Combined NZGS CSG NZSEE Meeting
6.00pm – 8.00pm, Thursday 4 November 2010
Lecture Theatre E1, College of Engineering, University of Canterbury
This Canterbury Technical Clearinghouse meeting summary was prepared by Bruce Deam on behalf of the NZSEE. This version (0.1 - 10 Nov) is waiting for corrections from the contributors. Click on contributor's names to send them an email message.
Nick Harwood (Coffey Geotechnics and NZGS Canterbury Branch Coordinator) presented the agenda for the meeting.
John Snook (John Snook Consulting and Canterbury Structural Group chair) asked for suggestions for issues that need discussion.
Nick Harwood is interested in harvesting the issues that people are facing to provide contribution to the NZGS to advance the profession.
Helen Rutter (Aqualinc Research Ltd) outlined some of the impacts the earthquake had on groundwater. Research groups are coordinating their investigations to avoid overlapping. While there is interest, there is no funding to understand how unconsolidated aquifers are affected.
Helen presented a large number of observations: Damage to bores; with subsidence around top of bore as the sediment column around bore collapsed, bores that ‘popped up’ and had casings damaged (close to the Greendale fault).Damage to pumps by sand, silt, rust and impact against bore casingWater levels generally rose (by as much as 40 m but mostly by 1 to 5 m) and appear to have dropped in the confined aquifer away from fault. The trends in some have been to continue to increase since earthquake.Artesian flows have tended to decrease (up to 75%), possibly due to declining groundwater pressure (although irrigation is only starting now).Surface water changes include short-term flow increases in some rivers and a possible longer-term increase in Halswell due to narrower channels and bed lifting.Unconfined aquifers only appear to have short-term effects (but more data is required).Water levels were higher than normal
They are still assessing what has caused the effects, particularly whether liquefaction has affected shallow bores. They will be collecting and collating data using summer students (which need funding) and need to inform the community.
In response to a question, Helen indicated that they are still to investigate areas of lateral spreading and change in land level. The Christchurch City Council has had problems with bores but their focus has been on repairing pipe breakages rather than monitoring the aquifer response.
Dave Brunsdon (Engineering Advisory Group) outlined the EQC's brief for the Group. The outline of the guideline document is the same as previously presented.
He indicated that the context is that the earthquake and its effects are complex. House performance still requires ongoing research to understand the performance. House repairs are governed by insurance and regulatory requirements. This includes the contracts with offshore reinsurers and needs to carefully balance the claims process (fair and reasonable entitlements) against the long term objectives (such as improving and adapting codes).
The group's draft is being prepared, will be reviewed at a workshop on 10 Nov to produce a draft for the DBH by 15 Nov as the basis for the guidance document. The initial review will be councils & insurers, followed by their contractors & their teams and eventually a wider audience.
Dave outlined some of the key features in the draft. The most significant was restoration methods for the three major types of foundations: timber floor on piles, floor and perimeter concrete foundation beam and slab on grade. The focus is on the foundation repair and rebuild process and is for land where the remediation has addressed future risks (i.e. land zones A, B & C).
The guideline addresses both relevelling (restoring the previous situation) and rebuilding (improving future performance during liquefaction). It provides options with 12-16 step method statements and indicative designs that still need engineering input but avoid each situation being reengineered from scratch. These include:Relevelling foundations and floors using, for example, edge beam jacking with portable jacks, piles and urethane foam [as described later in the meeting Ed.]).Rebuilding the foundation and floors by disconnecting and lifting the house, moving it, parking it and reversing the process once the foundations are restored.Full rebuild with new foundation solutions that reflect good engineering practice.
Designs are to accommodate up to 50 mm spread (with plan regularity limits) using details such as:A geotextile beneath well compacted fill and slab on grade. DPC layers beneath a slab-on-grade that would allow the slab to slide. Strong ribs (particularly for re-entrant corners). Waffle slabs – ensure foundation has sufficient flexural capacity. Driven timber pile to solid bearing.
Dave finished his presentation with draft threshold criteria for the restoration actions (for each foundation type and type of intervention): slope, perimeter beam cracking, floor stretch or local in-plane stress.
In response to a question about the level of site verification needed, Dave indicated that the Tonkin and Taylor suburb-level investigations are evaluating the deep properties and shallow investigations may be adequate for individual sites within those suburbs. They aim to have technicians trained to conduct the tests to cope with the large quantity of testing. There is also a section providing advice for internal and external chimney repair methods.
Mike Jacka (Tonkin and Taylor) indicated that they are remappping and local mapping 20 000 houses and engineering assessment teams 3500/6000 site investigations. CPT rig now working on 400 CPT tests 50-80 boreholes (targeted at ground treatment). Aim to complete by Christmas (200-250 m grid and closer around perimeter). Next part is damaged land repair categories. Areas with significant ground treatment – buffer zones required between house repairs being investigated.
Holding ground together under building expected to help
Nick Wharmby (Brian Perry Civil) and Graeme Quickfall (Hiway Geotechnical) outlined the range of ground improvement methods available in New Zealand, ranging from improvement, to semi-structural and full structural (see their presentation). Nick presented the least expensive, vibrocompaction (with a horizontally vibrating probe and water jets), was used at Pegasus, however silt layers can affect the performance. A second alternative, dynamic compaction (pounding the soil), is probably inappropriate and deep drains have been used elsewhere.
Semi-structural methods such as vibroreplacement (e.g. the 0.8 – 1.0 m diameter columns used in the AMI stadium) require good water management on the site. Graeme outlined the soil mixing methods, using twin augers and a turbojet mixer with a jet grout. A case study from a Kobe hotel used confinement cells constructed from 11 m deep overlapping columns showed that they performed well as a remedial option in Kobe. Mass stabilization using dry mincer (instead of grout) avoids dewatering and only requires low level vibration. Grout shafts, both compaction and permeation, can be drilled inclined under structure.
John Bush (Piletech) outlined how piles can be used under a house (e.g. Option 6 in the draft DBH guideline). Their aim is to develop a standard pile for the housing market (that can be installed by others). Relevelling uses a vertical pile beside the floor slab which is jacked up using a bracket at the top of the pile. Uretek can then be used beneath the remainder of the foundation. Another option is steel screw piles that are quiet to install and provide a permanent solution (Designed to AS2159 with sacrificial steel for a > 50 year life).
Nick Harwood presented evidence of a surface sand boil near Kaiapoi that reliquefied when poked. He asked what those attending were observing and found that there is still ongoing movement in buildings, settled ground behind retaining walls, opening stonework on the hills, brick buildings are still responding and pore pressures still appear to be elevated.
General Discussion: David Bell (University of Canterbury geologist) outlined how he had inspected the cavern behind Princess Margaret Hospital after the main shock and identified about 4 rocks that had fallen from crown. However, there was a significant collapse after the 24 October shallow M4.8 aftershock. Grant Wilkinson (Ruaumoko Solutions) asked about the earthquake damage signature as there is a need to quantify what buildings had been subjected to. Bruce Deam noted that there was a reasonable amount of information about the response spectra (e.g. on the NZSEE Clearinghouse, his summary for 22 Sept and John Xhao's presentation for 28 Sep ).Bruce Deam asked whether there was a need for solutions to be developed for consultants working on larger structures; mirroring the guidelines being developed EQC and DBH (e.g. when to ignore, inject or rebuild damaged concrete joints).Grant Wilkinson questioned the status of the 2/3 of current code requirement for insurers. It was suggested that this is likely to need resolving in the courts before commencing any engineering works. Another suggestion was to get the Insurance Council (John Lucas) to provide info. Yet another asked whether DBH needs to make a determination on the 2/3 to allow it to be tested in the high court.Grant asked about the relationship of reinsurance contracts to insurer's contracts with householders. It was suggested that there is no conflict as betterment needs to be worked through with reinsurements and that this is the reason that the perimeter ground treatment is financed by Government rather than insurers/reinsurers.Liquefied properties are thought to most likely be tagged as having been damaged.There is a reinsurers workshop to understand what has happened and is happening.Areas with no ground damage can have work start as soon as the aftershocks reduce.Insurance companies are applying stand-down periods. Home lenders seem to be happy with the situation (they want to maintain market share).
It was agreed to hold the next meeting on Wednesday 17 November.
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