306.1. Steel arches/
ribs and lattice girders
1. The Contractor shall provide method statements for the erection of arches to the Engineer for agreement. Arch materials shall conform to Section 207.
2. Steel arch ribs and full circle ribs shall be firmly fixed in their final positions against the excavation. Arch bases shall be provided with integral base plates of size to suit the bearing capacity of the ground and shall bear on rock or concrete of adequate strength. Arches and ribs shall be sufficiently clear of the excavation and the final internal profile of the structure to accommodate any required concrete cover.
3. The number of joints in the arch shall be varied to suit the Contractor’s method of working subject to the Engineer’s agreement. Steel tie bars and struts between arches shall be installed where shown on the Drawings.
4. A minimum 50 mm thick sprayed concrete layer must be in place before the installation of the lattice girders. Under no circumstance shall lattice girders be installed under unsupported ground.
5. Lattice girder segments shall be secured by use of temporary wood blocking, steel wedges, concrete spacers, mortar sacks and/or other appropriate means to maintain position during application of sprayed concrete. The means of support shall be subject to the approval of the Engineer. All wood blocking shall be removed during the next phase of excavation and any void infilled with sprayed concrete to ensure continuity of the primary lining.
6. Lattice girder segments shall have butt plates and the method of installation shall ensure tight connection of all elements.
7. Immediately prior to concreting, casting or spraying, the arches, ties and struts shall be rendered clean and free from deleterious matter.
8. Lattice girders shall be firmly fixed in their final position against the excavation prior to application of sprayed concrete. Lattice girders shall be sufficiently clear of the excavation and final internal profile of the structure to accommodate the required sprayed concrete cover.
307. Forepoling
307.1. General 1. Forepoling boards can be steel (trench sheets or similar) or timber shaped to suit and adequate for the purpose. They shall be driven from the supporting frame in a slightly upwardly inclined direction at the crown of the heading and should penetrate at least half a set beyond the next excavation cycle.
2. On completion of the excavation cycle the next supporting frame is to be installed and all necessary heading supports fitted and wedged tight including wedges between the head tree and boards prior to continuing excavation.
3. The next set of poling boards are then installed immediately below the previous set and above the head tree by removing the wedges one by one, and then the boards are driven as before. In the event that full penetration is not achieved, the boards can be fully driven to their required location during the excavation cycle.
4. Upon completion of each advance or at the end of each shift all head and side timbers should be fully grouted up.
5. Great care shall be taken to prevent the disturbance of face boards and supports in general during the forepoling cycle.
308. Explosives
308.1. General 1. The Contractor shall use explosives only in circumstances where it is safe to do so having due regard to the safety of persons, third-party property and the safety of the Works. Explosives shall not be used without the agreement of the Engineer.
2. The Contractor shall obtain all necessary licences and consents and shall provide secure storage facilities for all explosives and equipment in accordance with BS 5607:1998 Code of practice for the safe use of explosives in the construction industry and the requirements of the local police force and the Engineer.
3. Explosives shall be handled and used only by the Contractor’s duly authorised personnel. The names and qualifications of such personnel shall be submitted to the Engineer in writing in advance of any possible use of explosives.
4. Blasting operations shall be carried out only under the direction of an experienced operator. The Contractor shall appoint one competent person to be responsible for the security of explosives.
5. At an early stage, in advance of the proposed use of explosives, the Contractor shall notify the Engineer, third parties, statutory authorities and services which have an interest in or are likely to be affected by blasting operations, of the general nature of the operation. The Contractor shall subsequently give a minimum of 14 days notice to the Engineer and others described above of the proposed use of explosives. With this notification the Contractor shall submit to the Engineer a detailed method statement on all aspects of the proposed use of explosives, including the treatment of misfires.
6. Blasting shall be carried out carefully so as to avoid loosening or shattering rock beyond the required line of excavation, and loose or shattered rock (where it does not contribute to stability of the excavation) shall be removed by scaling down or other means before personnel will be permitted to restart operations after blasting.
7. Notices of blasting operations shall be posted on site. Before each firing, the Contractor shall give audible warning, clear the area, and shall take positive measures to prevent personnel from entering the danger area.
8. The Contractor shall monitor the results of blasting closely and, where it is proper to do so, shall propose changes to his blasting operation for the agreement of the Engineer.
9. Under no circumstances shall any holes be charged until completion of all drilling operations at the face.
10. After each blasting operation the tunnel drive shall be sufficiently ventilated to remove any nitrous gases and the atmospheric conditions shall be constantly checked prior to personnel accessing the excavated face.
11. No person shall be allowed to approach the face and no face operation shall commence until the Contractor’s authorised person in charge of the operation has given permission.
12. As soon as practicable after blasting and without undue delay the Contractor shall erect such support as may be necessary to safeguard the excavation and personnel.
13. The shot-firer must keep a record of the number of shots fired, their time of firing, type and weights of explosives used and the type and number of detonators used, together with a record of the post-blast situation for each and every location.
14. A copy of the record shall be available to the Engineer at the end of every shift on which shots are fired.
15. The Contractor shall comply with the following documents in respect of the use of explosives:
(a) Explosives Act 1875
(b) Amendment to Explosives Act 1875, dated July 1923 (c) The Manufacture and Storage of Explosives Regulations
2005
(d ) BS 5607:1998 Code of practice for the safe use of explosives in the construction industry
(e) Control of Explosives Regulations 1991
( f ) Carriage of Explosives by Road. Road Traffic (Carriage of Explosives) Regulations 1996
(g) PD CLC/TR 50426:2004 Assessment of inadvertent initiation of bridge wire electro-explosive devices by radio-frequency radiation. Guide
(h) Quarries (Explosives) Regulations 1988, as far as it is relevant to tunnel works.
308.2. Blasting vibrations
1. For structures in the proximity of blasting, the peak particle velocity shall be measured at the locations immediately adjacent to the structure nearest to the face being blasted or any other location where it is necessary to limit vibration.
2. Vibration monitoring proposals shall be submitted to the Engineer for his agreement.
3. The measurement of peak particle velocity shall be obtained from instruments capable of measuring along three orthogonal axes, one of which shall be aligned parallel to the centre line of the excavation and another shall be vertical. The Contractor is to provide supports for the measuring instrument if so required by the manufacturer’s instructions.
4. The measurements of the particle velocities shall be the respon-sibility of the Contractor. Copies of the readings in an agreed form shall be supplied to the Engineer.
5. Equipment for measurement of vibration shall be in accordance with BS 7385 Part 2:1993.
6. Prior to the commencement of blasting in any location, the Contractor shall demonstrate by the use of test firings, or by other means, that neither the peak particle velocities given in BS 7385 Part 2 nor those stated in the Particular Specification will be exceeded.
309. Groundwater
309.1. General 1. Unless specifically authorised or approved by the Engineer under the Contract, lowering of the natural groundwater table shall not be permitted.
2. Any proposals to lower the water table shall take account of the risks of causing settlement. Removal of groundwater shall not cause damage to the Works, nor to third-party property and shall not cause nuisance either by the removal of ground or by settlement.
3. Any impact on other groundwater abstractions shall be assessed, mitigated and monitored in line with the agreement with the holder of the abstraction licence and the Environment Agency.
4. The Contractor’s working methods and systems shall be designed to control groundwater to permit the construction of shafts, tunnels, breakouts and connections. Methods shall be in accordance with best practice as outlined in CIRIA Report C515 Groundwater control – design and practice. The Contractor shall submit proposals to the Engineer for controlling and mon-itoring any dewatering system, including monmon-itoring changes in groundwater level/pressures and settlement monitoring, prior to commencing installation and running of any dewatering system.
The dewatering system shall include a system for identifying ingress of soil material during the dewatering operation.
5. In planning temporary pumping systems, the Contractor shall take due consideration of water quality, pressure, quantity and variations in water levels.
6. The Contractor shall obtain the necessary consents from the appropriate authorities to abstract, recharge and dispose of groundwater. See Section 507.4.
7. In areas of contaminated land, abandoned mine workings and other possible water-filled voids, the Contractor shall take account of the potential hazards of inundation of the Works.
8. Any temporary works for the control of water shall be removed and the ground reinstated when they are no longer required.
9. Groundwater lowering as a method of ground stabilisation is further considered in Section 404.