We are rightly in an environment where almost every major contractor requires a utility survey prior to commencing work on site. Additionally, clients and consultants also have the completion of a survey as a prerequisite before any design or feasibility work is undertaken.
Even though it is now widespread and standard practice across the industry, the utility survey is still a misunderstood process to many.
Notwithstanding the technical aspects and limitations of geophysical survey methods, this article is for explaining the process involved in undertaking a utility survey to give clients a better understanding of what they should be expecting the survey practitioner to deliver.
What is the specification for an underground utility survey?
If you have not issued the utility survey provider with a required specification, they should be working to industry recognised best practice. These standards vary around the world. In the U.K., the standard is currently detailed in PAS128 Specification for underground utility detection, verification and location. In the USA it is ASCE Standard 38-20. These two in particular share similarities in relation to the different types of survey to be undertaken.
Detailed within them is the methodology used for undertaking the survey and the deliverables you should be receiving from a utility survey.
In the U.K., the benefits of a PAS128 survey are still not universally appreciated and some companies will offer surveys which are not to the PAS 128 standard. These may be generally cheaper due to the amount of information required by PAS. Ultimately though, the on-site process is broadly the same no matter what has been agreed. Despite the variation in the cost of a utility survey, there are certain steps which have to be undertaken to produce a complete and accurate utility survey. No matter where you are located globally, the utility survey methods should be broadly similar. As a minimum a survey practitioner should be using radio frequency location (rfl) and ground penetrating radar.
What are the different methods of a utility survey?
The utility survey fundamentals
The fundamental elements of a utility survey have never really changed. This is because to provide comprehensive and accurate survey data there are standards to which all survey companies should hold themselves, no matter what specification is being employed. In addition, the equipment types used in a utility survey vary in specification, but essentially are fundamentally pretty standard.
The survey starts with a services records search
All utility surveys should have a desktop study undertaken. This is to ascertain the availability of existing services records for the area. This is a critical step in the utility survey process and should not be overlooked.
It was not unusual a few years ago when we requested records the stock answer was, “You can’t have them. That’s what you’re here for”. Needless to say this attitude is at best unhelpful and potentially dangerous. Fortunately, the industry has progressed to the point this is a very rare occurrence.
For sites in highways and public spaces, the search can be undertaken by the survey company undertaking the job. The process involves contacting the relevant infrastructure asset owners that operate within the area and requesting the relevant plans for the area. Some of these records will be available free of charge and others will have to be paid for.
On privately owned sites the asset owner records do not always, and often rarely, provide any relevant information as to what is buried within the area. The organisation who has employed the survey company should ask the client for any records the site holds. Alternatively, the utility survey company can do this for you if you provide the relevant contact information.
Lift and trace with EM location will find many utilities
The principle element of a utility survey is still the lifting of covers along with the employment of electromagnetic location (EM) technology. A survey practitioner should lift all of the covers possible within the survey area and trace the services within using a signal generator and receiver. A utility survey company is more likely to use the higher end precision locator models from Radiodetection or Vivax, that the standard equipment available from a hire shop for instance.
Part of the EM process is to use a direct connection to services where it is safe to do so and a range of peripherals to aid in the location of the services. These will include cable clamps, sondes and tracer wires as a minimum. The use of these allows the isolation of individual cables or ducts and the location of non-conductive services such as drainage and fibre optic cables where access to the pipes and ducts is available.
Some chamber covers will not be able to be lifted by the survey company and they should notify you if this is the case. It is very likely that for some occurrences the survey company will be able to arrange additional equipment as necessary. These may include Gatic type covers, covers which may be padlocked and seized covers.
Locate services with further EM location techniques
When the location of services from the lifting of covers is completed the surveyor should employ the EM location equipment to induct a signal actively into any other available conductive services. This may include lamp columns, cathodic protection posts, earth points etc.
After the collection of all possible information from the methods outlined above an active and passive EM sweep of the site should be undertaken to identify further conductors which have yet to be located during the process.
Ground penetrating radar for utility surveys
Ground penetrating radar or GPR as it is most often referred to is a critical element of the survey process. This is also often the most misunderstood by people outside of the survey industry.
A GPR system is not limited to locating conductive materials such as cables and metal pipes in the way that EM is. Ground penetrating radar has the ability to detect services of all materials including concrete, clay and plastic pipes.
There are limitations as to what can be achieved with GPR due to the many external factors affecting the data. These include conductive soils such as clay, water content and salt content. In addition, surface type and condition play a major role in GPR data acquisition.
There are many GPR options available to survey providers. These range from simple single frequency systems with no data collection capacity to high-density arrays which are trailer mounted and towed by a vehicle.
The system chosen will depend on the survey specification required and the survey environment. This will have a significant effect on the GPR survey cost.
For simple sites and areas with a low density of buried utilities, it is likely that a mark on-site system will be sufficient. In this instance, it is the experience of the survey operative that determines the presence of GPR targets. There is no requirement for data collection and later post processing.
Most sites will require GPR post processing to be undertaken to achieve the best results from the survey. When buried utility infrastructure becomes more complex, recording the data for a qualified Geophysicist to interpret increases the chances of target location and removes the chance of the surveyor mixing up visible targets.
The recording of GPR data for post processing provides the additional benefit of having a traceable resource for quality assurance purposes. Where a mark on-site system is used, there may be no record of why individual targets were identified by the surveyor.
What information is produced from an underground utility survey?
A topographical survey is undertaken to collect the data marked on site. This is then processed along with other collected information to produce the final set of information produced from utility mapping. The final deliverable from a survey should include the following as a minimum:
- A report detailing the effectiveness of the techniques employed
- CAD and PDF plans of the located information
- Metadata detailing the types of service (I.e. High Pressure, High Voltage etc), depths and quality confidence of the indicated position
- Survey team who carried out the survey
- Explanatory notes detailing the potential limitations of the data
- Data of every chamber including photographs and sketches
Even if not issued to the client, the surveyor should be able to produce the following information on request:
- Details of different types of equipment used
- Raw GPR data for independant review
- Control network observations
Additionally a survey may include elements of the following:
- Survey data linked to 3D topographical survey
- Survey data included within a 3d laser scanning survey
- Services included into civil engineering design drawings
Complete and accurate information, along with communication between the survey practitioner and the client can ensure both sides understand the scope, the expectations and the limitations of any survey undertaken.
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