TC - Ross Wylie
Submission to the Nelson City Council on the Main Hall of the Trafalgar Centre.
1. Preamble. I have read a number of the reports provided by the Nelson City Council to the public to inform them of matters relating to the stability of the main hall of the Trafalgar Centre in the event of an earthquake. The raft of unpredictable earthquakes of various magnitudes which occurred in the Christchurch CBD and environs caused extensive damage to a wide range of buildings. One must assume that taking these lessons on board the design capability of engineers, architects and builders are better equipped now to design buildings for earthquake prone areas in NZ. However the speed and the method by which the Trafalgar Centre was closed to public use was astonishing, which in my opinion can be likened to punishment before the fact. This submission was prepared with assistance of well informed people and reports and is intended to provide a picture in the minds of the reader of the reinforced concrete framework of the main hall constructed to maintain its integrity according to the building standards required at the time of building. Considering the huge weight of reinforced concrete on this site, there has to my knowledge been no report of subsidence or sign of stress fractures in the reinforced concrete.
2. Start of construction. The work on building the main hall of the Trafalgar centre commenced about May 1971 following a comprehensive set of detailed plans prepared under the direction of Sanders and Lane, consulting engineers, Nelson. There were amendments to the plans/specifications as the centre’s consultants incorporated modifications to increase the overall effectiveness and strength of the structure of the building.
3. Frank Piling System. One notable change was the use of the Franki Piling System (1) replacing the use of precast concrete pile of a nominal length of 30 feet. This was a very significant development as it enabled piles to be cast to a length that struck a soil formation which met the requisite minimum loading bearing properties for the piles (49 tons). There were other advantages of this system a couple of which were firstly the capacity of the system to use a solid metal casing about 30 foot long and through which a drop hammer can densify soil of acceptable bearing capacity, at the bottom of the casing and in so doing forced a plug(s) of concrete below the casing to form a bulb or a mushroom type foot which improves significantly the bearing capacity of the pile and in so doing imbedding the bottom of the pile in the substrate; secondly when withdrawing the casing during the simultaneous process of charging the casing with concrete, a unique tamper was used to densify the concrete forcing it into and against the soil wall. Thus the pile diameter increased at least to the exterior diameter of the casing providing an extra thickness of concrete outside the metal reinforcing, thereby ensuring the design and shear strength of the pile.
4. Design features. The building design required piles along the outside perimeter of the building and these were interconnected with a concrete beam 3 foot deep over their pile caps then to the vertical columns and block work that formed the perimeter framework of the seating stands. Inside this perimeter structure, near ground level, there are transverse 3 foot deep concrete beams connecting with the top of the pile caps of either the single, double or treble piles and from which concrete columns were formed to complete the concrete framework of the seating stands. The concrete wall on the Eastern side of the main hall was buttressed.
5. Enhancing stability of the superstructure. The transverse concrete beams above the piles (inside the perimeter beams) were only formed where the reinforced concrete seating area was to be built and not underneath the main hall’s wooden floor. However, through them were ducts, which passed underneath the wooden floor, from the East side to the West side within which high tension cables were fed, tensioned, fastened and then grout forced into them to stabilise their position. This measure was a means of enhancing the stability of the ground positions of beams and pile caps and therefore firmly stabilise the architects ground positions of the opposing seating stands.
6. Integrated grid work of cohesive strength. This project then resulted in at ground level an integrated grid of reinforced concrete piles, pile caps, large beams and tension cables having a powerful cohesive force in resisting damage to it and the seating stands during an earthquake.
7. Roof of the Main Hall. Laminated wooden arches to which a metal roof is attached on the outside, cover all the inside of the main hall. To improve the stability of this roof it has been suggested that tie rods of sufficient strength be connected transversely between the bases of each of the arches and tensioned. This measure is to reinforce the cohesiveness of the current shape and structure of the roof and thereby prevent lateral widening of the bases of the arches and eliminating the potential spreading and possible collapse of the stadium roof as a consequence of an earthquake.
8. Soil profile in relation to liquefaction. Liquefaction of soil particles is well known now in New Zealand as a consequence of the earthquakes in the eastern suburbs and townships of Christchurch. Sand boils occurred together with building fracturing and slumping of concrete slab floors along with general settlement of the land. Whereas other areas of Christchurch were unaffected by liquefaction.
The next paragraph provides a subsurface soil profile of the land where the main hall of the Trafalgar Centre is sited. This soil profile is published in the Trafalgar Centre Geotechnical Report prepared by Tonkin and Taylor, June 2013.
9. Subsurface soil profile of the land under the main hall of the Trafalgar Centre (2)
1. Up to 3.8 m thickness of reclamation fill, consisting of reclamation fill, consisting of CLAYS, SILTS, SANDS, and GRAVELS, with layers of ash and also some organic material. The fill to the east of the Trafalgar Centre is known to contain domestic refuse. This material overlies.
2. Up to 2.0 m thickness of alluvial soils. Consisting of loose to dense sandy GRAVEL with minor silt overlaying.
3. Up to 1.5 m thickness of estuarine deposits consisting of SILTS, sandy SILTS, and clayed SILTS, overlaying .
4. Up to 16.0 m thickness of alluvial soils, consisting of loose to dense sandy GRAVEL with minor silt, overlaying.
5. Port Hills GRAVEL (PHG) to an indeterminate depth.
Assuming the heavy load bearing piles are a nominal 11m long the base of the piles are imbedded in loose to dense sandy GRAVEL with minor silt. The bed of each pile had to have a minimum of 49 ton load bearing capacity.
10. Soil Liquefaction (3)
10.1 Soil liquefaction describes a phenomenon whereby a saturated or partly saturated soil substantially loses strength and stiffness in response to an applied stress, usually earthquake shaking or other sudden change in stress condition, causing it to behave like a liquid.
10.2 Deposits most susceptible to liquefaction are young (Holocene-age, deposited within the last 10,000 years) sands and silts of similar grain sized (well sorted), in beds at least metres thick, and saturated with water. Such deposits are often found along stream beds, beaches, dunes, and areas where windblown silt (loess) and sand have accumulated. An example of soil liquefaction is earthquake liquefaction.
10.3 Therefore one can understand why the designers of the main hall of the Trafalgar Centre found that the best soil foundation material available for the piles is present in level 4 of the soil profile. In addition it appears that the components of this part of the subsurface soil profile are less than satisfactory for liquefaction because of the absence of well sorted silts and sands of similar grain size which are metres thick. As well, gravels are present in level 4 of the profile.
10.4 By the builders compacting soil on which a concrete pile will be cast using the hammer of the Frankie piling machine (refer para. 3 above for the technique) will reduce the pore space for water in the soil material, decreasing or eliminating the likelihood of the soil liquefying under the pile base during an earthquake.
10.5 Remember that there are specific criteria for soil before it will liquefy as is stated above and as the evidence in Christchurch has demonstrated. Should it liquefy anywhere above the base of the piles and if the liquefied soil moves laterally there is no proof that it will cause lateral movement of the piles nor is it likely that such movement of small particles will exceed the shear strength of the piles simply because of the flow characteristics of liquids.
10.6 Liquefaction of soil below the densified pile bulb would be a problem but not life threatening. It would be unlikely to occur as the characteristics of the soil there, as described in para.9/4 are very unlikely to be subject to liquefaction.
10.7 Buildings in Nelson which have been built on sandy-silt soils metres thick should be concerned about their building if an earthquake occurs of the magnitude or greater than that which caused liquefaction in Christchurch.
10.7 For a senior engineer of the Nelson City Council to say in public that the main hall of Trafalgar centre is sitting on piles 3 metres long when in reality they are about 11 metres long, one wonders whether any engineering report/assessment on the main hall, paid for by the ratepayers, has grasped the practical issues of this matter.
(1). Franki Piling System. http://en.wikipedia.org/wiki/Franki_Piling_System.
(2). Tonkin and Taylor Ltd, Trafalgar Centre Geotechnical Report June 2013
(3). Soil Liquefaction, http://en.wikipedia.org/wiki/Soil_liquefaction
Other reports read: Dunning Thornton – Trafalgar Centre Seismic Evaluation Peer Review addressed to the Nelson City Council; R.O. Davis – Geotechnical Consulting, Peer Review of Trafalgar Centre Seismic Evaluation addressed to the Nelson City Council
Acknowledgement: I acknowledge the knowledge and expertise of Kerry Neal whose understanding of this subject became a fountain of enlightenment.
93 Mount Street,