Challenging excavation of parking caverns in Jerusalem Mar 2021

Two parallel caverns of 20m wide x 30m high x 270m long lie beneath Shazar Road in the Temple Mount area of Jerusalem with just 3m to 12m overburden, passing within 4.5m of the new Hauma train station and within 7m of the planned K convention centre. Excavation of the parking and road caverns by the contractor Electra Group for owner Moriah Jerusalem Development started in April 2017 to designs prepared by Pini Swiss Engineers. Opening of the facility is planned for end of 2022.

Fig 1. Location of the caverns near the historic city center

Each cavern has a three-lane road in the upper part and about 700 parking places distributed over five floors beneath. The distance between the parallel caverns is 3.8m at the top and 5m in the lower area. With the start of the operation, traffic will be connected by six cross passages between the caverns and five vehicle exits and eight pedestrian connections to the Convention Center of Jerusalem, the Hauma railway station and other nearby structures. The caverns were excavated by drill+blast and simultaneously from both the east and west portal, under shallow cover and through weak sedimentary rock. The size of the caverns, the narrow distance between them, the limited overburden, the difficult ground conditions, awareness of nearby structures, and the intense potential earthquake activity of the area, make the Shazar project technically very challenging.

Fig 2. Cross section of the caverns in relation to Hauma Station

Fig 3. Design details and excavation sequences of the caverns

The geology consists mainly of dolomite and dolomitic limestone of the Bina-Weradim formation. The upper portion of the rock mass is highly weathered with extensive karst features. Above is a 3m layer of fill. The weathering of the 3m to 7m layer had it classified as soft ground. Below 7m, both intact and poor jointed, fractured and crushed rock has been encountered. Sound dolomitic limestone appeared at about 30m to 40m below ground surface. 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.

Due to the limited overburden, the poor ground conditions and the large cavern span, large settlements and, in the worst case, crater formation at ground surface were identified as the major geotechnical hazards during excavation of the top heading. A pipe umbrella presupport system in combination with full face excavation was applied over the majority of the 270m length of the caverns. 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 45cm 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 (Fig 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 rockbolts were installed beneath the top-heading foundation beams 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, the instability of the rock pillar presented a major hazard during bench excavation. To mitigate this risk, the maximum height of bench excavation was limited to 3m. Four rockbolting 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. Primary support of the walls consisted of 30cm thick shotcrete reinforced with two layers of steel mesh. Additional strengthening was applied in the areas of the connection passages and exit shafts.

Fig 4. Design detail of the caverns

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 45cm thick cast in place steel-reinforced concrete arch in the crown, a 90cm thick concrete slab across the top heading invert, 30cm cast concrete and installation of precast parking slabs designed to support the ground pressure under both static and dynamic conditions. A sheet membrane waterproofing is placed behind the final lining (Fig 4). The final lining was cast from the bottom up after completion of cavern excavation.

The potential of 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 prisms 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. Detected voids 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.

Cross passage of the parallel caverns in the top-heading

One of the most critical sections of the project was in the area adjacent to the new underground Hauma Railway Station where, for a length of 90m, the north cavern runs within 4.5m proximity to the station. The station was not part of the contract and was built before the cavern. The platform level of the station is lower than the bottom of the caverns (Fig 2). It was part excavated by drill+blast, starting from a circular shaft, and part from the surface outside the Jerusalem hill. Previous analyses by others identified that the excavation of the caverns may create 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, was judged unacceptable for the serviceability and ultimate state of the station, thus questioning the technical feasibility of the entire project.

Benching of the lowest of seven levels with a view into the two-lane vehicle exit and connection to the conference centre

The final design consisted of an intervention in the station, reinforcing the station wall using steel plates. This solution was combined with sequential excavation of the top-heading in the area and a risk management plan based on intensive monitoring of the wall behaviour during excavation. Deformation 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 started 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. Value of the construction contract and the total investment in the project was withheld and works are in the final stages for the parking caverns to start operation at the end of 2022.