The excavation shown in this photo is part of the construction of a metro station, beginning at the basement levels of an old masonry building and extending below its foundation. In this article, I will discuss the design, planning, and cost management of the station building in a metro project constructed under a Design & Build contract in Stockholm. Design & Build contracts typically define only the project’s outline and require the contractor to develop detailed plans. As a result, the contractor is directly responsible for the design. Since the contractor handles the project design, time is saved, and costs are managed more effectively. I will provide a detailed analysis of this later. As illustrated below, the entrance to the metro station will be through the basement level -1 of this old building, followed by a descent to basement level -2, then to the ticket hall, and finally further below the foundation. Additionally, the Client’s general planning indicated that we would perform an open excavation adjacent to the building, allowing for the creation of an elevator tunnel from the base of this excavation to the platform tunnel. Next, we needed to integrate the ticket hall building, which would be constructed inside the existing structure, with the station building being built in the open excavation outside, resulting in a cohesive station complex. Consequently, upon completion of the construction, we would have created a station that does not alter the external silhouette and remains entirely within and beneath the building.<\/p>\n
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We would conduct the design, planning, and cost processes for all these tasks concurrently and submit a bid for the tender, accompanied by a methodology document titled “Added Value.” During this process, we collaborated with SWS, a reputable design firm, to obtain technical support and jointly develop a preliminary design. Based on this design, we planned a deep excavation for constructing the open portion of the metro station (the section outside the building), commencing right next to the building, as illustrated in the picture below.<\/p>\n
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This deep excavation would extend below the foundation level of the building. As the excavation progressed, both horizontal earth loads and the loads imposed by the building would exert lateral forces on the excavation area. We planned to support the excavation by installing steel piles, referred to as RD Piles, around the entire excavation area, including the section that intersects with the building, and anchoring them into the bedrock. We also intended to install steel struts at intervals to counteract the lateral forces generated by both the building and the soil. This approach would ensure stability by supporting both the soil and the building, allowing us to carry out the excavation safely. We needed to complete this open excavation promptly because the specifications outlined a milestone for finishing the elevator tunnel that would connect to the platform. Waiting for the completion of the main line NATM tunnel before constructing a tunnel to connect to the elevator tunnel would be too late; therefore, the entrance to the elevator tunnel would be constructed from this deep excavation. While designing the excavation support system, we were simultaneously planning the excavation process. The excavation base was deep, and the boom length of a standard excavator could not reach this depth from the surface. While making these plans, we modeled them in real time, as shown in the photo below, to enhance detail visibility, improve planning, and prepare the methodology document more efficiently. As the excavation progressed and deepened, it became impossible to lower and maneuver an excavator into the excavation area because the strut beams we had placed were not wide enough for a standard-sized excavator. Consequently, using smaller excavators would require additional work time and more machinery.<\/p>\n
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To achieve this, we planned to use a long-boom excavator specifically designed for such situations. We held meetings with construction equipment manufacturers to discuss and finalize our plans for purchasing and renting excavators suitable for the excavation depth. The design for the exterior of the building was now nearly complete. Simultaneously, we were conducting planning and cost calculations alongside the design process. First, the site would be cleared, followed by soil tests, the installation of RD piles suitable for the soil parameters, and the placement of struts as the excavation progressed step by step until we reached the excavation base. Completing all of these tasks with a single subcontractor was not feasible. We held meetings with different contractors for soil tests, excavation support, and excavation. Simultaneously, we conducted a price analysis to estimate the approximate cost if we carried out these tasks in-house. While we continued our design and cost calculations outside the building, our planning progressed inside as well. The exterior work was relatively straightforward, but the real challenges awaited us inside. The -1st basement floor of the building served as the entrance to the metro station. Currently used as a parking lot, it would undergo only minor demolition and renovation work. Access from the street level was straightforward, requiring only the existing ramp. However, no ramp or road existed for vehicles to access the -2nd basement from the -1st floor. To lower the construction equipment from the -1st floor to the -2nd basement, we needed to cut the floor slab of the -1st basement and two columns without disrupting the building’s structure. Additionally, we had to support the building with an external support system<\/p>\n
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We planned several scenarios to lower the work machine one floor down (-1 to -2):<\/p>\n
Scenario: We would position a mobile crane on the street and this crane would extend its boom horizontally into the building from the basement entrance door and lower the machine.
\nScenario: We would cut the floor and then fill the basement with soil and lower the vehicles that way.
\nScenario: We would attach a hoist to the ceiling of the -1st basement floor and use it to lower the machine.<\/p>\n
We eliminated the first two scenarios for various reasons and ultimately decided to install four hoists in a balanced manner on the ceiling to lower the work machines from the -1st basement floor to the -2nd basement floor by connecting them to these hoists. We needed to balance the weight of the work machine load acting on the hoists, which impacted the upper floor. This required an additional steel support system. Each plan incurred additional costs and time, and as we continued our work and planning, we realized why the total work duration specified for this job was so lengthy. After lowering the construction equipment, we needed to cut the floor of the second basement level of the building, specifically the reinforced concrete at its foundation. Following this, we had to excavate the ground beneath the building. However, cutting the foundation concrete would shift the building’s center of gravity, leading to an eccentricity between the geometric center of the building and its center of gravity.<\/p>\n
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To prevent eccentricity, we planned to support the entire area to be excavated with RD piles by driving them into the ground from the foundation concrete surface inside the building before cutting the foundation. Therefore, after cutting the foundation, the building would remain unaffected, and the lateral loads would be supported by the RD piles. We modelled this situation as follows.<\/p>\n
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However, the height of the mast of the RD pile driver, which grips the pile, was greater than the height of the building floor. According to our measurements, the pile driver would not fit inside the building when positioned upright to drive piles. This is the best part of Design and Build projects! \ud83d\ude42 If we had only approached it from a designer’s perspective, without considering implementation or planning, we would have performed the standard RD pile calculations and left it at that. The contractor company would then figure out how to implement it. However, when you are the one who designs, plans, and implements\u2014essentially the one who will actually spend the money\u2014you consider all the details.
\nSince the mast length of the RD pile machines exceeds the height of the interior floor of the building, we needed to use a smaller machine. This allowed us to drive smaller diameter piles. Therefore, we coordinated with the design company to use micropiles instead of RD piles for excavation support. This way, we could drive the piles from inside the building, then cut the foundation and continue our excavation.<\/p>\n
And working hours. To minimize the impact of pile driving vibrations and excavation noise on residents of the surrounding buildings, we had to adhere to the working hour restrictions specified in the administrative document. This meant we could not work on certain days and during specific hours. Therefore, we carefully incorporated these time constraints into our cost planning. We developed our plan by selecting the most optimal scenario based on time, cost, ease of implementation, safety, and regulatory limits from various alternatives, allowing us to calculate our overall costs effectively.<\/p>\n","protected":false},"excerpt":{"rendered":"
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