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Automatic monitoring systemsIn the often hazardous environment of construction, automatic monitoring systems (AMS) play an increasing role in providing real time information on structural performance to help manage risk. An AMS has sensors to detect change such as movement, strain or load, signal conditioning to convert the output from the sensors into digital format, data loggers to store the information, computers and software to process and display the data so that action can be taken if required. The three most important features of an AMS are: resolution to observe change, stability to instil confidence and repeatability to understand cause and effect. (Accuracy often discussed at length is a difficult attribute to define in a constantly changing environment; it depends on how closely you observe the changes taking place with the changes in the environment). There are currently two main types of AMS: a) fixed sensors, b) optical targets. CMCS deal mainly with fixed sensor monitoring but have considerable experience in carrying out optical surveys, so both systems are first considered in a simple illustration. Tunnelling under or close to structures can cause damage due to excessive ground movements during excavation. Sensors fixed in the ground close to the tunnel best quantify the ground movements taking place. If ground movement is minimised by using appropriate tunnelling procedures then the effect on the structures would also be minimised. Similarly if stress levels in critical elements of a structure must be kept to safe limits during work on the structure, then sensors directly attached to the elements would be better at ensuring the limits were not exceeded. In general fixed sensors are best suited for directly observing the prime mechanism of concern, while optical systems only have the resolution to observe relatively large external movements. AMS systems are often used to control the construction process by specifying allowable levels or allowable rates of change in physical parameters. These levels or rates must be specified by experienced personnel who would consider the initial state of stress of a structure, normal diurnal or seasonal related changes, as well as the safe serviceable limits of the structure or component. Setting up the AMS well before the start of any construction is an important factor if normal behaviour is to be separated from the effect of construction. There are currently two widely used fixed sensors in the Construction Industry:
The contribution made by CMCS and its key personnel in the development of AMS systems using these key sensors is briefly outlined in the following sections. Development of the electro-level AMSIn the early 1970s, Cooke & Price whilst at the Building Research Establishment (BRE) developed an instrumentation system using electro-levels (ELs) to use in their research projects on pile soil interaction. 
The EL consists of a sealed glass tube (or more recently a ceramic container) partially filled with an electrolyte. Depending on the type of EL three or four electrodes are fitted into the tube and are used to measure the resistance of the liquid in each half of the tube. When the EL is horizontal the volumes in each half of the tube are equal and the resistances are equal, when the EL is tilted the volume of electrolyte and hence the resistance increases in one side of the tube and decreases in the other. The change in resistance is used to determine the change in angle of the tube. The linearity and sensitivity of the EL depends on the type and amount of electrolyte and the geometry of the tube. A series of laboratory tests were carried out to demonstrate that deflections of simply supported and cantilevered beams under load could be calculated from a series of ELs mounted on the beams. Loads applied to the beams were accurately back figured from the monitored changes in slope along the beams and the EI values of the beams. Cooke & Price used the system extensively in the late 70s and early 80s in some of the most comprehensive investigations into piled foundation performance carried out to date. During these projects the ELs were read manually with an analogue AC signal conditioning system. The readings at each loading stage of the pile were hand written into a record book and later transferred to a main frame computer via punch cards. 
In the late 1970s I F Wardle brought to the research team computer skills and an ability to understand communication between electronic devices. As small affordable site computers and analogue to digital (A to D) converters became available, data was recorded, stored and processed on the site computer. In the early days due to variations in switching resistance each sensor had its own signal conditioning units making a large system of 50 or sensors very expensive. With the improvements to multiplexers the sensors could be switched and read by a single signal-conditioning unit reducing the cost of the system significantly. Price & Wardle then developed the system from a research tool for use on laboratory systems into a field monitoring system that could be used on full-scale structures. To prove their worth as field instruments Price & Wardle compared the system with electrical resistance strain (ERS) gauges to monitor the performance of piles under lateral loads. Lateral loading tests were carried out on 'I' section beams in the laboratory at BRE and later on full-scale 'I' section steel piles fitted with ERS gauges in France. The French pile tests were carried out by J F Jezequel, one of the best field research scientists in the world, see publication 22. The figure above shows how the instruments compared in the tests. After the tests the EL system was completely recovered from the piles and had only cost a tenth of the ERS installation which was unrecoverable. 
While ERS gauges can be used on steel piles vibrating wire (VW) strain gauges are extensively used in concrete piles. To test the performance of the system in concrete piles, lateral loading tests were carried out with piles also fitted with VW gauges. The results from tests on two 2.4 m diameter concrete piles laterally loaded to 2400 kN are given in the Figure opposite. At low loads both the VW and ELs show the same bending moment distribution, but at higher loads the VW gauges become unreliable because of tension cracks in the concrete. The ELs monitored over a larger area of pile are not affected by the local cracking so provided more accurate information at higher loads and were also recovered at the end of the tests, unlike the VW gauges, see publication 24. Having proved their worth on simple structural elements it was then possible to use ELs with increased confidence to monitor the behaviour of more complex full-scale structures, which included diaphragm walls, marine jetties. Since 1993 CMCS have continued to develop the electronics and software to improve reliability so that it can be used to aid in the management of risk on safety critical projects. A summary of some of the types of projects the system has been successfully deployed on are given in the form of Information Sheets in this brochure. New improved load cell for the Construction IndustryThe main problems with using traditional load cells in construction are the large collapse potential, cost of making large cells, i.e. 2 or 3 m in diameter, the adverse environment and the required working life of the cell in that environment, possibly 10 or more years. Whilst at the Building Research Establishment, G Price & I F Wardle designed a load cell for use in concrete structures. The cell has negligible collapse potential, low cost, is readily available, easy to fabricate into any shape and load capacity and has reliability measured in decades. The basis of the new load cell is that it is made from a series of standard capacity strain gauged elements (VW) units. The cells can be formed directly in the structure with load being transferred in and out of the sensing units through plates, or bars cast in the concrete. The collapse mechanism is limited to less that 3 mm, the thickness of the soft membrane used to force the load through the sensing elements. Long-term stability of the new cell is achieved by making the sensing element completely watertight and preconditioning the wire used in the VW gauge. To fully test the design and the installation procedures a 1 m diameter cell was constructed and installed at the bottom of a 3 m deep pile. After the pile was fully load tested in the field it was removed from the ground and the whole pile load tested at the Building Research Establishment in the 10,000 kN grade 'A' load-testing machine. The results of the loading test showed the load cell had a resolution of better than 0.1% full scale. Many other tests were carried out, including long-term water tightness tests by placing sensing elements and load cells at the bottom of a 50m deep water filled shaft. Because of the limited collapse potential of the new load cells they were deemed safe to be installed in working piles, previously additional piles had been used to monitor loads. The final tests on the cells' suitability were to prove they could be constructed and installed in working piles under commercial conditions. The following installations proved they could be.
The photograph (above left) shows the construction of 1.8 m diameter load cell; the sensing elements are attached to a base plate and are fitted with pressure pads above. The soft membrane that forces the load through each of the units can be seen as white on the bottom of the cell and around the units. The circular steel sheet around the cell formed the shuttering for the concrete that later filled the cell. A screw collar fixed to anchors can be seen in the centre of the cell, that was used to attach to the 'Kelly bar' of the piling rig to the cell for lowering to the bottom of the pile bore. To release the cell the 'Kelly bar' was turned anticlockwise. An inflatable seal around the cell sealed it in the bore, stopping the pile concrete from bridging out the load cell.
In 1981 one of the settlement reducing piles under the foundation of the new QEII Conference Centre at Westminster (London) was fitted with a new design load cell. The photograph (above left) shows the top of one of the working piles being prepared for a new type load cell. Sixteen 1,250 kN sensing elements making up the 20,000 kN load cell were screw connected on to the threaded re-bars protruding through the pile concrete. The shuttering placed around the top of the pile was used to form a flat concrete surface onto which the soft membrane was fitted. Top link bars were then screwed on the sensing elements to link the pile firmly into the 2 m thick raft cast above the pile. The graph shows the loads monitored by the load cell (from 1981 to 1995). 
A 2.5 m diameter hand dug caisson pile used to support the new development over Charring Cross Station in London was fitted with three shaft load cells of the new design. The photograph shows the soft membrane and sensing elements with top pressure pads being fitted in the pile. The top section of the pile through the gravel down to the top of the London Clay was coated with a 'slip coat' to reduce the load transfer at ground level. The load cells were installed between the 24th and 31st of March 1988. The table below shows a summary of the loads monitored by the cells at the various levels down the pile. The only variation to the construction sequence was the placing of concrete inside the concrete casing, i.e. a metre of concrete was poured on top of each cell and allowed to cure for 6 hours before the remaining concrete was cast. This was to stop the cellular soft membrane being crushed by the 15 m or so of concrete above some of the cells. During the installation of the concrete linings special concrete and polystyrene inserts had been made to force the load through the cells. The latest sets of readings on the cells are due in 2003
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