Question 3
Propose an overall schedule for the collective project, assuming we have full-block building permit and would like to construct all at one time.
Conventional vs. Hybrid Top-Down Construction
After submitting our RFQ response, we have continued to evaluate both a conventional excavation with bottom-up construction and a hybrid top-down method for construction of the project. Consistent with our initial findings, a hybrid form of top-down construction provides schedule efficiencies and accelerates tower construction by approximately six months. A summary comparison of the conventional construction schedule and hybrid top-down schedule with potential time savings is shown below. Detailed schedules for both methods of construction are also included.
Hybrid Top Down Construction Summary
Our approach for a hybrid top-down construction methodology is detailed here. In summary, our hybrid top-down approach is based on tower superstructure construction proceeding prior to the basement substructure and excavation and foundations as described below.
Shoring Traditional soldier pile and wood lagging, tied-back with anchors, are used to provide temporary support of excavation. The soldier piles are installed prior to excavation, then the lagging and tie-back anchors are installed as excavation proceeds.
Tower Columns and Core Walls Starting the towers prior to completing the basement excavation and construction is made possible by supporting the tower columns and core walls from structural steel kingposts (plunge columns) that become socketed caissons at their base. These columns are installed from the ground surface prior to excavation beginning. The kingposts are normally concreted into concrete columns as the basement slabs are constructed.
Basement Retention Walls Basement retention walls are constructed as conventional shotcrete walls against earth shoring, constructed after completion of basement excavation and foundation level.
Sequencing The tower construction is typically started from the ground floor or first basement level, following the completion of the tower column kingposts. While the tower construction is started upwards, the soil excavation and shoring continue below simultaneously. The soil is excavated below the framed slab constructed at the ground level and removed from the non top-down (non-tower) portion of the footprint. After completion of excavation and shoring, the basement structure, including basement walls and intermediate floor slabs, are constructed from the bottom of the basement back up to the surface. The towers’ structure can generally carry upwards 10 to 15 floors before the full excavation and subterranean structure is completed, but this is structure specific and will need to be confirmed by the structural engineer.
Dewatering Approach
The development will require dewatering—temporarily lowering the groundwater level—to:
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Intercept seepage water which would enter the excavation and potentially interfere with construction
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Improve the stability of the excavation, particularly the vertical excavations in a soldier pile excavation prior to placing lagging boards between soldier beams
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Prevent heaving of the portions of the excavation bottom in soil
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Reduce lateral earth pressure on temporary support of excavation elements (such as soldier piles)
The site groundwater conditions based on groundwater measurements provided in geotechnical exploratory borings and in wells indicate a range of water levels, measurements at some locations indicating groundwater below the planned bottom of excavation, and other measurements indicating groundwater above the planned bottom of excavation. In general, groundwater appears to be present at around Elevation 5190, which is around 10 feet or less above the surface of bedrock, as shown in a schematic cross-section in Figure G1. Therefore, a limited depth range of dewatering is anticipated at the site to accomplish the necessary excavation.
Figure G1 Generalized Cross-Section of Site illustrating generalized top of bedrock, groundwater level, shoring, bottom of excavation, and anticipated groundwater lowering.
Figure G2 First Image: Components of eductor system; Second Image: Educator system around perimeter of soldier beam excavation (photographs by Malcolm Drilling)
In order to accomplish dewatering, two alternatives are available:
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Cut off groundwater from laterally moving into excavation plus trenches/sumps to dewatering the base of excavation
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Lower groundwater at and in the vicinity of site sufficiently to allow permeable shoring.
A groundwater cutoff could consist of a slurry wall, secant piles, or similar methods.
A system to lower the groundwater level could consist of:
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Deep conventional groundwater wells around perimeter of excavation, combined with trenches/sumps as necessary on interior of excavation.
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Groundwater eductors around perimeter of excavation, combined with trenches/sumps as necessary on interior of excavation.
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Wellpoints installed within interior perimeter of excavation when excavation has proceeded near groundwater level, combined with trenches/sumps as necessary on interior of excavation.
At the subject site, with a limited depth zone of groundwater overlying rock, traditional groundwater wells are less effective because they are not as capable of lowering the groundwater below the surface of bedrock, and therefore would need to be spaced much closer than in a standard dewatering operation. Figure G1 illustrates traditional groundwater wells placed outside the shoring line. Because of the difficulty of dewatering with standard spacing of traditional wells, it is likely more effective to utilize groundwater eductors or wellpoints as described below.
Eductors are installed from the original ground surface, outside the planned perimeter of shoring, but at a close spacing (typically 2 ft to 10 ft on center) and have a smaller diameter than traditional wells since there is no need for a downhole pump. The eductors then extract groundwater at each location using a vacuum created within each well using venturis created by recirculating pumped groundwater into the well. Therefore, after installation of each well with associated supply and return lines, a supply manifold and a return manifold is connected to a tank and pumping system. Eductors have the capability to lift water a sufficient height to utilize at the top of the planned excavation at the site. The location of eductors is shown in cross-section in Figure G1 and is similar to traditional wells but at a closer spacing. This is illustrated in Figure G2.
A wellpoint system is similar to an educator system in that it operates by vacuum extraction of groundwater and consists of closely-spaced small diameter wells. However, a wellpoint system only requires vacuum lines from a vacuum manifold (rather than both supply and return manifolds of an educator system). The wellpoints have a limitation of pumping head such that it is not generally practical at a depth of greater than about 15 feet. Therefore, the wellpoint system could be installed on the interior of the excavation after the excavation has advanced to within proximity of the groundwater. Then the wellpoint system can be installed from the interior of the excavation, through the shoring, between soldier beams, with the vacuum manifold hung from the shoring. This is illustrated in Figure G3 and is shown in cross-section in Figure G4.
Figure G3 First Image: Vacuum wellpoint system installed at top of relatively shallow excavation; Second Image: Vacuum wellpoint system installed within deeper excavation (photographs by Malcolm Drilling)
Figure G4 Generalized Cross-Section of Site illustrating generalized top of bedrock, groundwater level, shoring, bottom of excavation, and anticipated groundwater lowering using wellpoint system.