Probing Sinkholes and Small Openings - Determining Areas of Interest

Version 1.2

Written by Mike Doknjas RFT

Original October 3, 2014, Revised January 6, 2017

The Target Audience:

Karst Specialists


This brief is written for assisting Karst Specialists in determining the most probable location of subsurface voids, to determine search intensity and concentrate probing efforts. These techniques have shown good results in the Quatsino Limestone and Parsons Bay formations on Vancouver Island and have been used in the field by the author since 1983.

After reviewing various karst assessment reports produced for industry and government on harvesting land base, Ive concluded that many small openings and sinkholes are only assessed based on the surface expressions. Typically, just visual assessments, generally dimensional and current feature states are noted within the report. Probing or digging were not mentioned or noted in the field.

Its all about risk. When purchasing a home, an inspection of the attic or crawl space generally takes place. Likewise, its also prudent to locate a septic system before driving a heavy vehicle in close proximity. You can get away with not doing it only so many times before your luck runs out.

The majority of sink holes and selected small openings located on karst of Vancouver Island and inner coast islands are possible windows or access points to the subsurface.

Determining which features should be probed or hand excavated

We have all heard in the housing and business market, location, location, location. Same with karst - location does determine where one should concentrate energies and focus selected probing or enlarging small openings to gain access to the subsurface. The larger the historical water flows the larger the subsurface system. Fracture beds are the exception.

Step One

Produce an accurate karst feature map with terrain data of the selected area under assessment to determine drainage basins (LiDAR data works the best). Areas where no surface streams or known cave structures are located are better. Typically, on the East Coast of Vancouver Island, a basin greater than 25 hectares void of surface streams, has a high degree of probability of containing a subsurface flow large enough to create a man size passageway. Basins smaller than 25 hectares can also produce medium size features, especially near contact zones where the limestone lens are typically thinner and more altered. For the West and North Vancouver Island, reduce the area by the annual rainfall percentage difference from the East Coast of the island. Map streams entering a limestone area including losing or gaining streams.


Step Two

Calculate a typical drainage pattern where a stream should be located using existing slopes and gullies and consider the local annual rain fall amounts. Isolate a smaller zone on the lower slopes of this hydrological catchment area and include the upslope feature area. This will become the main focal area. A line of sinks or gullies or the base of bluffs can refine the search further. Limestone ridges and volcanic intrusions are generally subsurface obstructions to the downward trend of the subsurface water flow. Limestone fractures, fissures, bedding plane and dip all need to be considered.

Step Three

An intense search should take place over the entire focal area. A search system such as a grid pattern should be considered, to ensure the entire area is searched. Local vegetation and topography will determine grid spacing and the number of persons required. Locate and map all sinks, pits and exposed limestone in the main focal area. Probing should be performed in all sinks and pits within this area. Most probing is performed by using a 6-8 long diameter aluminum metal pipe accompanying with a longer clean out rod or plug. The probe is inserted in all accessible areas at the bottom and sides of sinkholes for the purposes of finding cavities. Ive been using a one meter pattern for probing. On positive leads, a 1.5-2 PVC diameter pipe is inserted to further assess the cavity. A small camera and light can be inserted to assess whether it warrants digging. With multi-opening caves on moderate slopes a draft can be felt through the pipe under certain circumstances. Many caves have been discovered using this technique. Note the air flow in the diagram below with multiple entrances and differing elevations.


Entering a sinkhole is very dangerous work. Each sink must be assessed for dangers. Always work in pairs with one person on topside out of the sink at all times. Visually inspect the path you intend to use for travel into the sink from the opposite side of the sink to determine if any overhang exists. Using the probe, test the ground in front of you as you enter the sink rim zone all the way down. If the bottom of the sink has no vegetation this could mean it has a false floor - a thin organic layer of sticks and needles covering a deep pit. Some sinks are too dangerous to descend into and not all are for humans. For sinks with steep sides or very loose wall material, consider scaling and attaching yourself to a rope before proceeding down the sink. Full vertical gear is required for the person entering the sink and the safety person on top side. It is recommended that only those with vertical caving experience undertake this. The safest side is generally the side with the gentlest slope, as shown in the photo below left. The photo on the right shows an unsafe approach to a sink.

When openings or cavities are found, if they can be safely and easily enlarged by hand, they should be, closely monitoring for changes in air movement and direction. If the dig becomes a false lead, place the material back, flag with date and depth dug.

The majority of all sinkholes, pits and small openings are potential windows or access points to a subsurface passageway. The size of these passageways are determined by past hydrological influences, geological state, bedrock position and remnant of glaciation infilling.

Before you start probing, spend a day on a probing course. The author of this brief generally holds 3-4 training days per year.

Does digging damage the karst feature? What about the GAR orders? Think about the consequences and damage to the hidden subsurface feature and/or the safety of the forest work force. In order for a professional to prove due diligence, all reasonable effort must be made to determine subsurface size, depth and state.