Subsurface Void Measurement Surveys

On numerous occasions Laser Surveys has been asked to identify the location and extent of possible large voids below or behind structures. In the past, if the voids can be accessed by a surveyor safely this has not been a problem.

But what happens when the void/cavity or underground feature e.g. sub-surface tank, chamber, large cavity behind a wall, large egg-shaped culvert, etc. cannot be either readily accessed or safely accessed and someone wants to know the size of it?

Previously the position and approximate depth has been ascertained from the surface by using Ground Penetrating Radar techniques, but with all technologies this has its limitations, mainly due to varying site conditions including depth, surface construction and presence of rebar. Our clients would ideally require more information, including an accurate size and shape of a void. Previously this was not achievable. Now there is a solution!

Firstly, after locating the approximate position and extent of the void/cavity from GPR or previously held data, a borehole is drilled in to the void or cavity. Laser Surveys will then deploy a miniature, ruggedized 3D laser scanning system into the void via the borehole or other small access point. Once in the void the laser head opens out to measure the 3D shape of the void and its surface reflectivity. The void can then be measured remotely.

The survey can either be viewed in real time at the surface or safe distance away, or post-processed to produce 3D point cloud, surface model, volume, sections or AutoCAD points output. This information can be further supplemented by deploying one of our CCTV cameras.

Applications where man-entry may not possible and the laser system can be utilised -

• Sub surface voids and cavities
• Undercrofts
• Underground Chambers and Tanks
• Voids behind walls and
• Duct Surveys
• Inaccessible roof spaces
• Bored Piles
• Subsidence
• Undersides of bridges (when safe access is available on top but not below)
• Stope Surveys
• Ore Passes
• Egg culverts
• Water Tanks and Fuel Tanks
• Culverts
• Shafts and Bunkers
• Underground Caverns
• Industrial production facilities where access is limited or unsafe e.g. vessels
• Better targeting of adjacent boreholes

Key Benefits of using Laser Scanning System

• Survey potentially dangerous underground voids in complete safety
• View results in minutes not days
• Robotic automatic operation
• Flexible deployment methods
• Flexible automatic operation
• Small and easily transported
• Precise and accurate cavity/ void measurement
• 50mm/ 2.0" diameter allows deployment down narrow boreholes
• Rugged stainless steel construction
• 360° Spherical coverage with no blind spot

What do we ideally need to know for a quotation enquiry?
(We realise you might not have all the answers but the more information we have the better)

• Is the void/hole flooded?
  The instrument is very expensive and cannot be deployed in flooded or fast flowing water.
• What is the diameter of the hole?
  Our equipment is 50mm/1.97 inches in diameter - so 100mm will be adequate.
• What is the condition and length of the hole?
  Ideally we like the borehole clean of debris, the collar cased and a core liner in stable ground.
• Can we get our equipment close to the area?
• Is the area clear of obstructions or debris, particularly around the deployment area as we do not want any debris falling down the hole when we deploy?
• Are there any existing drawings/photographs/other record information, or drawings relating to either the structure we are scanning or the area above?
• Is there a nearby power source?
  The equipment is self battery powered but depending on the length of time deployed and the amount of scanning required etc these may need to be re-charged.
• Do you require the survey data related to a previous survey?
  If a topographic survey of the surface exists we can relate our findings to it both in grid and level datum.
• What are you hoping to achieve with the survey?
• Do you require an onsite real time display only or would your require volume calculation, sections, dimensions, flythrough, 3D model, 3D point cloud, AutoCAD points, wire mesh etc.

Further information

The laser is a Class 1 eye safe and has a stated range of 150m/500 foot.
Do not worry if the range is not long enough - you just drill another hole (we can direct where it would be best placed by looking at the laser scan).

Accuracy is dependent upon site conditions and how the scanners position is established.
The manufactures stated accuracy is 5cm/1.97inch with resolution of 1cm.

Laser Surveys do not drill the holes.
Unfortunately, this is beyond our expertise and insurances as we are a land surveying company specialising in mapping services.

If the client is drilling holes, how does he know that they will not hit anything?
Laser Surveys also offer underground services location services as well as rebar interval locating to reduce the risk of this.

The scanner relies on clear lines of sight.
Blind spots can result in missing data e.g. An obstruction in the void or a column. To resolve this issue a new borehole or access would be required towards the other side of the obstruction.

Moisture/dust particles in the air can reduce the reflectivity of surfaces.

Extreme humidity or a dust storm can completely obscure a scan.

Positional accuracy depends upon the depth of deployment.

How deep can it go?
Typically to 100 metres on rods but potentially further on an armoured cable and winch system

For deep or horizontal holes where the unit has to be deployed by armoured cable, the inbuilt compass is used to orientate the survey. The compass can be affected by other external influences e.g. Generators, pipes.

The unit cannot 'snake' round tight bends.

It will be the Surveyor's decision as to when the unit is deployed or removed.
The kit is extremely expensive and cannot be readily purchased.

If you would like more information or would like a quotation please contact

Steve Hawkins on 01708 438403 Email: london@lasersurveys.com
or
Chris Wilds on 01886 83173 Email: worcester@lasersurveys.com.