You might imagine that digging a hole is an easy business when installing a bus shelter or post. But there are complications that are hidden under the surface. This short article will give you a flavour of what goes on underground.
What’s under the ground?
Let’s start with what a footway is constructed from. On the top you will usually find asphalt (also known as bitumen, macadam or tarmac). This is around 60 to 100mm in depth. Below this is a compacted sub-base material which comprises an aggregate of sand, gravel and stone. The sub-base can be as deep as 600mm or beyond, but hidden within the sub-base are utilities. These are cables and pipes carrying electricity, gas, water and telecommunications. On busy roads utilities are almost always going to be present, and they can make a massive difference to installing bus stop posts and shelters.
How do you search for utilities without using X-Ray Specs?
We use a slightly less cool way of seeing beneath the surface with a C.A.T (Cable Avoidance Tool). This scans through the footway to show us where cables and pipes are located. It gives some idea of depth and can identify the type of utility we’re dealing with.
When we find these, we mark where they are, because soon we’ll be digging near them. Our installers will also check utilities plans against what they have found. But to be clear, utilities plans are often just a very rough guide for shelter and pole installation because they are not detailed enough. Moreover, utilities companies never guarantee them to be accurate. A utilities plan will give an idea of what cables and pipes are around the area, but they are not accurate enough to explain exactly where or how deep they are.
How is the footway excavated?
To neatly get through the top layer we cut through with a disc cutter (or cut-off saw). This creates a square of tarmac which can be removed. For the next couple of hundred millimetres we might excavate using a hydraulic breaker (aka, a jackhammer) or manually. If there is nothing detected and the ground is hard, the breaker makes the dig easier by breaking up the sub-base.
Below this level our guys use insulated digging tools to manually excavate. Extra care is taken within a 500mm radius of any utility readings that have been picked up on our initial scan. The C.A.T is repeatedly used by the guys throughout the course of the dig until the required depth of excavation is met.
Can you always dig to the right depth?
No, often there are pipes and cables in the way. Utilities should be 450mm down or lower but regularly we find them nearer to 300mm and sometimes even closer than that. If they are near the surface, they should be marked with tape and sand. But this is not always the case. So, for an installation crew it means being exceptionally careful, requiring highly-trained and experienced operatives. Sometimes, a cable can be moved to the side a little to create the space to fit the leg in, but pipes are pretty fixed. If a shelter has 5 holes to be dug it can be tricky to get all 5 excavations to match up, but there are ways around this problem.
The quantity, width and depth of excavations differ depending on the product, style and size of our installation. Other factors that can change the dimensions of the excavation requirement include underground utilities and sloping sites. For example, if we can’t get the full depth required due to underground cables, we can make the length and width of excavation larger to compensate for this. A shelter or pole needs a large mass to hold it in place, so if you can’t go down, you have to go out sideways.
Is that it?
When the holes are dug, it is time for the shelter or pole to go in. In the next article we will explain what makes up the foundations and how the resurfacing materials are laid.
Now you know the main intricacies of excavating for bus shelter installation you’ll appreciate it’s not a job for amateurs. During excavation or disturbing the earth Externiture take great care to avoid damaging any below ground services. We are experts in our field with fully qualified, specialist operatives.
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