Ask An Engineer

Ask An Engineer

Q: Why is there a black market for sand? Is there one in NZ and will it become a problem?

T+T's Chief Coastal Engineer Richard Reinen-Hamill has an answer:

A: The black market for sand is driven by construction and land reclamation. However, with sand, not all sand is suitable.

For example, you would think sand for concrete in construction projects in the Middle East would be easy and cheap – you would just use sand from the Sahara Desert, right? Unfortunately Sahara sand
is too smooth and fine-grained and so it has to be imported from further afield.

It’s “suitable sand” that’s increasingly in demand and has become a valuable commodity.

There is a global black market for construction materials to build at a lower cost or to increase construction profits. The international sand and gravel market in the US and UK alone is in the order of $10 billion (USD) and markets such as the Middle East, India and China are all increasing demand for construction aggregate (gravel, sand and rock material used for construction).

In New Zealand, the aggregate market is more regulated and controlled and distance to market from international sources to New Zealand reduces the likelihood of importing bulk materials such as sand.

When Tonkin + Taylor conducted sand source investigations for the beach replenishment of Wellington’s Oriental Bay (in the early 2000s) we were offered beautiful white sand from Western Australia at a low cost, but due to mana whenua concerns and biosecurity risks, we sourced rock and crushed it into sand from a cliff near Nelson.

Q: If you were to fly to Fiji for the Christmas holidays, how many trees would you have to plant to offset your carbon emissions?

Sustainability Engineer Kate Boylan, with help from Environmental Consultants Roger MacGibbon and Kate Draper, has an answer:

Firstly, it feels like my duty to say, that offsetting your emissions is great, but reducing your emissions is the best! So, flying to Fiji (or anywhere) less regularly would be the best thing to do. However, your bags are packed and, being the good human you are, you would like to offset the negative impacts of your holiday flight. Carbon offset calculating websites are becoming popular, but we’ll use good old distances and emissions factors here.

To calculate the emissions, we use an ‘emissions factor’ which allows us to estimate emissions from a unit of activity data (e.g. litres of fuel used). In New Zealand, the Ministry for the Environment (MfE) provides emissions factors for most activities. Many human activities produce multiple Greenhouse Gases (GHGs) at once, however, to allow for meaningful comparisons, we commonly express GHG emissions as carbon dioxide equivalent (CO2-e).

Given you are just one person on the plane taking up just one seat, you are only responsible for a small portion of the flight’s overall emissions. If you are in an economy seat, you are responsible for fewer emissions than those in business or first class, purely because you take up less space.

The flight to Fiji, according to Google, is 2160 km. Assuming you can drag yourself away from the beautiful white sand beaches at the end of your stay, a return trip is therefore 4320 km. Because these flights are less than 3700 km (each way), it is considered short-haul. The emissions factor provided by MfE for an average passenger on a short-haul flight is 0.162 kg CO2-e/, including Radiative Forcing Factors (where Radiative forcing factors help account for the wider climate effects of aviation, including water vapour and indirect GHGs (MfE, 2019)).

Therefore the emissions are the total flight distance multiplied by our emissions factor; 4320 km x 0.162 kg CO2-e/ = 701 kg CO2-e.

MfE also sets out standard ‘sequestration’ rate (carbon sucking up ability) emission factors for some of our common forestry carbon sinks. Planted forests can remove 33,807 kg CO2-e per hectare, whereas naturally regenerating forest can remove 5,097 kg CO2-e per hectare.

So, to offset your Fiji holiday flight emissions of 701 kg CO2e, we would need to plant 207 m2 of planted forest, or regenerate 1376 m2 of natural forest. Let's assume you’re looking to plant some fast-growing trees (e.g. radiata pine), where standard forestry spacing is initially around 1000 stems per hectare.

You would, therefore, need to plant 21 trees to offset your return holiday flights to Fiji.

If you are interested in some further reading, check out the MfE measuring emissions resources online, or for more information on specific sequestration rates see the Carbon Look-up Tables for Forestry in the ETS.

Kate (with help from Roger MacGibbon & Kate Draper)



What’s not to love about the great Kiwi outdoors? Cool, pristine bush, magical vistas and crystal clear waters await those of us who love to roam. But just how safe is it to drink straight from our streams and rivers? We asked one of T+T’s hydrogeologists, Dr Jeremy Bennett.

A: Water, water, everywhere – but which drops can we drink? It depends – water has a ‘memory’ of where it has been, or more specifically, what it has travelled through. Generally, the closer you are to where the water came from (i.e. the sky), the cleaner the water is likely to be. If you’re up the top of one of our many mountain ranges, then the water in the streams has not travelled that far and won’t have too many ‘bad memories’.  As you move down to lower areas, particularly where there are more people or animals, then it is highly likely that the water will be unsafe to drink due to the presence of microbiological contaminants like E. coli and Giardia. When you get sick from these nasties, the vistas are not magical.

The risks of waterborne disease are lower in conservation areas, and many trampers and hikers do drink directly from streams without too much trouble. However, you need to think carefully about where the water is coming from (its catchment). Are there farms or wild animals upstream of your location, or is the area often used by people? Even in the most pristine bush environment, there could be something just around the corner which will make the stream water unsafe to drink without treatment (dead possums are a common one). The Department of Conservation recommends that you boil water if you have any doubts about its quality and Wilderness Magazine has some good tips on collecting water during a tramp.

Q: How were the pink and white formations at Ōrākei Kōrako formed?

Senior Engineering Geologist Kevin Hind has an answer:

A: The light pink/white formation you see in the centre-right of the image is a ‘sinter terrace’. You may have heard of the famous pink and white terraces, which were formed in a similar fashion. These terraces form when warm spring water rises up through the earth’s crust. This water becomes enriched with minerals such as silica and chloride as it flows through the surrounding rock. As the water reaches the earth’s surface it cools. This cooling results in the minerals solidifying creating the terrace-like structure. The bright orange and brown are the result of bacteria and microbes thriving in the warm spring waters.

With summer almost upon us and tens of thousands of people already heading for the beach, we asked T+T Coastal Geomorphologist Eddie Beetham:

Q: How do you spot a rip current and what causes them?

A: An area of low wave height and limited breaking located next to an area of larger breaking waves will likely be a rip. A danger of rips is that low energy sections of beach are often more inviting for bathers than a section with breaking waves. Always watch the beach for a few minutes to identify rips before entering the water and swim between the flags.

Rip currents form when waves break on irregular sandbars spaced along the beach. This forces an offshore current to develop between sandbars, as wave energy piling up at the coast is released. The process of wave breaking creates a local area of elevated water level called ‘wave setup’ landward of the breakpoint.

If smaller waves are breaking at a neighbouring section of the beach, a setup differential is established and this drives an alongshore current from areas of high setup (where waves break) to areas of low setup (small wave locations). Setup driven currents converge in lower energy locations between sections of wave breaking and form a seaward directed rip current. The velocity of a rip current typically increases with larger waves and high-speed rips can erode a channel between sandbars that enhances the development of a rip cell.

Q: What happened to all of the debris (rocks/dirt etc) from the Kaikōura Earthquake landslides? Where did you put it after it was taken away? Is it used for other work somewhere? Where does all of the stuff that’s taken out of tunnels go too – like the Waterview Tunnel while Alice was boring the holes?

A: This is a key part of what engineers do – we work with the materials we are given, to use them in the most sustainable, practical ways. Where possible engineers will always try and reuse, recycle or repurpose local material from landslides and major excavations.

These two projects, in particular, dealt with almost unprecedented amounts of soil and rock.

Our Engineering Geologist Nick Peters and Geotechnical Engineer Peter Millar provide their answers below.

Waterview Tunnel

The bulk of the material from Waterview was used to backfill the Wiri Quarry in preparation for an industrial subdivision on the site.

The material that was excavated by Alice (the tunnel boring machine) resulted in a wet slurry, which required substantial drying and conditioning before it could be compacted as engineered fill. The basalt rock that was excavated from the northern end of the tunnel was used for a range of applications including coastal and road engineering following crushing and processing

- Peter Millar

Kaikōura Earthquake Landslide

The landslides that occurred as a result of the Kaikōura Earthquake in November 2016 inundated SH1 and the Main North Line both north and south of Kaikōura. The volume of landslide material generated by these landslides was many hundreds of thousands of m3.  Following the earthquake, the North Canterbury Transport Infrastructure Project (NCTIR) Alliance cleared the landslide material from the road and rail and implemented solutions to allow the transport corridor to be reopened.

Although much of the material was stockpiled, the landslide material was also used in a number of ways, including:

- As fill along sections of SH1 where the road was to be realigned

- Forming small bunds and stopbanks

- To create laydown areas for plant and machinery and other areas like helicopter landing pads

- Some of the trees that came down in the landslides were extracted and made available to local people, for example for carving

Other landslide debris material was stockpiled because it contained organics (i.e. bushes and trees etc) or was too wet to use – this was in part, due to helicopter sluicing that was undertaken to remove unstable material from the slope faces.

- Nick Peters

This week we're switching things up and instead of Asking an Engineer, we're Asking an Ecologist!

New Zealand lizards are unusual in that only one, the egg-laying skink which lives near rock pools at the coast, lay eggs. The others are viviparous – they give birth to live young. Vivipary is thought to be an adaptation to New Zealand’s cooling climate during the ice ages. As lizards are ectotherms (do not generate their own heat but need to absorb it from their surroundings), staying within a warm mother ensures the survival of young.

Due to their viviparous nature, native lizards have fewer young than their egg-laying overseas cousins, generally two for geckos and between two and seven for skinks. This is in contrast to the introduced plague skink from much warmer Australia that can lay up to 80 eggs!

Many of our lizard species survive in cold climatic conditions, including alpine environments.

What are the weird holes that you see in the side or top of hills, for example in the Port Hills in Christchurch?

Great question! The answer is that the terrain up in the Port Hills is made up of Loess (rhymes with purse). Loess is composed of wind-blown dust or silt that has been loosely compacted. Most areas of loess were likely formed during glacial periods where there was very little vegetation. Because the build-up of dust/silt is not very well compacted, the loess is extremely prone to being eroded or removed from the hillside, particularly in heavy rain.

This can result in tunnels or gullies forming underneath the soil/grass surface - this is called piping. Some of these are large enough to fit a small person - but you should never get in one!

Loess is found widely across the South Island around the Canterbury Plains and on Banks Peninsula. Areas in China and the United States Midwest have loess deposits tens of meters thick!

Rebekah Robertson (Geologist)


Q: Can you please tell me if you can hurry up the pedestrian crossing by pressing the button lots of times to tell the machine there are lots of people waiting?  I have this terrible feeling that you can’t and that’s an urban myth.




A: Hi Trent,

Unfortunately not. Pushing the button more than once doesn’t make any difference to the lights at a pedestrian crossing. This is because the lights are programmed in a specific sequence made up of a number of phases. When you push the button the pedestrian phase is called, meaning the pedestrian light will turn green next time it appears in a phase in the sequence. However, once a phase is called pushing the button again doesn’t do anything because the call has already been made.



Q: Obviously it's more expensive to Uber than to Bus but if your only concern was sustainability, would it be better to Uber to travel to work in an electric car or ride in a fossil fuel-powered bus?


Marie, Wellington.


A: Hi Marie,

That’s a great question, thank you for sending it through. Our Sustainability Engineer, Kate Boylan has an answer:

We believe that despite the emissions that our diesel bus fleets emit, they are the more sustainable option. The more people we have on each bus, the less emissions there are per person when that is split evenly. Also, given a standard bus can take around 50 passengers, that’s around 50 cars that are not travelling on the same roads. Even if they are electric or hybrid cars; fewer cars means less congestion, which would make it easier for our buses and public transport vehicles to get around. Less congestion also means that the buses have to stop and start again less often, which saves on fuel, and therefore emissions. Less congestion also means that everyone travelling would save time, and we all know that time is precious, and time is money.

Fewer individual cars on the road saves space. If everyone used a bus, train, or active modes of transport to commute, we would need less road space overall. Just Google image ‘road space cars vs bus’ for some visual examples of this. We would also need less space to be used for car parking at workplaces, this would save you or your company money, as that space could be utilised for something far more useful.

However, vehicles on roads pollute more than just fuel emissions. Vehicles often pollute rubber and brake pad particles to the surrounding environment of roads. Stormwater run-off from our roads flows directly out to sea with these contaminant particles in tow. Buses also contribute these pollutants, however, as we’ve seen visually above, fewer buses are required to transport the same amount of people. Fewer individual cars on the road could also reduce the wear and tear on the road. Currently, high-traffic roads need resealing every seven years. If there was far less traffic on these roads, they could be replaced less often. This would require fewer materials and construction works, therefore a dramatic saving in construction emissions and embodied resource emissions.

As a fare-paying passenger of your local public transport system, you are supporting your local council or transport body. This revenue can help increase the investment in more efficient and better public transport vehicles and overall systems. After all, every dollar you spend is a vote for what you want to see in the world. Our councils are already starting to look into Electric and Hydrogen fuel buses, so they could be cleaner than your Uber in the near future.

Lastly, from a high-level sustainability perspective, buses and public transport represent changing people’s habits to a more sharing and circular economy. Instead of owning cars, electric or otherwise, could we hire them as we need them?

We hope that answers your question - see you on the bus!



Q: How come the Japan vs Scotland game at the Rugby World Cup was able to go ahead at Nissan Stadium in Yokohama, despite the massive floods caused by Super Typhoon Hagibis? 

Steve, Ruakaka

A: Hi Steve! It comes down to some pretty nifty engineering. The stadium is built on massive pillars that lift it above the flood level of the river. You might remember seeing that the adjacent Shin-Yokohama Park was totally awash, even while the pool game was on. The Park acts as a large flood retarding, or storage basin for the Tsurumi River, and, during a flood, overflows into the Park as it did during Hagibis. After the flood a drainage gate feeds water from the Park back into the river. Result? One flood-beating stadium - so the players at Yokohama stadium get to keep their feet dry! 



Q: After reading about how raw sewerage leaked into Wellington Harbour last week when the stormwater drains became blocked by a fat berg, I started wondering about whether peeing in the shower could contaminate our harbours and sea life.

I occasionally pee in the shower – I suspect lots of people do but I’m quite happy to put up my hand and admit to it. But now I’m wondering whether my peed-in shower water goes down the stormwater drains and out to sea untreated?

I’d really like to know, from one of your engineers, whether I should continue to pee in the shower or not? 


John (not my real name)


A: Hi John,

Fantastic question! Assuming your house has been plumbed correctly, your shower water (and shower pee) will make its way into the same sewer system as the flush from your toilet. Once your pee has journeyed down the sewer line, it will be treated and made safe to return to the environment.

As I mentioned above, this is all assuming your house is plumbed correctly. In some cases, wastewater is finding its way into the stormwater network, which ends up in the rivers and beaches that we swim in and our fish life live in.

Tonkin + Taylor is working with local councils around the country to track down and resolve these types of issues.  I encourage you to ask your local council about the water quality at your swimming beaches or check out websites like Auckland’s  Safeswim helps keep people informed about public health risks at bathing beaches using a live and forecasted safety rating.

Now, back to the pee. The shower could well be the best location for your pee! By peeing in the shower you are actually conserving the water that would have been used on a toilet flush. As to whether you should pee in the shower or not, if you are confident your plumbing and sewer is in good shape, then I don’t see any reason to change!

 - Ben Perry, Water Engineer


Q: Dear Engineers

I have been following Gareth and Jo Morgan’s motorcycle tour through Eastern Europe on Facebook – it’s quite interesting.  I now have a question for your geologists, the Morgans are in Russia and travelling in an area 30 metres below sea level, where does the water from streams and rivers run to when you’re that far below sea level? It can’t run uphill to the sea, can it?

Peter, Invercargill

A: We asked hydrogeologist (groundwater scientist), Dr Jeremy Bennett to respond.

“Great question, Peter! Water cannot run uphill… or can it?

The answer to your question is hinted at in the salty soil of Jo Morgan's Facebook post. The Caspian Sea does not have any natural outlet so the only way is up – evaporation! As the water evaporates, the concentration of minerals gets higher – this is one reason that the Caspian Sea and surrounding area is so salty. A similar process is used in our own backyard to produce table salt at Lake Grassmere near Blenheim. Other places where water evaporates instead of flowing to the ocean (endorheic basins for those who like big words) include Lake Eyre in Australia and the Bonneville Salt Flats in Utah, where fellow Southlander, Burt Munro set the under-1000cc speed record in 1967”

Q: Hi, I have just moved into a place with a swimming pool and am getting quite upset because I keep finding drowned honey bees, bumble bees, worms and skinks in it. Also the occasional German wasp - but who cares about them, right? Is there a way I can prevent this? Would a cover work? And what attracts them to it? Any advice would be much appreciated, it’s so upsetting to see dead bees.

Yours sincerely - Jennifer (Takapuna)

A: Hi Jennifer,

It’s a question that often pops up during summer and we've asked our ecologists to help you with it. Herpetologist (lizard expert) Dr Matt Baber and entomologist (insect expert) Dr Briar Taylor-Smith explain the issues below.

In most of urban Auckland (including Takapuna) there are two species of skink: native copper skinks (Oligosoma aeneum) and the introduced Australian plague skinks (Lampropholis delicata). Copper skinks are nocturnal and don’t move far from the bush and rocks in which they hide away. On the other hand, plague skinks – which are named so because they are found in very high numbers – are active during the day running about on driveways and other open spaces, and are therefore more likely to become trapped in pools. 

Bees need water for drinking, diluting their food and for hive ‘air-conditioning’. They like reliable water sources, so once a colony has decided that it likes your pool, the workers will keep coming back for more. In natural settings, such as streams and lakes, there are places for small animals to stand and drink. The problem with swimming pools is that water doesn’t come all the way to the top, so they have nowhere to stand and if they do fall in, they have no way to escape.

You can help out flying insects by floating things in your pool, such as corks, sticks or kick boards, which will provide a platform for drinking and a gentle slope that will allow them to climb to safety if they do fall in. However, to save lizards and worms (which tend to sink to the bottom), it’s best to invest in a pool cover.

You can create an alternative water source for bees and lizards by placing pebbles in a shallow container. Fill it with water leaving only the tops of the pebbles exposed to provide a safe landing spot for bees. Keep the container in a shady spot in your garden and top it up regularly, especially in hot weather.

Hopefully the bees will find and begin to prefer that as a water source quite quickly and you won’t find as many of our precious critters submerged in the pool.


Q: I’m a big fan of Mid-century Modern/Palm Springs architecture... and especially love the stone walls that feature on the more expensive homes built in that style in the 1960s and 70s.

There is a particularly spectacular one on St Johns Rd in Auckland - it is unusually colourful. Would one of your geologists know why it is so different to the other ones around Auckland? - Rachel, Mission Bay.

A: Great observation Rachel! Engineering geologist Kevin Hind and geology graduate Rebekah Robertson have an interesting explanation!

Those rocks have travelled a long way to feature in this stone wall.
They are particularly colourful because the vast majority of them appear to be schist, and schist is predominantly found in the South Island – people may know it as “Queenstown Stone”. 
Schist is typically formed across an entire region through a process called ‘metamorphism’. In many places throughout New Zealand we have a rather boring rock called ‘greywacke’ that is quarried for things like the rock you see under railway tracks (or aggregate). Greywacke around, and under, the Southern Alps is exposed to high pressure and heat, as it is squeezed along the Alpine Fault, turning it to schist. The colours of the schist typically reflect what’s called the different ‘grades’ of metamorphism, meaning the amount of pressure or heat the greywacke has been exposed to. For example, a blue coloured schist would typically have been exposed to higher pressures than a green coloured schist.                       

As the pressure and temperature changes, so does the rock's mineral make up, texture and colour. The colours and the shiny nature of the schists come primarily from the presence of a mineral called ‘mica’ – which, fun fact, is also typically what make things like eyeshadows and other make-up shiny.
The mustard/orange coloured rock in this wall is volcanic and called ‘ignimbrite’ - it has been stained by the presence of iron in its makeup that has multiple segments of other rocks imbedded in it. This type of rock is typically formed from huge super-violent eruptions that send clouds of hot ash, pumice and other rock fragments across the landscape at hundreds of kmph. Perhaps the ignimbrite in this stone wall flew from Taupō all the way to Auckland?
If you have a question for one of our engineers, please contact us via


Q: Can wind seriously blow a building over?

- Anna, Lower Hutt

A: That’s a really good question, thanks Anna. Alex Vink, Structural Engineer has an answer!

In a word, yes wind can absolutely blow buildings over. What’s really interesting is all the ways wind can cause structures to fail. 

Wind is mainly caused by the rotation of the earth, which causes the air in the earth’s atmosphere to move. When the wind blows, structures like buildings and bridges obstruct the air flow. This causes the air to flow around the obstruction and generates a pressure or drag on the obstruction. Engineers need to design their buildings and bridges to make sure that they can withstand this pressure.

In a storm, the air particles can move very quickly. This can generate large pressures and drag on buildings. If these buildings and their foundations have not been designed to be strong enough to withstand these large forces that are generated, they can topple or collapse. This happens quite commonly in the Pacific Islands during cyclones.


It’s not just large wind pressures that can make buildings or bridges collapse. During the life of a bridge, it will experience a lot of changing wind with different pressures. It may only experience a couple of really big storms, but lots of small gusts of wind can cause the bridge to fail due to a phenomenon called fatigue. Some materials, like steel, get weaker when they are exposed to many cycles of loading (like changing gentle breezes) and when they experience enough cycles, they can break. This is a bit like bending a paper clip back and forward until it breaks.

Perhaps the most famous example of a bridge collapsing due to wind is the First Tacoma Narrows Bridge collapse. The wind caused the suspension bridge to vibrate excessively without the wind speed increasing because the bridge was not able to absorb all the energy from the vibrations. Eventually these vibrations became too big and the bridge became unstable and collapsed. This phenomenon is called aerodynamic flutter. Engineers need to consider these effects when designing bridges.

If you have a question for one of our engineers, please contact us via

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


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.