You cannot just put anything in a hole. Naturally you go to your trusted geologist when you want to fill in a hole, for safety, to build something, or to remove a hazard. The following provides a quick guide to what a hole is and how to determine what to put in it.
How big is the Hole, dear geologist?
The first thing that gets talked about with holes are their dimensions, including width and depth. These are relative terms that don’t have exact definitions. However, for many purposes, consider the following:
Small diameter is between 3 inches to a foot wide, such as wells and test borings. Wide is somewhere between 1 to 300 feet wide, such as pits and excavations. Very Wide is 300 feet or more across, an example being a quarry or reservoir.
Shallow is up to 20 feet deep. This is the typical depth of building foundations and utilities. Intermediate is around 20 to 200 feet. This is the typical depth for drilled borings in environmental and geotechnical studies. Deep is on the order of 200 to 1000 feet, the typical depth of water supply wells, surface mining activities, and environmental remediation projects. Extremely deep is anything greater than 1000 feet. This is the typical realm of oil and gas, geothermal resources, and some very deep mines.
What’s on the outside, dear geologist?
The next big consideration with how to describe a hole is what the hole is not.
A Deep hole may be surrounded by Rock. If the surrounding rock is sufficiently solid, we call it Bedrock. Other holes are surrounded by soft natural material called Soil. Soil is a substance made up of decomposed rock and Organic Matter.
Still other parts of the hole may be surrounded by artificial materials, called Fill. Fill is anything that people place in or on the ground, that did not get there through natural means.
And what is on the inside, dear geologist?
You need to know whether the hole has water. Holes without water are called Dry.
When it comes to water, the most important consideration is its origin. Water landing in the Hole directly from the sky is called Precipitation. Water derived from precipitation that flows overland into the hole is called Stormwater Runoff. Water entering the bottom and side of the hole is called Groundwater.
And how much water is there, dear geologist?
The amount of water in the hole can be measured by determining the hole’s depth and dimensions of the hole. This is done with a measuring tape or electronic water level probe.
And how did the water get there, dear geologist?
Note that the location of water in a hole is not necessarily the same as the surrounding water table. Some holes are filled with water but the surrounding earth is dry. This could be due to recent direct precipitation or stormwater runoff. Other holes are filled with water, but the water level in the hole is higher than in the surrounding earth. This may be due to artesian pressure from groundwater outside of the hole.
And what if there is other stuff in the Hole, dear geologist?
Before filling in a hole, the contents of the hole need to be determined. Unnatural chemicals in the hole and surrounding groundwater are considered Contaminants. If there are living things in the water or soil, we call that Biology. If there are ancient human objects, we consider that Archeology. All of these factors need to be considered before filling in a hole.
And why is there a Hole, dear geologist?
The most well known hole formed in nature is the Sinkhole. Sinkholes are formed in areas underlain by certain types of bedrock such as limestone and dolomite.
Other holes are formed due to water main breaks in urban areas, not necessarily related to the bedrock. These holes can be just as dangerous as sinkholes.
A Well is defined by EPA (40 CFR 144.3) as a “bored, drilled, or driven shaft whose depth is greater than the largest surface dimension; or, a dug hole whose depth is greater than the largest surface dimension; or, an improved sinkhole; or, a subsurface fluid distribution system.”
And are there any requirements for filling the Hole, dear geologist?
There are lots of government codes and rules, commonly termed Regulations, about how to fill in holes. There are many regulations associated with construction activities, mining, and drilling. In some cases, these regulations require advance planning and notices to government agencies, called Permits. The permits and associated regulations often include requirements about what to put in the hole.
If a wide hole has water in it, it might fall under government jurisdiction. This determination is made based on the history, biology, surrounding soil, and origin of the hole. If the surrounding soil is saturated, or has specific biology, the area surrounding the hole may be considered a Wetland. There are specific regulations and permits required to place fill in wetlands.
And what if there are no regulations, dear geologist?
Many holes and what to put in them fall outside of regulations. Make sure these holes are properly evaluated before adding fill.
And what should we put in a Hole with Groundwater, dear geologist?
Aha! Your question is a good one. The best solution we have come up with for what to put below the water table include:
• Rock can be placed in a hole below the water table provided the rock does not contain harmful minerals. At typical construction sites, such as buildings, roads, treatment plants, and schools, excavations below the water table are generally filled with rock.
• Grout is an extremely fine-grained fill material that when placed into a hole, prevents the flow of groundwater.
• Cement is a type of grout that hardens to the extent that it is similar to Rock.
• In specific circumstances, Soil can be placed below the groundwater table. This has been done, especially at very wide holes like quarries. Considerable planning is required including groundwater monitoring and permits for such activities.