Ask an engineer?

Ask an engineer?
Photo credit: HBO

Q: If The Wall from the Game of Thrones was built in real life, how deep would the foundations need to be? Would it actually be possible to build it out of ice? NB: The Wall is meant to be 300 miles long, over 700 feet (213m) high, and made of solid ice.

- Grant, Tikitere

A: This is a goodie, thanks Grant. Mark Thomas, Senior Geotechnical Engineer has got the answer!

Mark: "How come I get the question about fictitious magic stuff?!

I know more about soil and rock than I do about ice. I know glaciers get way thicker than that, (~1500m thick) and the Greenland and Antarctic ice sheets are in the order of 2000 to 5000m thick! But they don’t have vertical cliff faces nearly that high.

In order to be strong enough to form a 213m high vertical face, the ice would need to have a theoretical mass strength of at least 2MPa UCS, which doesn’t even take into account defects within the ice. On top of that, Martin Truffer has pointed out that ice doesn’t hold its shape, and I guess that’s where you need a bit of Old Gods magic to keep the whole structure from falling apart.

Building a structural “foundation” for the wall wouldn’t really cut the mustard. You’d really want to dig down and build the wall on solid bedrock. Depending on the strength and fabric of the rock, you might have to dig tens of metres deep to support the steep sides of the wall. Given the size and weight of the wall, you’d probably also induce quite a few localised earthquakes during construction. That could be a problem for nearby masonry castles and forts due to their brittle construction; but less likely to be an issue for a typical wildling hut.

It’s a bit strange that the wall was built out of magic ice. If I was in charge of the wall’s construction, I would have opted for magic rock or magic concrete, which would be much more robust in both a structural sense, and in its resistance to dragons and long summers. Donald Trump, if you’re reading this, give me a call".

- Mark Thomas, Senior Geotechnical Engineer

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Q: Why is Ramp 4 out of Waterview SO BIG?!

- Andrea, Meadowbank

A: Thanks Andrea, our roading expert Matt Arcus has the answer on this one for you!

At the Northern end of the Waterview Tunnel, the interchange is made up of a complex set of ramps. Each individual ramp crosses over multiple carriageways but ramp 4 crosses the most carriageways.

In order to cross these carriageways AND meet the required road standards and design criteria, ramp 4 needs to be much taller than the other ramps. And with height comes the need for a bigger supporting structure and extra road length to address the change in height (so that driving down ramp 4 isn’t like being on one of the Big 5 at Rainbow’s End).

The roading standards and design criteria are mostly linked to the design speed of the road. Some of the biggies are:

  • Minimum curve radius: how tight you can make the curves - so that the forces keeping you on the road oppose the centrifugal forces trying to throw you off!
  • Superelevation: the amount of pavement banking around the horizontal curves to counteract the effect of centrifugal forces acting on the vehicle
  • Stopping sight distance: the amount of road you need visible in front of you to identify a hazard, react and then come to a complete stop
  • Maximum vertical grade and vertical curve requirements: how steep we can make the road and how much vertical curvature is needed so that we can see over a crest in the road
  • Vertical clearance requirements: how much head room we need between crossing carriageways to allow larger vehicles to pass beneath

Another significant part of designing any road is safety. This can add additional constraints to the design, such as making the shoulders wide enough to see around the bends, or safely accommodate a broken down car.


Q: How long will the eruption of the volcano on Kilauea in Hawaii last?

- Paul, Onehunga

A: Good question! One of our resident Volcanologists Alec Wild has got an answer for you!

The short answer is we don’t know! It’s difficult to determine how much magma beneath the volcano is going to be ejected, and over what timeframe, as we cannot directly observe it.

Kīlauea Volcano on Hawaii’s Big Island has been erupting on and off since 1983. However, this most recent phase has formed long cracks in the ground called fissures, along the East Rift Zone to the east of the main vent. The rift zone is a thin strip of land that is being separated, allowing magma to travel up and down. Once the magma reaches the surface and forms fissures, it produces fire fountains and lava flows. Of particular note are the fissures that have formed in a residential subdivision, Leilani Estates, resulting in the evacuation of residents and damage to properties and infrastructure.

The lava level from Kīlauea main summit has dropped, which indicates that the lava could be moving along the rift zone. Volcanologists are monitoring the lava level, seismic data and ground deformation to try and predict where the magma is moving and where it might come up to the surface.

This eruption is considered, based on activity and affected area, similar to the 1955 event which occurred in the same rift area, and lasted 3 months. During the 1955 event with lava flowing both north east and south west of what is now Leilani Estates. Although this is similar, we cannot definitively say that this eruption will be of the same duration.

- Alec Wild, Natural Hazards Specialist, MSc Quantitative Volcanic Hazard and Risk Modelling




Q: I have been transfixed by that dramatic, jagged, golden cliff face that features in the British TV programme, Broadchurch. Would one of your engineers kindly explain (in layman's terms) how it was created.

- Shirley, Remuera.

A: Great question, Shirley. Our legendary engineering geologist Kevin Hind was dead keen to answer!

"The steep cliffs of West Bay in Dorset on England’s Southern coast are made of extremely weak and highly erodible sandstone. They originally formed from layers of sand, mud and shell fragments that settled onto the ocean floor sometime between 183 and 174 million years ago.

It took a whopping 860,000 years for the full 43 metre thickness of cliff to develop! The build-up of sedimentary sand layers happened extremely slowly, forming at an average rate of one metre per 20,000 years. All those horizontal layers of sediment are clearly visible in the cliff face - just like they are in the cliffs of Auckland, which formed in a very similar way.

Years of wave erosion has formed the cliff face itself, most of which occurred since about 6,000 years ago, at the end of the last ice age, when sea levels stabilised at their current levels.

The corrugated look of the cliffs is caused by two things. Firstly, the geology contains two sets of strong sub-vertical joints, along which cliff collapse generally occurs. This results in tall, flat-faced columns, which stand out from the overall cliff face with recesses between them.

This close up image clearly shows the sub vertical joints in the rock formation, this effect is called ‘buttress and groove’.

Secondly, extensive landslips that occur at the top of the cliff within the soil and weathered rock. These landslips occur on a regular basis along the length of the cliff, giving it that serrated and ‘bread knife’ appearance. This, combined with the rock buttresses, give the cliffs their remarkable corrugated appearance".

Unique geology and erosion combined to create the ‘bread knife’ appearance of the cliffs along the Jurassic Coast.

So what about that golden yellow colour?  

"That’s actually the result of an oxidation process enhanced by beautiful cinematography. The rock is naturally a grey-blue colour, but when exposed to oxygen, the mineral ‘pyrite’ (also known as ‘fool’s gold’) changes to yellow limonite – snap – there’s your yellow stone".

- Kevin Hind, Senior Engineering Geologist




Q: Why doesn’t my bike trigger some traffic lights?

Sometimes I have to wait ages for the lights to change when there’s no traffic - Linda, Tauranga.

There are a number of reasons why this could happen, however, the most common is that your bike might not contain enough metal to be detected by the electro-magnetic sensor loops used by the traffic lights system.

You can usually see these sensor loops at the lights, in the pavement as dark rectangular lines near the stop line of each traffic lane.

Other reasons why your bike may not trigger the traffic lights include that you haven’t ridden over or near enough to the sensor loop, or the sensor loop’s sensitivity setting is not high enough. 

To reduce this issue in New Zealand, bicycle-specific sensor loops are sometimes installed where bikes at traffic lights are more common. Some cities also have cycle stop boxes (see photo below) at intersections, with painted symbols to show where the cyclist should stop for detection - Transport Engineer, Jeremy O’Neill.