -Serials Section] on [21/12/16]. Copyright © ICE Publishing, all rights reserved.

Wireless Sensor Networks for Civil Infrastructure Monitoring

21.2.

Solution

Figure 21.1 shows the basic network architecture, where sensor data flow from the network to a data sink, where such data are transmitted to a remote server by way of 3G and are subsequently made available to suitable applications (such as a graphical user interface, or a model). Given the short (12-month) development time for the monitoring system, most of the hardware, software and mechanical parts were off the shelf. The data sink and wireless sensor nodes were designed to be resistant to the tropical environment conditions in Singapore, where temperatures can rise to 35°C and 99% relative humidity above ground, and even more inside the excavation (40°C or higher). The size of the nodes needed to be small to reduce the likelihood of being damaged on site, and thus power-harvesting capabilities were unavailable. The sensor node is based around the Zolertia Z1 platform, with a bespoke strain gauge board (see Figure 21.2). It is backwards compatible with the ubiquitous Telos platform, but includes larger amounts of program memory, and better access to hardware pins with which to interface the selected strain sensor board. The platform is a combination of the MSP430 microcontroller and a 2.4 GHz CC2420 802.15.4 radio, and supports an external antenna. The Z1 supports both popular open-source WSN operating systems (OSs), TinyOS and Contiki OS, out of the box. The static strain measurement was chosen to be resistive, instead of using more costly vibrating wire gauges. Because the strain in a strut is assumed to be axial, only one strain channel is required per sensor. A custom printed circuit board was developed to support a single Wheatstone bridge circuit in a quarter-bridge configuration, which could be read by a lowpower 16-bit analogue–to-digital converter (ADC). To save energy, half of the bridge was configured to use 10 kΩ resistors. The bridge can be balanced using a manual potentiometer. The low-power ADC was connected to the Z1 by way of the inter-integrated circuit (I2C) digital interface. The strain measurement resolution is