Subsurface Survey
Detecting underground utilities before excavation. El-Eman Office provides comprehensive surveying including utility detection.
Stages of locating utilities before excavation
A subsurface survey does not begin with instruments but with gathering information. Utility drawings are requested from the owning bodies, such as water, sewerage, electricity, gas and telecoms, to identify likely locations. The site is then visited to identify manholes, covers and visible signs that indicate routes. Instruments are then used to scan the area, trace probable routes and mark them on the ground with spray paint, and finally their positions are recorded with coordinates and levels on one drawing. This sequence reduces surprises when digging begins.
The limits of the technologies and how they complement each other
No single technology detects everything. Ground-penetrating radar (GPR) gives an excellent picture in dry, sandy soil, but its accuracy drops in wet clay and salty soil. An electromagnetic locator traces metal cables and pipes only and does not see plastic pipes that carry no signal. Surveyors therefore combine technologies and compare the results with old drawings.
- Plastic pipes may need a tracer wire or a signal passed through them to be detected.
- The measured depth is approximate and affected by soil type and signal velocity.
- Neighbouring or crossing services can be confused, so they are checked at manual exposure points.
Limited hand exposure and excavation safety
Hand exposure by digging small trial holes remains the most accurate way to confirm a service's position, type and depth, and it is used at critical points. During mechanical excavation close to detected services, a safe distance is kept and soil is removed carefully near the route, with the locations marked for workers and operators. A briefing for the team before work is recommended, covering the detected services and the emergency steps if damage occurs.
- Gather all available drawings and records before the survey.
- Choose technologies according to soil type and the expected services.
- Mark the routes on the ground and record them with coordinates.
- Confirm critical positions with hand-dug trial holes before mechanical excavation.
The detection report and how to read it
The detection report should describe the method, the instruments and the area covered by the survey, and the areas that could not be surveyed and why. In the drawing, services are shown in colours and symbols, with confirmed services distinguished from probable ones. An area showing no services should not be taken to be truly empty, since services may be beyond the instrument's capability. It is therefore advisable to read the report together with the person in charge of excavation, to identify the points needing manual exposure before mechanical work.
Frequently asked questions
Does radar detect everything underground?
No. Its results are affected by soil type, moisture and salinity, and it may need supporting instruments and manual confirmation.
Do I need a survey on empty land?
Open land may have had old services or long lines pass through it, so it is better to enquire before deep excavation or large foundations.
How accurate are the recorded depths?
They are estimates, normally stated with a margin, and are not a substitute for direct exposure when high accuracy is needed.
