Cockburn Gateway Stage 3 – Top-Down Construction

Bassam Matty BSc MSc FIEAust Director, Airey Taylor Consulting

Abstract: This paper discusses a unique construction method adopted at the early structural design stage which delivered 20% reduction in total investment cost while eliminating the need to underpin the adjoining building. The development comprises 19,200m2 of underground parking, 21,200m2 of ground floor retail and 19,700m2 of part retail and part car-park upper floor. The cost reduction was achieved through the use of the top-down construction technique and a unique retaining wall system developed by ATC. Top Down construction commences at Ground Floor level and progresses aboveground while the soil is simultaneously mined underneath. The design of special steel-encased pile shafts, for belled-end CFA piles to form the basement’s columns and the use of short piles with Micro-Fine cement grout to construct a proprietary reinforced cemented-soil-block retaining system are discussed. The Top-Down construction technique combined with the use of hollow core planks for the first floor resulted in a total reduction of formwork exceeding 80% when compared with conventional construction. The use of post tensioning significantly reduced the concrete used. A special fibre reinforced floating floor topping was used to facilitate the next stage of development. The plinths for the car parking lots were designed using synthetic fibres to enable future crushing and re-use as road-base for the next stage of the development. Proprietary Styrofoam square formers topped with fibre-reinforced concrete were used to construct a lightweight floating floor for the retail area which also allows for the future re-planning and service reticulation of the first-floor car park retail area without impacting the ground floor retail operation. Keywords: Construction, Sustainability, Innovation, Top-Down, Design.

1. Background

The Cockburn Gateway Shopping Centre is located in the City of Cockburn, Western Australia, approximately 21 kilometres south of the Perth City Centre and 17 kilometres west of the City of Armadale. The development site is located on the south western side of the Kwinana Freeway and Beeliar Drive intersection. The existing Gateways Shopping Centre is located to the south of the site and stage three of the development will extend the shopping centre to the north replacing car parks which are adjacent to the existing Centre and over a vacant sand area to the far north of the 7ha relatively flat site.

The stage-three development of Gateways Precinct Shopping Centre, Cockburn is the first stage of the planned expansion of the existing Gateway Shopping Centre. The plan is to expand the shopping area from 26,460m2 Net Lettable Area (NLA) of shop/retail floor space to accommodate a total of 55,050m2 (NLA) and the expansion of the current non-shop retail floor space from 10,600m2 (NLA) to 49,490m2 (NLA)(1).

The stage-three development comprises 19,200m2 of underground parking, 21,200m2 of ground floor retail and 19,700m2 of part retail part car park upper floor. The top floor is designed with due consideration to the future utilisation of the entire upper floor as a retail area.

The ground sub surface profile consists of varying thicknesses of sandy fill overlying naturally occurring Bassendean sand with interbedded Coffee Rock bands. The historical groundwater level is approximately 10m below the existing ground level(2).

Figure 1. Existing Shopping Centre on Cockburn Gateways Site.

2. Construction Method

2.1 Constraints

The estimated cost of the proposed development, utilising conventional construction, was assessed by the project Quantity Surveyor to be in the range of $ 106m-112m. This cost made the investment commercially non-viable. Additionally, conventional open excavation for construction of the extensive basement imposed challenges to the operation of the existing shopping centre due to impacts on car parking bay availability, the northern entry, site access and the main northern service route that feeds the northern section of the centre.

2.2 The proposed solution

We proposed an alternative construction method using a tailored Top-Down construction technique that addressed the operational restraints of the shopping centre and substantially reduced the construction cost and project delivery time without compromising the architectural intent. The alternative we presented was based on our past experience with a smaller sized building with comparable site geology, where we successfully developed and implemented similar techniques. The alternative construction method proposed allows construction of the above-ground structural elements to precede the basement excavation, thus facilitating overlapping construction activities and early site access to other trades to fit-out the retail areas while the mining of the basement progresses.

The architect configured the basement using 8550mm grid spacing in both directions with columns located at the grid intersection; this arrangement facilitates locating three spacious car-parking bays between the columns, with 2700mm clear car-bay width, leaving 450mm structural space for the columns. The same columns continue to support the 1

st Floor slab, with the exception of the elimination of 6 columns to form a

column-free 17100mm span at the major retail.

A post-tensioned floor plate was selected for the Ground Floor slab. The slab was designed to be cast directly onto the compacted sand. The post-tensioning provides economy by reducing the slab thickness

requirements for punching shear and long term deflection as well as facilitating the driving of heavy construction equipment on the slab at an early age, prior to excavating the basement as the pre-compression introduced increased the slab flexural capacity.

Assessment of the soil bearing capacity revealed the need for large diameter piles driven to a depth of 9.0~10.0m to achieve the geotechnical capacity required to support the column loads. The use of small shaft, enlarged base (belled-end) piles was investigated; 500mm diameter pile shaft with 900mm diameter belled-end base was selected. As the selected pile shaft was larger than the 450mm structural space considered by the Architect, a grouted pile was chosen to facilitate the installation of steel CHS caisson to form a smaller shaft for the exposed section of the pile shaft (407mm O/D).

3. Top-Down Construction 3.1 Overview

The use of Top-Down Construction is conventionally employed in civil engineering to construct ‘cut and cover’ tunnels in developed areas where confining the construction space and limiting the soil movement without the use of anchors is essential. Top-Down Construction is also occasionally employed for the construction of multiple level underground basements in High Rise buildings to gain a head-start for building the superstructure and overlapping construction activities. Top-Down may also be utilised to reduce the risk of buoyancy under the action of high water table forces by introducing enough resistance through the weight of the superstructure prior to casting the base.

In Top-Down Construction, the walls are constructed first to support the excavation and form the final external structural walls. For buildings and wide tunnels, permanent piles or wall elements are installed internally between the soil retaining walls to reduce the span of the roof and floors. Next the roof is constructed and tied into the walls; the surface is then reinstated, in the case of tunnels, or the building superstructure construction commences before or during the undercover excavation. The remainder of the excavation is completed under the protection of the top slab. For multi-level underground construction, casting the floor precedes the excavation under the slab level. Upon completion of the excavation, the base floor is completed and tied into the walls. The finishes are installed within the completed structure.

Top-down construction offers several advantages when compared to traditional open excavation (bottom-up) construction:

• The temporary support of excavation walls are used as the permanent structural walls; • The structural slabs will act as internal bracing for the support of excavation thus reducing or

eliminating the amount of tie backs required; • It requires less width for the construction area; • Easier construction of slabs and reduced formwork need, as it is cast on prepared grade rather

than using bottom forms; • It results in lower cost by the elimination of the separate, cast-in-place concrete walls within the

excavation and reducing the need for tie backs and internal bracing; • It results in shorter construction duration by overlapping construction activities.

Top-down construction can impose a number of challenges and disadvantages including:

• Inability to install external waterproofing outside the external walls; • Potential water leakage at the joints ; • A risk that the exterior walls (or centre columns) will exceed specified installation tolerances and

impact the interior space; • Limited space and restricted access for excavation and construction of the floors below.

Figure 2, Cut-and-Cover Tunnel Bottom-Up (a) and Top-Down (b) Construction Sequence (3)

3.2 Identifying project specific Top-Down challenges

To address the space and access constraints, it was resolved that mining the dry Bassendean Sand under the post-tensioned slab using small excavators was suitable as long as proper ventilation for exhausting the fuel fumes and adequate confined-area work training for the workers was employed. The amount of services and finishes are typically limited in a basement and falls outside the critical path of the project planning, thus the extra time required to complete mining of the sand under the Ground Floor slab can be easily accommodated.

This leaves two main drawbacks of the Top Down construction technique, being the installation tolerance of the internal columns and the waterproofing of the external envelope. Both of these issues are addressed through the design adopted.

4. The Foundation System

4.1 Pile/Column Design

Replacing the columns and footings with belled-end piles centrally located under the Ground Floor columns, at the grid intersections, with high positioning accuracy required careful design consideration. The Piling Code, AS 2159-2009 allows for 4% inclination to the vertical (4) in addition to out of position tolerance; if such tolerances were to be allowed, the highly loaded pile shaft had to be designed for a significant bending component in addition to the axial load due to the fact that the pile/column loses its lateral restraint from the surrounding soil following mining of the basement’s soil.

To address the pile shaft size and verticality issues, rigid steel CHS caissons with centrally located UC sections and interlaid reinforcing cage were chosen to replace the conventional steel reinforcing cages of the piles within the basement’s excavated zone to form composite columns capable of providing the required load capacity (Figure 3). Extending the UC sections 1300mm above the Ground Floor slab level provided the required lap length for the Ground Floor columns’ reinforcement; at the same time, the above-ground leaver provided a measure for checking and adjusting the caissons verticality during installation.

To achieve the fire rating of the basement’s columns, the interlaid reinforcing cages inside the CHS caissons were sized so that, for each pile, the central UC section and the reinforcing cage were capable of providing the fire design load capacity when acting compositely, while the CHS caisson was considered sacrificial. Upper and lower vent holes were provided in the caissons to relieve the hot gas pressure so that bursting of the CHS caissons in the event of fire is prevented(5)(6).

Figure 3, Details of the rigid shaft and the belled-end pile reinforcement.

4.2 Pile/Column Detailing

The pile reinforcement was detailed to have independently fabricated site lap spliced sections, a bottom reinforcing cage and an internally reinforced rigid caisson. This arrangement facilitates the lifting of the rigid caisson from a central hole located centrally to the UC section without damaging the rather flexible bottom reinforcing cage. The single point lifting allows the caisson to act as a plumb bob enabling higher accuracy in achieving the column verticality. To control the pile cut-off level, 75PFC sections were welded to the central UC section to guide the installation and enhance the punching shear resistance of the slab.

Figure 4, Caisson lifting and cage splicing.

4.3 Pile/Column Installation

To achieve high control in positioning the piles, the piling contractor elected to cast in-situ concrete guides cast around cylindrical polystyrene foams centred to surveyed pegs. Surveying the top level of the concrete guide prior to pile installation provided the necessary means to achieve high control on the cut-off level. The technique halved the out of position accuracy recommended in AS 2159-2009. Further adjustment was achieved by controlling the lowering of the caisson into the grout. Post construction survey of the projected UC sections of the piles confirmed the conformance of the installed Piles/Columns with the tolerances for structures and members recommended in Clause 17.5, AS 3600-2009(7).

Figure 5, Pile installation through the cast-in guide.

5. Soil Retention

Using Continuous Flight Auger Piles (CFA) along the external perimeter of the basement and casting the Ground Floor slab prior to excavating the soil offered the advantage of increased Pile rigidity through the rotational restraint imposed on to the pile’s top end by the slab (Typical connection - Figure 6). With the increased rigidity it was possible to reduce the diameter and depth of the reinforced piles and use Micro-

fine cement injection to grout the soil block between well seperated reinforced piles (Figure 7). The Micro-fine cemented block formed an infill between the reinforced pile while the reinforced piles provided the necessary wall strength and stiffness.

Figure 6, Slab / Retaining wall typical detail.

Figure 7, Reinforced cemented-block retaining wall.

Waterproofing the external envelope was achieved through the design of a free-draining cavity wall around the external envelope. This was achieved by building an internal masonry leaf off the basement slab and maintaining the basement slab short of the piled wall to create a free-draining trench (Figure 8). This formation was used to construct a plenum to circulate the air in the basement and exhaust the fumes.

Figure 8, Detail of the free-draining cavity wall.

6. Superstructure

Stainless steel high-capacity shear connectors were used to prevent double the number of columns along the movement joints of the building. To optimise the design of the post-tensioned floor slab, the joints were located 1600mm away from the support line to provide negative bending moment to one end span and shorten the end span of the adjoining plate. (Figure 9 – detail; in Figure 10 – implementation)

Figure 9, Use of shear connectors at movement and construction joints.

Figure 10, movement joints location and edge detail.

The ability to drive heavy construction machinery on the Ground Floor slab prior to mining the soil provided the opportunity to maximise the use of precast pre-stressed hollowcore planks for constructing the 1st Floor deck; this eliminated the need for long reach cranes. Cast in-situ post-tensioned band beams were designed to be monolithically cast with the hollowcore plank topping. This arrangement facilitated the use of linear formwork towers to support the tray for casting the beam base while supporting the installed precast planks (Figure 11).

Delayed mining of the basement meant large linear loads, as a result of supporting the planks, topping and beams, were directly transferred to the soil-supported ground floor slab, completely eliminating the need for back-propping. The band-beam and precast propping arrangement (Figure 11) significantly reduced formwork needs for the upper deck, and, by casting the ground floor slab on compacted soil and the use of proprietary reinforced cemented soil-block retaining, we achieved an 82% reduction in the formwork needs for the project when compared with conventional cast in-situ construction.

Figure 11. Typical band-beam detail with linear formwork arrangement.

7. Future-proofing the deck

To facilitate the planned future expansion of the retail area (which will incorporate the 1st floor car-parking

deck as future retail space) a floating floor was used. The space provided through the floating floor allows reticulation of the services above the structural floor and connection to pre-installed service risers. This configuration will facilitate the freedom of future service locations and will not affect the trading of the ground floor retail area. The 290mm thick waffle floor was designed with 65mm topping (Figure 12 below) which allowed us to incorporate commercially available 1080mmx1080mmx225mm polystyrene void former pans, typically used for construction of raft floors for housing on clay soils. To construct the slab with reduced workmanship the use of steel fibre reinforced concrete at 15 kg/m3 fibre dose rate, without supplementary reinforcing bars or mesh, was used. This configuration provided the required strength to resist the retail area design loads. The large void ratio of the waffle configuration assisted in reducing the design dead loads for the structure.

An independent plinth to falls was documented for the car-parking area (Figure 12) for drainage. The plinth is designed to be fibre reinforced concrete utilizing synthetic fibres. The use of the synthetic fibre achieves sustainability as it allows the plinth to be easily recycled as crushed road-base material, for the full removal of the plinth and replacement with a 290mm thick waffle is planned for the next phase of the development.

Figure 12, Floating floor and car-bay deck plinth.

8. Conclusion

The tendering process revealed the reality of the advantage offered by the tailored top-down construction technique as opposed to conventional bottom-up construction for this particular project. Four contractors

were invited to price the job. Three of the contractors provided their bid for constructing the building as documented with a variation of less than 5.5% between them, while the fourth bidder was fully against the use of the top-down technique and offered an alternative bid to re-design and construct the development at a confirmed price of $95.5m. The project was awarded to the lowest bidder for a total construction cost of $79.7m, which is 16.5% lower than the alternative bid and about 25% lower than the QS estimate for the projected project cost using the conventional construction referred to earlier.

The capital cost saving is a direct result of the top-down construction technique which lead to achieving the following cost cutting advantages:

• Maintaining road access to the existing shopping centre during construction of Stage Three; • Reduced need for temporary car parking relocation; • Elimination of the need to alter the main service supply route to the operating shopping centre; • The existing shopping centre is able to remain 100% operable during construction of the new

extension; • Significant reduction in the requirement to underpin the existing structure; • 82% reduction in formwork usage; • Reduced excavation volume and backfilling requirements; • Reduced concrete volume; • Significant saving in the cost of retaining the soil; • Maximisation of off-site production; • Eliminating the need for waterproofing of the retaining walls; • Shorter construction duration by effective overlapping of construction activities and providing early

start to critical path activities.

The approach achieved higher sustainability through the effective reduction in concrete and formwork usage and reduced excavation volume and compaction extent.

This design offered the owners the advantages of significant reduction in capital cost, lower holding cost and much earlier occupation by their principle tenant.

9. Acknowledgement

The writer would like to acknowledge the support provided by Mr Peter Airey and all Airey Taylor Consulting staff for their hard work and dedication in delivering this project. Mr Stuart Coutts and the engineers of Belpile Australia Pty Ltd were critical in devising the installation technique for internal pile set- out and installation within very tight tolerances that made the construction technique viable.

10. References

1. “Gateways Precinct Local Structure Plan”, The Planning Group - WA, Report 708-034, February 2012.

2. Geological Survey of Western Australia. 1:50,000 Environmental Geology Series,Fremantle.

3. C. Jeremy Hung, PE, James Monsees, PhD, PE, NasriMunfah, PE, and John Wisniewski, PE, “FHWA Technical Manual for Design and Construction of Road Tunnels – Civil Elements“, Report No. FHWA-NHI-10-034, March 2009.

4. Piling – Design and Installation, AS 2159-2009, Standards Australia.

5. Michael G. Goode, “Fire Protection of Structural Steel in High-Rise Buildings”, Building and Fire research Laboratory, NIST GCR 04-872, National Institute of Standards and Technology, July 2004.

6. S J Hicks BEng, PhD, G M Newman BSc(Eng), CEng, MIStructE, MIFireE, “Design Guide for SHS Concrete Filled Columns”, The Steel Construction Institute, 2002.

7. Standards Australia, “Concrete Structures”, AS 3600-2009, Standards Australia Limited, Sydney, Australia