Representing one of Israel’s largest and most ambitious urban renewal plans, the Jerusalem Gateway Project will feature an extensive development of office, hotel, convention and retail space centred around the main entrance to the city from Tel Aviv and the coast. Located close to the Temple Mount, it will also serve as a transportation hub catering for the high-speed train to Tel Aviv, as well as two light rail lines. Forming part of its below-ground development will be the impressive 20m wide, 30m high and 270m-long Shazar caverns for traffic and parking which will be part of the first phase of the project.

Running parallel beneath the Shazar Road with only 3-12m overburden, the two caverns pass within 5m of the 2019-completed Hauma train station and within 7m of the proposed K convention centre.

Excavation of the parking and road caverns began in April 2017 by contractor Electra Group for owner Moriah Jerusalem Development to designs by Pini Swiss Engineers. The facility is planned to be operational at the end of 2022.

Both caverns are multi-deck affairs: each has a three-lane road in the upper part and about 700 parking places distributed over five floors below (Figure 1). The distance between the parallel caverns is 3.8m at the top and 5m in the lower area. When the project is in operation, traffic will be connected by six cross passages between the caverns, five vehicle exits and eight pedestrian connections to the Convention Centre of Jerusalem, the Hauma railway station and other nearby structures.

Drill and blast was used to excavate the caverns; crews worked through weak sedimentary rock from both east and west portals simultaneously. Technically, the Shazar project is very challenging from multiple aspects: the size of the caverns, the close distance between them, the minimal overburden, the difficult ground conditions, the proximity of existing structures, and the potential earthquake activity in the area all make for a complex project.

GEOLOGY

Dolomite and dolomitic limestone of the Bina-Weradim formation are the main geological components. A 3m layer of fill lies over the upper portion of the rock mass which is highly weathered with extensive karst features (Figure 2). From 3m-7m, the weathering of the layers was classified as soft ground. Below 7m, both intact and poor jointed, fractured and crushed rock has been encountered. Sound dolomitic limestone appeared at around 30m to 40m below ground level. Major karst cavities, both empty and filled with clay, have been encountered, with one of the largest having to be filled with more than 750m3 of concrete.

During excavation of the top heading, the major geotechnical hazards were identified as the minimal overburden, the poor ground conditions and the large cavern span, the large settlements and, in the worst case, crater formation at the ground surface.

A pipe umbrella pre-support system in combination with full-face excavation was instigated over the majority of the 270m cavern length. Partial excavation of the top heading, with a central drift enlarged with two side drifts, was added and applied over the most critical areas. The primary support of the top heading consisted of a combination of steel ribs and a 450mm layer of shotcrete lining.

The instability of the rock mass below the top heading was another significant hazard. To mitigate this risk, a 1m2 longitudinal foundation beam reinforced with a longitudinal coupled reinforcement was installed at the base of the top heading span on both sides (see Figure 3). Each beam was interrupted only by the junctions of shafts and cross-passages. During the casting of the in-situ final lining, the beams were used as the foundation for the gantry crane. Prestressed rock bolts were installed below the top-heading foundation beams in order to reduce further the risk of vertical settlements of the top heading.

Due to the narrow distance between the caverns, and between the caverns and the new Hauma Station, with about 4.5m distance over a length of about 90m, the instability of the rock pillar presented a major hazard during bench excavation. To mitigate this risk, the maximum height of benching was limited to 3m. Four rock bolting patterns were designed to cope with all possible geological scenarios. The patterns included cross-bolting between the caverns using Gewi bolts with a preload of 110kN. The primary support for the walls consisted of 300mm-thick shotcrete reinforced with two layers of steel mesh. Additional strengthening was installed in the areas of the connection passages and exit shafts.

The final lining was designed without taking into account the favourable effect of the primary lining and support. To face all hazards and load combinations, the final lining is a 450mm-thick, cast-in-place steel-reinforced concrete arch in the crown; a 900mm-thick concrete slab across the top heading invert, a 300mm thick cast concrete plus the installation of precast parking slabs designed to support the ground pressure under both static and dynamic conditions. Sheet membrane waterproofing is applied behind the final lining which was cast bottom-up after the excavation of the cavern had been completed.

The potential for earthquake activity was investigated thoroughly for both the construction and operation phase of the caverns. During excavation, the behaviour of the caverns was monitored by installed optical prism total stations and prism arrays, load cells and extensometers, and by monitoring the force in the prestressed bolts. Systematic probe drilling was carried out to identify the presence of karst features in the pillar and beneath the benches. Any voids detected were filled with cementitious grouting. The longitudinal foundation beams were designed to cope with the worst-case scenario of a badly-filled karst cavity extending underneath the top heading foundation.

One of the most critical sections of the project was located in the area adjacent to the new underground Hauma Railway Station where, for a length of 90m, the north cavern runs within 4.5m of the station. The station was not part of the contract and was built before the cavern was excavated. Furthermore, the platform level of the station is actually lower than the bottom of the caverns. It was partly excavated by drill and blast, starting from a circular shaft, and partly from the surface outside of Jerusalem hill.

Previous analyses by others identified that the excavation of the caverns could have created unacceptable stresses in and around the narrow rock pillar between the cavern and the station. These stress levels, combined with the fact that the station walls are unsupported over a height of 8.5m on the vertical span, were judged unacceptable for the serviceability and ultimate state of the station, thus questioning the technical feasibility of the entire project.

…AND FINALLY

The final design consisted of an intervention in the station, reinforcing the station wall using steel plates. This solution was combined with the sequential excavation of the top-heading in the area and a risk management plan based on intensive monitoring of the wall behaviour during excavation. Deformations measured in Hauma Station during cavern excavation remained within the expected ranges and no cracks appeared in the station walls.

Excavation of the top-heading in both caverns commenced in April 2017 and was completed in August 2018. Benching in both caverns started in February 2018 and was completed in July 2019. Casting of the final lining started in August 2019 and the works are now in the final stages for the parking caverns to begin operation at the end of 2022.