Foundation Issues Encountered At the National Art Gallery (From The Architect’s Point of View)

Fig6_NAG Rendering_Studio Milou

Fig. 1: Discovering the tilted City Hall building: The spacing of the new columns from the existing wall varies at different levels due to the existing slanted wall.

Foundation Issues Encountered At the National Art Gallery (From The Architect’s Point of View)
By Sandy Liew

Have you ever wondered what it is like to work on a major A&A development? No, we are not talking about cosmetic renovations or minor retrofitting work – the experience that I have with the National Art Gallery project (affectionately known by the project team as the “NAG”) is an unbelievably challenging one.

The NAG project, which is currently under construction, involves the redevelopment of the Former Supreme Court and the City Hall buildings, which are both gazetted as national monuments under the URA guidelines. The external envelopes of both buildings are to be conserved, and right beneath the two existing buildings is a new 3-storey basement. Ask any of the project team members and they will tell you that calling it a “redevelopment” is an understatement!

The biggest challenge in any A&A development lies in the uncertainties posed by the existing building, both architecturally and structurally. This is even more so for the Former Supreme Court building, where the majority of the existing internal structures is being retained and strengthened for its new usage. While working on this project, we’ve encountered some interesting structural issues, some of which are unique to the NAG. In this article, references will be drawn from the project to illustrate some common issues that are pertinent in many A&A developments.

Discovering the tilted City Hall building
The existing City Hall building, which had been standing on shallow foundations, was discovered to have settled inconsistently within the building’s footprint. Fig.1 shows the slightly slanted walls of the building’s internal courtyard facades. Due to the slanted alignment of the existing walls, the spacing of the new columns from the existing walls is inconsistent at different levels, which also means that the corridor width is inconsistent at each level. This issue recurs in the project whereever new structures are positioned next to existing structures.


Fig. 2: The Contractor’s proposal for the removal of parts of existing walls (to be reinstated later; highlighted in red) to allow access for the piling machine. New piles are highlighted in purple.

Are you ever worried about how your builder’s piling machine moves on site?
In planning for piling works for the NAG, access for machinery works during construction is an important consideration for both the Consultants and the Contractor. Fig.2 shows locations of piles that are inaccessible by the machine due to existing structures or wall openings that are not big enough for machine entry. Together with the Consultants, the Contractor proposed the temporary removal of some existing structures (to be reinstated later) to accommodate the machine route (refer to Fig.2 & Fig.3). This had to be done carefully, as the building is a gazetted national monument, and it is the architects’ responsibility to ensure the safekeeping of historical finishes and elements in accordance to the conservation guidelines.


Fig. 3: Photos showing portions of existing wall proposed by Contractor to be removed for piling works access

Architects, the piling layout is not just the engineer’s business!
In addition, the existing foundations had to be carefully considered in designing the new piling layout. Fig.4 shows instances of conflicts between new and existing pile foundations at the Former Supreme Court. Possible solutions include shifting columns away from existing structures, or alternatively, creating pile-cap “hunching” to accommodate the depths of existing pile-caps (refer to Fig.5 for illustration). These solutions will undoubtedly affect the architectural layout and the usable floor area, and thus need to be closely consulted with the building user.

Making space for services in the existing building frame
The headroom of spaces is affected not only by the new structures, but also by existing ones. Interestingly, we found that the headroom of some spaces in the Former Supreme Court is limited by existing beams that are deeper than the new beams that have been added for structural strengthening. The coordination for M&E services will need to take into careful consideration both new and old structures, in order to achieve the most optimal headroom for the space.

The above instances from the NAG serve to shed light on some common structural issues that could be encountered in A&A developments.

Fig. 4: Plan showing example of new pile caps in conflict with existing pile cap

 

Fig. 5: Section showing a typical hunching detail of the new pile cap to avoid conflict with the existing pile cap.

A Summary of Useful Pointers for A&A Developments

1. Understand the building
For A&A developments, it is imperative to include architectural and structural investigative surveys at the early stage of the projects. In crafting the scope for survey works, the following should be noted:

(a)   The alignment of existing walls or structures, especially around the building facades or periphery where new columns or structures are likely to be inserted.

(b)   The height and sizes of existing foundations, also at locations mentioned in (a). If new pile foundations are positioned close to or on top of existing foundations, the dimensions of the existing foundations will be particularly crucial for the engineers. When structural survey works are carried out by surveyors prior to construction, it is often not possible to expose all the foundations within the existing building. Hence, it is useful to select areas for localized openings to obtain more accurate surveys.

It may be worthwhile mentioning that the survey works for the NAG, which were carried out during the early design stage, were constrained by various factors. For instance, the requirement to conserve existing architectural finishes and ceilings limited the extent of works that could be opened up, and access to some spaces was difficult because they were still being occupied by users.

2. Respecting the building’s geometry in the design
The surveyed information should always be closely referenced when designing new structures. As a rule of thumb, it is advisable to allow some buffer or construction tolerances between the new and existing structures to minimize conflicts in setting out. It is likely that the Contractor will also require the buffer as working space.

3. Having fun with coordination work for building services
The essence of coordination work is in creating spaces for services within the constraints of the existing building geometry.

The clear headroom survey is usually included in the architectural survey. However, it is often worthwhile to allow for (localized) opening-up works to existing ceilings and finishes by surveyors in order to obtain more accurate structural headroom surveys. The information will be useful to all disciplines, especially when it comes to services coordination.

4. Preparing your Contractor for the bumpy ride!
It is important to prepare the Contractor by highlighting potential issues during the early stages of construction. In the case of the NAG, the Contractor was briefed on the uncertainties in the condition of the existing building before the start of the project.

As soon as they came on board, the Contractor was instructed by the Consultants to carry out verifications of measurement on-site prior to the execution of works. The verification works included checking the setting out of new columns against the existing building alignment; ensuring that corridors and staircases that are adjacent to the existing tilted façade walls meet the required widths at all levels etc. The verification of setting out for new structures should be carried out way in advance of structural foundation works. The Contractor was also briefed on critical areas where dimensions are to be strictly governed, such as fire escape corridor widths, new lift shaft sizes, etc.

So, what’s the fun?
Through working on the NAG, the team learnt that preparation works and investigative surveys carried out in the early stage of the project, however comprehensive, will almost never fully reveal the condition of the entire building. The key challenge in A&A projects is in the constant checking and uncovering of previously unknown existing building and site conditions, before and throughout the construction period.

From my point of view as an architect, the NAG construction site is an ever-dynamic one. We constantly discover new elements or “surprises” – as we like to call them – during our site walks. Whenever an issue arises, it is often difficult to have an off-the-shelf solution. The challenge is in generating ideas and tailoring solutions to each problem. These unpredictable challenges are precisely the most interesting feature of the project.♦

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