Health & Safety Executive (HSE) concerns in the UK regarding the occupational health and safety risks arising from heavy physical work, including the use of vibrating tools in a confined space, together with the requirement to evacuate tunnels quickly in a rescue situation, has been the catalyst for new design guidelines endorsed by the British Tunnelling Society (BTS) and the UK Pipe Jacking Association. The best practice consensus provides guidance for designers of internal dimensions and indicative drive lengths for tunnels and pipejacks below 3m diameter and covers a range of mechanical and manual excavation techniques for both tunnelling and pipejacking.
Compliance with the document does not relieve designers of their responsibility to consider all other risks to include manual handling, noise, heat, vibration and confined space working, which in itself includes safe means of access and egress.
In parallel with the joint guidance document the Pipe Jacking Association has published its own document: ‘Guidance on the Design of Hand Excavated Pipejacks’. The document is also aimed at clients, designers, specifiers, and contractors, and covers the limitations on, including lengths, of hand excavated pipejacks. The Pipe Jacking Association recommends that hand excavated pipejacks should only be undertaken if mechanical excavation methods are not reasonably practicable, and then only at limited lengths. The major driver for the guidance note, as with the HSE document, is to avoid the long term health risks of hand-arm vibration and noise exposure.
A completely revised and updated ‘Introduction to Pipe Jacking and Microtunnelling’ has also been issued by the UK’s Pipe Jacking Association and this now incorporates the advice issued by the Health & Safety Executive. All the documents referred to are available for free download from the PJA website www.pipejacking.org.
The jointly issued guidance for designers is based on two tables (see page 20), which provide indicative drive lengths for pipejacks and tunnels below 3m diameter.
The guidance should be read in conjunction with BS 6164:2001 and is intended to be used only by those competent to design pipejacks and tunnels. As stated, complying with the guidance does not relieve the designer of the duty to consider the risks arising from the foreseeable hazards of pipejacking/ tunnelling including manual handling, noise, heat, vibration and confined space working. Neither does it relieve the designer of the duty to ensure there is potentially adequate space to allow a safe means of access and egress along with adequate working space within the tunnel/pipejack. The minimum diameter required for construction may in some cases be determined by the criteria above, rather than by consideration of the hydraulic requirements for, or the intended use of the pipejack/tunnel.
Indicative drive length and the number of drives of that length, have been determined from consideration of access and escape requirements. Again, complying with the guidance does not relieve the designer of the duty to consider the risks arising from the range of foreseeable emergency events which could occur and which could necessitate escape or rescue of those underground.
The drive lengths given in Table 2 are indicative. Designers should note that for entries not marked * it is acceptable to exceed the indicative drive lengths by up to 25% however exceeding these lengths by over 25% should be avoided. Exceeding the indicative lengths by over 75% should be considered to be not acceptable.
All hand dig is categorised as “not acceptable” or “avoid” – the lengths given in Table 2 for items marked * are indicative and are already in the category “avoid”.
Definitions
What do the definitions used in the tables actually mean? ‘Acceptable’ means that designers should undertake an assessment of the risks normally associated with small size pipejacking/tunnelling and specify the appropriate mitigation measures. ‘Avoid’ is a signal to designers that they should undertake a robust technical assessment and risk assessment to justify their decisions to deviate from ‘acceptable’ criteria. Designers should identify appropriate risk mitigation measures. They should seek the advice of the Planning Supervisor/Co-ordinator and only proceed if the Planning Supervisor/Co-ordinator is satisfied that due attention has been paid to health and safety in undertaking the design and that appropriate risk mitigation measures have been identified. Contractors being asked to construct a pipejack/tunnel in this category should also seek advice from the planning supervisor/co-ordinator on the adequacy of their risk mitigation measures. Not acceptable is a clear indication that designers should not specify the use of pipejacking/tunnelling of this size and construction method. An alternative design solution should be sought.
Pipe jacking guidance
With specific reference to hand excavated pipe jacks the Pipe Jacking Association has proposed even more onerous conditions as outlined in Table 3.
The reason that the Pipe Jacking Association has applied more specific guidelines than the HSE is based on its experience of the technique and also taking into account diverse ground conditions.
One of the main drivers for the Pipe Jacking Association recommendations is the need to avoid the use of hand held vibratory tools to minimise the long term health risks from Hand Arm Vibration Syndrome (HAVS) and Noise. Consequently, mechanised excavation should be adopted wherever and whenever possible and the requirement for work methods and practices to ensure that the hand-arm vibration Exposure Action Level of 2.5m/sec2 and the noise Exposure Action Level of 80dB(A) (New limit from 6th April 2006) is not exceeded in an 8hr period.
The practical difficulties of on site measurement of vibration exposure, caused by changes in ground, personnel, efficiency of the equipment, maintenance of equipment and changing trigger times, to demonstrate proper management control of HAVS are considerable and costly.
Assessments should be made at the preliminary design stage and if possible design out the risk and where excavation by hand is proposed, prior to work commencing on the actual project.
These are only assessments and due to the significant variables, a certain amount of measurement of actual vibration exposure is inevitable to monitor and modify working arrangements to ensure compliance.
The introduction of The Control of Vibration at Work Regulations 2005 and the publishing of the HSE Guidance L 140 on these regulations, requires the pipejacking (and the tunnelling) industry, both designers and contractors, to further consider the need and desirability for hand excavation techniques.
The regulations introduce the Vibration Action and the Vibration Limit Exposure Levels of 2.5m/sec2 and 5m/sec2 normalised for an 8hr period.
Exposure above the Action Level will require the introduction of preventative measures and planned health surveillance and monitoring programmes.
Reference should also be made to ‘The Management of Hand-Arm Vibration in Tunnelling, Guide to Good Practice’, published by the British Tunnelling Society and which is available free on their website: www.britishtunnelling.org
The Pipe Jacking Association
The Pipe Jacking Association has been established for over 30 years and represents the leading contractors, pipe suppliers, and machine manufacturers in the United Kingdom pipe jacking and microtunnelling industry.
The Association is either represented on, or works closely with, all the key bodies involved with education and training, specifications and standards, and research in the tunnelling and pipe jacking industry. These include British Standards Institution and CEN; British Tunnelling Society; Construction Industry Training Board; the Departments of Civil Engineering at Oxford and Cambridge and Newcastle Universities; Transport Research Laboratory and the Health & Safety Executive.
Pipe jacking underway Pipe jacking underway Table 1: Nominal internal diameter of pipeline or tunnel linings Table 2: Indicitive drive lengths (e.g. between shafts) and max number of drives Table 3: Conditions for hand excavated pipe jacks
