The SUMO Survey is an adaptable utility survey designed to meet the needs of a wide range of clients looking to minimise the risk of service strikes. With years of experience in providing the SUMO Survey to clients, we discuss the question; can SUMO locate all underground utilities?
Addressing the question directly, yes in theory SUMO can locate all underground utilities, however, there are always limitations. Having developed a proven methodology over many years, there are various survey techniques which make up a SUMO Survey. A combination of techniques applied in a systematic manner, such as manhole-cover lifting, electromagnetics (in both passive and active mode), Ground Penetrating Radar (GPR) and statutory plan analysis help to make up our advanced methodology. We can use each survey technique’s strengths to our advantage. This way we have the best chance for the detection of buried utilities no matter their material type.
Why does SUMO use different utility survey techniques on a site, wouldn’t it be better to just choose the most suitable one?
With a vast range of applications and differing site conditions, it is not always as simple as a ‘one survey fits all’ approach. Plus, with the use of lots of survey techniques, you can gain the optimum ‘picture’ of what’s beneath the ground. Try and think of it like having multiple perspectives on something. The more perspective you can get, the better ‘picture’ or understanding you can get.
There are also certain limitations to each individual survey technique when used in isolation. This is the very reason why SUMO’s utility survey offers a combined approach in order to have the best possible chance of locating all utilities.
What are the limitations of each individual survey technique?
The SUMO Survey involves 4 key elements including:
Manhole-cover Lifting, Electromagnetics, Ground Penetrating Radar (GPR) and statutory plan collation. Below we will discuss them as individual survey techniques. Whilst highlighting their limitations when used in isolation.
Manhole-cover lifting
Lifting a manhole cover can give a surveyor 100% visual confirmation of the location and depth of buried utilities within the confines of a manhole. But the restrictions to this survey technique are that the cover could be full of water or debris. This means it could even be sealed shut or covered by an obstruction, therefore stopping the surveyor from using this technique. Also, this technique only allows for the confirmation of utilities within the confines of the manhole itself. So as soon as the utility is outside of the surveyor’s visual reach it is mapped using alternative survey techniques. Once out of the surveyor’s sight, the pipe could change material, taper or change depth. For this reason, we use alternative survey processes to confirm the location of utilities once they are outside the confines of the manhole.
In the instance where assessing manhole covers is not achievable, SUMO would use alternative survey techniques to determine the depth and location of buried utilities. The diagram below illustrated manhole-cover lifting.
Above: Diagram demonstrating how a surveyor can lift a manhole covers to verify the location of utilities within the confines of the manhole.
In this instance, the manhole is free of limitations such as water and debris. This means that the surveyor can confirm and measure the depth to the utilities.
Above: Diagram showing the inside of a manhole.
This example shows the inside of a manhole. The green fluid in this image is Drain Trace, a coloured fluid used to trace the location of pipes.
Electromagnetics (in active and passive mode)
In passive mode, the Electromagnetic Locator (EML) can detect cables which emit a power or radio signals. It does this by detecting the magnetic field emitted by the pipe or cable, but it does not detect the utility itself. In passive mode, the surveyor will walk the site with EML and will mark on the ground where a signal is then located.
A good way to explain the use of EML used in passive mode is using the example of a street light. At night the street light will turn on, meaning the cable to it is powered up. It will then emit a magnetic field detected using EML. During the daytime, the street light will not be supplied with power. This means there will be no magnetic field emitted and the cable cannot be detected in passive mode.
In active mode, SUMO introduces a known frequency into the pipe or cable which can be traced. We create a magnetic field by clamping onto the metallic pipe or cable (i.e. anything that can carry a current) and inducing a current which is then carried through the service and traced from above ground. Introducing a known frequency to the utilities is only achievable where there are metallic pipes or cables exposed. Exposed pipes are usually found through an opened chamber or during excavation.
Duct and drainage can be mapped using a tracer wire which is a length of copper wire sheathed in nylon, known as a sonde. The sonde is threaded into a duct or drain and traced from the surface but, any blockages along the pipe route would limit our findings.
What’s more, where utilities are in close proximity to each other, cross induction may occur. This is where the signal can ‘jump’ from one cable to another due to their proximity. An example of cross induction is a communications cable which runs next to a power cable. The signal emitted by the power cable could transfer to the communications cable. This causes a false reading on the communications cable. The communications cable would then be detected by the equipment, but, it would have to be labelled with caution as it has been detected as emitting a power signal; even though it may have previously been visually identified as a communications cable by the surveyor lifting a manhole-cover. Where this occurs we would highlight an ‘area of concern’ as there are multiple cables in one area which cannot be identified.
Ground Penetrating Radar (GPR)
Above: This photo shows the High-Density Ground Penetrating Radar equipment that SUMO utilises.
GPR has the ability to detect any pipe or cable regardless of the material. This is where we have the best chance of locating plastic and fibre optic cables. It works by emitting a pulse of energy into the ground. It travels through the subsurface until it hits something anomalous to the norm. At which point some of the energy reflects back to the receiving antenna. The time that it takes for the signal to travel back to the receiving antenna is measured and this can be converted into depth estimates. Where these anomalies appear consecutively in adjacent radar traverses it is interpreted as a linear feature (which suggest a pipe, duct or cable).
GPR, like any other detection device, has its limitations. Unfortunately, these are due to the law of physics and are out of the surveyor’s control. The GPR unit needs to be operated on relatively flat ground as the antenna (the section of the equipment which emits and receives the radar signal) must be also kept flat on the ground. It needs to allow enough ground coupling for the signal to penetrate the earth and return. If the ground is uneven, the signal will dissipate before it has a chance to enter the ground. In turn, this causes a weak signal to be emitted into the ground. This makes a somewhat accurate detection of the depth of features unachievable. Where an area of land is too uneven or overgrown, we would not be able to detect plastic and fibre optic cables, so there would be limitations in this area.
GPR antennas come in a variety of frequencies. High-frequency antennas will detect anomalies of a few centimetres but will have a very poor depth of penetration. For example, a 1GHz antenna can penetrate to shallow depths of approximately 0.5m and will be able to detect individual rebars within concrete. Whereas a low-frequency 200MHz antenna may penetrate to 3m+. But it will only detect larger items and it is because of this, it is not suitable for locating all services. A mid-range frequency is generally employed for a utility survey and this usually penetrates the ground to around 2-2.5m depth. But, this is affected by the ground conditions. For instance, dry sand is a very good medium for radar signals (a typical depth of 2.5m or upwards may be achieved). In contrast, very dense clay is a poor medium (a typical depth of 1.0m may work), as the clay absorbs the radar signal.
Depth estimates are generally very good i.e. +/- 10% accurate. As a rule of thumb, the GPR will only detect anomalies which have a diameter which is 10% of their depth. This means that a 100mm diameter service will be detected at 1m, but it may not be at 1.5m. Therefore, one of the most difficult things to locate is plastic domestic gas service which are very small, but often at a depth of over 0.8m and therefore unable to detect. In this scenario, we would note an ‘assumed route’ on the drawing.
The final limitation of GPR is that it can prove difficult to pre-determine how the site may affect the penetration of the signal. As ground conditions can differ over several metres as well as changing throughout the course of the site visit. The GPR system that SUMO utilises is self-calibrating and uses a multi-frequency antenna. Over many years of experience, we trust this achieves the best results irrespective of the ground conditions.
Statutory plan collation
If available, we will utilise statutory plans. These plans can help us to gain a better understanding of the existing infrastructure. It often gives information on the presence of the following features:
Telecommunications
Fibre Optics
Gas Pipelines
Water Mains
Sewers
Electric cables
There are unfortunately restrictions, as the accuracy of these plans can often vary between utility providers. We sometimes find that the plans supplied to SUMO are very accurate. They may even supply us with information such as material type, pipe diameter and other useful measurements. But, we also often encounter statutory plans which will lack information. For example, we may only be given a general area where the utility could be located and what type of utility it is, proving somewhat insufficient for our needs. In an urban context, we find the accuracy of plotted utilities is generally within 2-4m. But in a rural context, it can be 40-60m out. Despite the varying accuracies of the plans, SUMO uses most of these resources where possible.
Plans can often help the surveyor to label a buried utility marked as an “unknown” pipe or cable and can act as a checklist of information. Statutory plans allow the surveyor to re-confirm that the survey work is as accurate as possible. For example, if the surveyor believes they have located all the pipes and cables within the survey area but the statutory plans show that there may have been one missed, it allows the surveyor to double-check their working. If after re-confirming their work on site and still being unable to locate a utility which is present on the statutory plan, they can inform the client that there is a possibility that a pipe or cable in the area has gone undetected. In the situation where the location of a utility is not possible, but it is shown on a plan, we would mark the route as “assumed” on the ground or drawing to warn the client of its presence. This further highlights the technique’s limitations that clients must remember that statutory plans are not always accurate. It could, in fact, be the plans which are in error and not that the surveyor has been unable to detect all the utilities.
So, by combining individual survey techniques SUMO can locate all underground utilities?
In theory, yes!
SUMO has developed a proven methodology which combines the benefits of many survey processes to produce the best quality and most accurate survey data. Having more options available means that there is a greater chance of achieving the most accurate data. It also allows SUMO’s utility surveys to be very adaptable. If one technique is unsuitable for a site due to its limitations, then we have an array of alternatives to work around this.
However, there are always limitations. On some sites, every technique will work well. On others, we find there are plastic cables at depth, uneven ground, clay geology, blocked manhole etc. Which all affect the outcome of the survey.
As well as our combined survey offer, we always ask that the client gives us as much information as possible about the site conditions. This gives us the best chance of locating all utilities by applying the right tools for the job.
SUMO has also invested a lot in the training of surveyors to make them multi-disciplined. This allows for further efficiencies and associated cost savings due to a reduction in the workforce needed to survey the site.