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Good evening! This year seems to have had more snow than others in recent memory, with the Midlands & North getting multiple dumpings. So I thought I’d tackle a relevant topic – the science of gritting! #uksnow #highways #roads
We occasionally see these orange & yellow beasts spewing out grit that might ‘damage paintwork’, and often see indignation of ‘roads not gritted’ when there’s snowfall. So how does this actually work?
Gritting falls under ‘winter maintenance’, and is the responsibility of the highways authorities of the UK – in England Highways England manage major A-Roads & most motorways, and county/city/borough councils manage local roads.
So what is grit? Grit is rock salt, pure and simple – the same as table salt you eat your dinner with! But table salt is cleaned and purified whereas rock salt is in its raw mined form, hence its orangey-brown colour from impurities and dirt.
Most UK grit comes from Winsford mine in Cheshire – it has over 160 miles of tunnels and a massive salt reserve. The mine has been here in various forms from the 1800s. Salt can also come from mines in Northern Ireland.
The Winsford mine has so many tunnels it rents them out as storage space, currently housing parts of the National Archives, police force documents, soil samples from Crossrail, bank records, and even film archives!
The salt is processed into grains of 6-10mm, the process of stocking up grit stores is year-round. You may see some of these grit stores on your travels, often in service & maintenance areas. Small yellow grit boxes are maintained at problem locations eg. steep hills or junctions
The winter maintenance season typically runs from October to March – teams receive weather forecasts, mostly from the Met Office, at various intervals such as daily 2-to-5-day outlooks, and 21-day longer-range forecast, to help plan resources and operations.
Furthermore, authorities may have networks of weather stations, which measure air temperature, wind speed, precipitation, and some measure road surface temperature too! In Hampshire and Scotland embedded road surface temperature sensors are also being trialled.
Water freezes at 0c. However road surfaces hold temperature far longer than air, and many factors such as sunlight & wind exposure, trees cover, location & material type affect what the surface temperature will be.
Urban areas are often warmer due to the ‘heat island’ effect, whilst rural routes, higher altitudes, north-facing slopes, valleys, or windswept areas may be more susceptible to lower road temperatures and/or increased risk of snow and ice.
Grit works by preventing water molecules joining together when freezing to form ice patches. The rock salt, in small particles, dissolves on contact with moisture to form a ‘brine’ solution. The brine solutions spreads the salt further across the road surface.
Vehicles wheels and pedestrian foot traffic also grinds & breaks the particles down, helping them dissolve easier and spread them across the road surface further. The more traffic, the more the salt is broken down and spread about.
Timing is key: grit when dry; the grit gets blown off the road by draught from vehicles. Moisture helps stick the particles to the road surface. Grit too late; you may miss rush hour & lots of vehicles spread & dissolve the particles. Gritting usually is 4 hrs before ice forecast
As the salt dissolves into water and spread over the road surface, any rainfall will dilute the brine solution, reducing its effectiveness and also wash it off the road surface, conveniently allowing ice to form if temperatures remain low.
Gritters spread the salt at different rates to ensure efficient usage. Gritters covering 3-lane motorways will dispense at a greater spread rate than on rural roads, also depending on how severe the forecast is. Spread rates vary between 10 to 40g/m2 depending on travel speed
Most authorities operate gritting routes based on priority – a primary network of main roads that need to remain open to let people travel, reach hospitals, motorway junctions etc. Secondary routes usually include at least one route into a village, special schools, problem hills.
Gritters take approx 3hrs to discharge loads, mileage covered varies depending on spread rate. Authorities cover around 200-500 miles in their gritting routes. You may see gritters not spreading as they are returning to the depot, or another gritter has already treated this route
Gritter fleets have GPS & sensors to monitor spread rate, & ensure wasteage is minimised such as avoiding treating the same section again unless necessary. Some fleets also measure surface temperatures as they travel for more accurate data, and adjust the spread rate as necessary
One thing I can’t say loud enough: GRIT DOES NOT STOP SNOW OR ICE FORMING. It helps prevent it, but at extreme low temperatures the salt won’t work.
Snow is tricky. If you see slushy snow, it means the grit is working. But large flakes or heavy snow can settle over grit particles or the slushy layer. Or without traffic to mix the grit into the snow, only localised patches of snow are affected.
Ploughs are fitted to the gritters to help clear snow. Ploughs are a specially-angled shovel, angled to fire snow off into the verges under the force of the vehicle motion. They have a rubber blade fitted to the base to avoid damaging manhole covers, gully gratings and cat’s eyes
However the rubber strip means around 30-40mm of snow is left on the surface, but this is usually enough for vehicles to drive in. Farmers may be employed under service agreements to help plough rural areas with blades fitted to tractors where needed.
Bridge decks are also tricky – having an open space under the roadway means air passes beneath and makes the road cooler, so grit is less effective. The M5 Avonmouth viaduct was infamously closed in 2016 due to ice on the bridge https://www.bbc.co.uk/news/uk-england-somerset-38442975
Other treatments exist. ‘Pre-wetting’ or ‘pre-treating’ involves spraying the brine solution directly onto the road surface, i.e the salt already dissolved. Transport Scotland’s recent trial concluded that a 50:50 mix of grit and brine solution is most effective form of treatment
On several occasions, molasses sugar has been mixed into the grit & brine to help it stick to the road surface. In 2007 in Wales this caused queues when sheep starting licking the roads to get a sweet frozen treat!
http://news.bbc.co.uk/1/hi/wales/north_east/6317693.stm
http://news.bbc.co.uk/1/hi/wales/north_east/6317693.stm
Other products include urea, a waste product from animals and humans – basically pee! It is less corrosive & damaging to bridge (see my Clifton Bridge thread), but costs around 16x more than salt! Other chemical products exist, use is minimal due to cost and environment concerns
The big takeaway from this: although I've called this thread the 'Science of Gritting', and there's lot's of techniques and research, procedures and working groups - it's not an exact science. The weather is unpredictable and changeable
For example, today we had several inchies of snow on site. 5 miles west of us, there was no snow at all. 20 miles west and the air temperature was 5 degrees warmer. Bear this in mind next time you see snow or ice on the roads! Best advice is to slow down & drive carefully.
I hope this has helped understand how we manage our roads in winter! Check out your local council's webpage for more info, most have pages on gritting.
Image credits:
https://newsimg.bbc.co.uk/media/images/46950000/jpg/_46950840_gritter_pa466.jpg
https://www.baildontowncouncil.gov.uk/images/1036325/large.jpg
https://www.weatherforschools.me.uk/images/grendon_road.jpg
https://lh3.googleusercontent.com/proxy/h7tC0kyi0lD9xbgxGobMIWxMvo7ZOKK_qGnx_kXMaotJtMY3eJ4r2CwaWwtzpwQ_j4wUVQ8f7Wq5fMQFWXCbh9TEKyIa9g178JxKaFyPPoqRp7mNEwY1mgMk0SG1BnqzlcSVBVGuRy8pltUdZLz6qDZk3Y6_6bct
https://www.sandwell.gov.uk/images/gritters4.jpg
#uksnow #gritting #grittertwitter #roads #highways #snow #ice
https://newsimg.bbc.co.uk/media/images/46950000/jpg/_46950840_gritter_pa466.jpg
https://www.baildontowncouncil.gov.uk/images/1036325/large.jpg
https://www.weatherforschools.me.uk/images/grendon_road.jpg
https://lh3.googleusercontent.com/proxy/h7tC0kyi0lD9xbgxGobMIWxMvo7ZOKK_qGnx_kXMaotJtMY3eJ4r2CwaWwtzpwQ_j4wUVQ8f7Wq5fMQFWXCbh9TEKyIa9g178JxKaFyPPoqRp7mNEwY1mgMk0SG1BnqzlcSVBVGuRy8pltUdZLz6qDZk3Y6_6bct
https://www.sandwell.gov.uk/images/gritters4.jpg
#uksnow #gritting #grittertwitter #roads #highways #snow #ice
Bonus gritter action: https://twitter.com/russiankiwi3/status/1353390203243794434?s=19
