The meeting was opened by Danny Sherban of Yodfat Engineers, who outlined the background of the project. The Kishon River lies on the outskirts of Haifa, Israel, and is subject to flooding at certain times of the year.
These floods were particularly severe in 1991-1992 and again in 1995-1998. As this is a residential area with important highways and railways running through it, the domestic disruption and structural damage caused by the intermittent floods has been a huge problem. Various flood-protection schemes were implemented between the years of 1993 and 2000. These consisted mainly of embankment protection and river bed dredging to clear away detritus. These measures proved to be inadequate, therefore a scheme was developed for a diversion canal with capacity and gradient to provide an adequate channel for the relief of similar floods in the future.
Diversion canal and box jack
Rania Gideon of Yodfat Engineers continued with the technical details of the design. The critical item for the canal construction was to pass under the Histadrut Road, an extremely busy suburban dual-carriageway. The authorities were adamant that this should not be done in open cut.
The idea of a box-jack under the dual-carriageway was very appealing as it eliminated disruption to traffic and the need for expensive service diversions. Of particular note are the general density of services and the proximity of an oil pipeline to the excavation.
The box-jacked underpass consisted of twin 5m x 3m x 56m long precast reinforced concrete boxes. The ground consisted of heterogeneous clays and sands with a water table lying about 1.5 to 2m below the ground surface. The twin box culverts were cast in a sheet-piled excavation adjacent to the dual carriageway.
The key elements of the temporary works were as follows:
the casting/jacking pit and water cut-off;
The sheet-piled cofferdam was driven to a depth of 16.5m and excavated to a depth of 8m with one level of wallings secured by ground anchors at 2.5m centres. Well-pointing was used to stabilise the ground around the excavation and the invert and reduce the inflow of water.
Ground Freezing
John Belton of British Drilling and Freezing continued with a detailed description of the ground freezing operations. The freeze agent chosen was brine. Nitrogen was considered, but there was simply not enough available in Israel and it was significantly more expensive to buy than in the UK.
The tunnel face consisted of sand in the lower part of the section, this was overlain by a ‘fat’ clay with a high natural moisture content. It was estimated that, with freeze tubes at 1m centres, it would take up to 10 weeks to freeze the clay and up to four or five weeks to freeze the sand to about –35°C. 120mm of heave was predicted in the clay, this was of major concern to the client.
British Drilling and Freezing prepared the specification for drilling and a local company carried out the drilling subcontract. A total of 77 freeze tube holes, 90mm in diameter and 55m long were drilled, with seven observation holes containing thermocouples to monitor the progress of the freezing in the ground.
The array was essentially drilled horizontally with some vertical holes. All the freeze holes were pressure tested on completion. Some 50% were found to be leaking and had to be sealed to prevent brine escaping into the surrounding ground during freezing. The hot weather in Israel, with daily temperatures reaching 40°C, meant doubling up on the cooling capacity of the freezing plant. The sheet piles also affected the freeze, acting as giant radiators. They had to be screened from the sun and the wind to reduce thermal effects. The freeze was achieved in 74 days.
Problems with circulation of groundwater through voids created during drilling and by the dewatering operations hampered the freeze at times. Once the ground was frozen, the circulation of brine was confined to the perimeter tubes to maintain the freeze. A problem also arose during the removal of the sheet piles to allow commencement of tunnel excavation. The closeness of the cut piles to the underside of the box caused a crack in the ice wall, which subsequently resulted in flooding. This was tackled by the installation of an additional freeze pipe, which helped to stem the water. After a short period of wet working conditions, operations continued.
The freezing of the two boxes took place consecutively with Box 2 freezing taking place during the construction of Box 1.
Tunnelling and Jacking
Jack Knight of Charles Haswell & Partners gave an account of the actual tunnelling and box-jacking operation. The boxes sat in the launch excavation and were to be advanced by six No 307 bar jacks using wire ropes. An additional section of slab was cast at the rear of the excavation to deal with the jacking forces. The excavation was advanced using a roadheader with a backacter clearing the invert. Mucking was planned to be by conveyor, but in practice a Bobcat was used.
The initial cut under the road was the most critical activity. This went well, however, the strength of the frozen materials had been underestimated. They lay at around 60-70MPa – like a substantial rock. The roadheader was therefore underpowered for the required operations and only advanced 1.5m/day instead of the planned 2.5-3m/day. An anti-freeze lubricant was needed to assist the sliding over the frozen ground. There were no problems with the boxes sliding past each other.
A sophisticated automatic total-station settlement observation system had been installed for monitoring heave on the highway. This was not a success because of the frequency of the traffic and the vibration it generated. Manual levelling was used, despite the elevated danger to personnel from traffic on the busy highway (2000 cars/hour). The calculated heave was exceeded with 200mm being observed. This had the effect of producing cracks in the highway tarmac and some bumps.
At the conclusion of the jacking operation, the boxes joined up with cast in-situ channels at the other side of the road. Despite the normal day-to-day operational problems, the job went according to plan and proceeded to a successful conclusion.
Chairman, Peter South, then invited questions from BTS members for discussion.
