Drogheda is a medieval town situated about 50 miles north of Dublin near the mouth of the river Boyne. The Main Drainage Scheme included short sections of pipe jacking in a project that mainly involved open trench excavations for construction of Drogheda’s sewerage system. The pipe jacking techniques were specified for the critical areas where access was too restricted for open cut trenches. The project was well under way when the contractor started to consider how to carry out the pipe jacking work and was unable to obtain Contractors All Risk Insurance for damage to third party property. Mr Bateman was called in by the contractor to carry out a risk assessment and develop the method statements.
The pipe jacking section of the route passed along a 4 m wide alleyway known as Bessexwell Lane and the contract stated that a 1,000 mm diameter concrete pipe was to be at a depth of 3.5 m to invert of the pipe. The buildings on either side of the lane were over 200 years old and in poor condition with no real foundations.
Investigations
In order to carry out the risk assessment, additional ground investigation was specified that included boreholes, cone penetration tests (CPTs), ground probing radar and cross-hole tomography.
Results from the additional testing allowed greater confidence in the analysis of jacking pressures, friction forces and ground movements at the tunnel horizon and above in order that a more complete risk analysis could be carried out. The CPT was used to identify the lenses and bands of sand that were present, but in the critical areas adjacent to buildings only thin bands of sand were found in the clay above the tunnel crown.
Risk assessment
A range of options was reviewed including open-cut trenches, open-face pipe jacking, thrust boring and microtunnelling. The open-cut, open-face pipe jacking and thrust boring techniques would involve high risk of extreme ground movements, resulting in significant safety risks and damage to property. The potential disruption and ground movements caused by ground treatment, dewatering and underpinning would have been excessive and were similarly discounted.
Settlement and damage assessments were carried out following the guidelines set out in the CIRIA Report PR30 and the Guide to best practice for microtunnelling published by the UK Pipe Jacking Association. The slope of the ground and tensile strain were then used to categorise the risk of damage which was found to be very slight.
The risk assessment undertaken concluded that microtunnelling would provide the lowest risk solution but with a residual risk from buried walls and foundations.
Construction
A Herenknecht AVN 800 was fitted with a rock cutting head, so that the machine would deal with the walls and also operate in the soft ground conditions. These proposals were reviewed and agreed by the project insurer before work could begin. In addition to the critical drive in Bessexwell Lane, from MH121 to MH120 there were two other drives to be carried out using the microtunnelling technique. It was decided that these drives would be constructed first in order to learn more about the material/machine operation.
The machine was launched from MH123 towards MH121. Monitoring was continued on an hourly basis checking levelling pins installed through the concrete road slab and “÷telltales’ on adjacent structures.
An incident occurred when suddenly the material would not pass through the slurry pipes and up to the separation plant. The machine pushed forward a few centimetres with only slight increase in jacking pressure but with ground heave that caused the concrete road slab to lift and crack. The problem was created by heather blocking the holes in the mucking chamber and was solved by careful back flushing of the machine head. It is thought that the heather was a form of soil reinforcement used in medieval times to allow construction on the very soft material. A number of stones were also excavated which indicated the presence of a footpath.
The ground had heaved by 75 mm almost instantly when the slurry pipe blocked. It was considered that the proximity and the poor structural conditions of the buildings meant that they would be extremely sensitive to ground movements much less than 75 mm.
The Herenknecht microtunnelling system allowed a great degree of control over face pressure, rate of progress and rate of spoil removal. It was decided that the machine control, though dependant on the skill of the machinery driver, was sensitive enough to control ground movement. The problem lay in monitoring the progress and effects of the microtunnelling machine, particularly in such mixed conditions and the possible presence of obstructions. It was decided that some form of real time monitoring using electronic sensors linked to a computer in the driver’s control cabin would be required. All of the instruments incorporated electrolytic tilt sensors, which consist of a precision bubble-level that is sensed electrically as a resistance bridge. As the instrument is tilted, the resistance changes, and this can be easily and accurately measured.
Within Bessexwell Lane, a novel approach was devised which involved installing individual electrolevels in angled plastic lined boreholes drilled through the concrete slab. The boreholes were drilled at 2 m centres, at a 45å¡ angle to horizontal so that the electrolevels would be positioned at 1 m below ground on the centre line of the microtunnel to indicate both settlement and heave, as shown in Figure 2.
In addition to ground monitoring, tilt meters and beam electrolevels were installed on the walls of the buildings within the zone of influence. Software was developed to display the results of the monitoring on a laptop computer located in the control cabin. Each sensor was monitored on a 15 second interval so that any movement greater than 0.1 mm was registered on the lap-top immediately.
Monitoring Results
The monitoring was installed a few days prior to the commencement of microtunnelling operations. This enabled any background movement picked up by the highly sensitive equipment to be identified. Generally, very little movement occurred although it was possible, on many of the sensors, to identify the approach and passing of the microtunnelling machine
Sudden ground movements did occur and were immediately identified by the monitoring devices so that the driver was able to modify the rate of progress, slurry pressure and rate of spoil removal to minimise the extent of movement. A typical example is shown in Figure 6 where heave occurred when an obstruction, possibly a boulder, wall or foundation was encountered. The ground moved by 9.5mm immediately, before levelling out as the machine stopped and pressures were relieved.
Overall, no significant structure movement was observed and ground movement was noted on only three or four occasions. Maximum ground movement did not exceed 10 mm.
Conclusion
The use of risk management techniques in planning, design and construction of projects is a given in recent years. The Code of Practice is a powerful tool that underlines the importance of the skills and experience of the people carrying out the risk assessments in addition to providing a clearly defined process.
Ground movements due to tunnelling operations are inevitable, even from small diameter microtunnelling operations. The extent of movement and risks associated with the movement must be assessed and the appropriate degree of control and monitoring applied. In the case described above – the risk of movement was high and the consequences of ground movement were not acceptable with the possibility of disproportionate impacts on properties. A high degree of skill and experience was required to recognise and mitigate the risks that were identified in planning or became apparent as the work progressed.
This article is a summarised version of a paper entitled A risk management approach for microtunnelling projects by Geoff Bateman, BSc CEng MICE. The paper was presented at Trenchless Australasia 2008 – the National ASTT Conference and Exhibition held in Sydney this year.