NZ Geomechanics News

News – In Brief

Biggest ever meteorite collision in the UK 

Evidence for an ancient (1.2 billion years old) meteorite strike was first discovered in 2008 near Ullapool, NW Scotland by scientists from Oxford and Aberdeen Universities. The thickness and extent of the debris deposit they found suggested the impact crater made by a meteorite estimated at 1km wide was close to the coast, but its precise location remained a mystery.

In a paper published in Journal of the Geological Society in June 2019, a team led by Dr. Ken Amor from the Department of Earth Sciences at Oxford University, show that the crater is located 15-20km west of a remote part of the Scottish coastline, buried beneath both water and younger rocks in the Minch Basin.

Using a combination of field observations, the distribution of broken rock fragments of impact debris known as basement clasts and the alignment of magnetic particles, the team was able to gauge the direction the meteorite impact material took at several locations, and from this they plotted the likely location of the crater.

It is thought that Earth’s collisions with an object about 1 km across (as in this instance) occur between once every 100,000 years and once every one million years but estimates vary.  One of the reasons for this is that our terrestrial record of large impacts is poorly known because craters are obliterated by erosion, burial and plate tectonics.

Summarised from:
(10 June 2019)

Footnote: The Minch is a strait in north-west Scotland that separates the north-west Highlands and the northern Inner Hebrides from the Outer Hebrides. 

We’re delighted to announce that Misko Cubrinovski has been appointed as chair of ISSMGE Technical Committee 203 (Earthquake Geotechnical Engineering). This is a great honour for both Misko and New Zealand.

Liquefaction-prone ground needs to be mapped Update from MBIE


We’ve changed B1/AS1 to require robust foundations for liquefaction-prone ground. This change is already in place in the Canterbury region, and will now be extended to all of New Zealand. This will provide clarity to both councils and engineers, ensuring new buildings are being built safely and strongly enough to withstand liquefaction risks.

Why we needed the change

The focus on liquefaction and lateral spreading is a result of the experience of the Canterbury earthquakes and responds to recommendations made by the Royal Commission of Inquiry. Currently, this limitation on the application exists for the Canterbury region only. Specifically, this change is to update the definition of ‘good ground’ within the definitions of the B1 Acceptable Solutions and Verification Methods document, and specific references to the term ‘good ground’ within B1/AS1.

What this means

Foundation solutions on land prone to liquefaction and/or lateral spreading would need to be consented as a Verification Method or an Alternative Solution. TAs/BCAs will have the flexibility to develop, implement and enforce policies on how they address land instability risks in their regions. Affected stakeholders include: developers, home owners, geotechnical contractors, engineering consultants (geotechnical, civil, structural), TAs and BCAs, suppliers and manufacturers, and MBIE. 

These stakeholders will all be invariably affected by the change, some positively, and some negatively but by large we have received positive feedback saying this change is very much necessary and are supportive. The sector believes that the proposed changes will increase the cost of building upon liquefaction-susceptible land but will be offset by a gradual increase in the level of seismic resilience and corresponding reduction in post-earthquake disruption to New Zealand’s residential housing stock.

This change will affect a number of different stakeholders (as mentioned above) but doesn’t come into effect immediately as there is a transition period of 24 months (November 2021).

Feedback from the consultation indicated the purpose/intent of the transition period didn’t allow sufficient time to TAs/BCAs and for the sector to be in line with the proposed change. Accordingly, an awareness campaign will commence targeting the general sector and advising key stakeholders of Building Code update changes. Education will be targeted towards regional councils, territorial authorities, building consent authorities and engineers helping to make the transition as smooth as possible.

Key outcomes of this change include:

1.)  Better understanding of communities’ seismic risk.

2.) Achieve greater resilience by appropriate initial geotechnical investigations.

3.) Increase sector efficiency through communication, collaboration and education.

NOTE: MBIE/MfE liquefaction planning guidance serves as the basis for completing future maps. MBIE along with different stakeholders (MfE, EQC) consider it is in everybody’s interest to have a consistency of mapping approach across New Zealand. The existence of the relatively comprehensive New Zealand Geotechnical Database (NZGD) also provides the opportunity to better define and update the level of hazard and hazard boundaries nationally.

The biannual Building Code update programme

Twice a year, MBIE consults with industry to ensure continuous improvement to NZ’s Building Code clauses. The next consultation for biannual Building Code updates will open in February 2020. Find out more about the Building Code update programme:

SESOC / NZGS Piling Specification Panel – Correction

The Panel of Experts for the review and update of the Auckland Structural Group “Piling Specification” document has been confirmed as below. Paul Berriman was inadvertently omitted from the list published in Geomechanics News (page 6) in June 2019. NZGS apologises for the error.

Panel Chair: 

  • Anthony Fairclough: NZGS, Christchurch

Geotechnical Engineering Panel Members: 

  • Andrew Langbein: Tonkin & Taylor Ltd, Auckland 
  • Nicola Ridgley: Beca, Auckland Martin Larisch: Golder, Waikanae 
  • Andy Dodds: Arup, Auckland 

Construction and Supplier Panel Members: 

  •  Nick Warmby: March Construction, Christchurch 
  •  Paul Berriman: Brian Perry Civil, Auckland
  •  Malcolm McWhannell: Brian Perry Civil, Auckland 
  •  James Harrison: Fulton Hogan, Christchurch 
  •  Lian Ching Oh: Firth Industries, Auckland 

Structural Engineering Panel Members: 

  •  Michael Robinson: Beca, Auckland 
  •  Rob Presland: Holmes Consulting, Wellington 
  •  Ryan Clarke: Dunning Thornton, Wellington 
  •  Tessa Beetham: Aurecon, Wellington 

As previously communicated, the Panel is to review, update as appropriate and reissue the Auckland Structural Group “Piling Specification” as a national document jointly published by SESOC and NZGS. 

The following Brief is modified from an article published in GeoDrilling International, August 2019

Ensuring SPT hammers provide accurate data

Every structure built relies on proper support and a strong foundation. A properly supported foundation is essential to longevity for any structure. A common means of analysing soil strength and conditions is by using standard penetration testing (SPT). This soil exploration tool uses an SPT hammer to drive a drill string with a split-barrel sampler attached at the bottom of the string. The split-barrel sampler recovers soil samples and the bottom of the borehole after it has been advanced to the required sampling depth. Using this test provides clarity to the engineers and is essential to understanding foundation conditions.

SPT hammer efficiencies vary, which influences the resulting N-values. For this reason, many authorities, including the Federal Highway Administration in the US, require SPT hammer calibration. In addition, period calibration is required by many US Departments of Transportation.

ASTM D1586 recommends that a measured N-value be normalised to 60 per cent drill rod energy, N60 by multiplying it by the ratio between the measured energy transferred to the rod and 60 per cent of the theoretical potential energy. This compensates for the variability in efficiency, and therefore, improves the reliability of soil strength estimates used in geotechnical designs.

But how can we be sure of correct test results? The following advice comes from GRL Engineers, an SPT calibration specialist.

To perform the SPT calibration, attach an SPT rod, instrumented with strain gauges and accelerometers, to the SPT drill string rod. As the drill string is driven into the ground, the strain gauges and accelerometers obtain force and velocity signals with each hammer blow. The signals are transmitted to a pile-driving analyser that displays the force, velocity and energy transmitted to the drill string, calculates and displays the maximum transferred energy value, and stores the complete time record of force and velocity for all SPT hammer blows. GRL typically acquire several SPT energy measurements per hammer at a given test location, in accordance with ASTM.

With this testing, engineers can provide a quantitative calibration report presenting transferred energies, energy transferred ratios and the SPT N60 value for each sample interval tested.




NZ Geomechanics News
NZ Geomechanics News>Issue 98 - December 2019

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