Surface Subsidence in Jefferson County as a Result of Mining

The mining of coal and clay in Jefferson County has already caused surface subsidence in some areas and has created the potential for subsidence in other areas. It is possible to map the subsidence which has occurred in the past and to estimate the extent of the subsidence potential which exists today.

Figure A. Extent of Mining Map (Golden) Dotted areas are extent of underground coal mines. Locations marked 15, 16, 17, 18 are sites of mine subsidence. Location 19 is the site of differential settlement of fill material over reclaimed clay strip mine (hash marked area). Location 6 is the site of a horizontal mine opening.

For a larger map of the study area, click here (this image is 190 KB).

Despite the common frequency of mining-induced subsidence in Jefferson County, the amount of surface area affected to date has been quite small. Moreover, the subsidence which has occurred has been, with one exception (Colorado School of Mines, family housing area), in rural areas where the main land use has been pasture and range land. Subsidence in such areas has often gone unnoticed and until now has caused little concern.

Types of Subsidence Occurrence

There are a number of general types of subsidence occurrences in Jefferson County. These types are based on the shape and size of the subsidence features produced, the proximity of the mine to the surface and the mining method used.

Circular Subsidence Pits Over Air Shafts and Man Shafts

Subsidence over abandoned shafts usually produces a circular or nearly circular pit whose sides slope toward its center. The diameters of most pits are between five feet and 20 and are a function of the dimensions of the original shaft. The depths of these pits range from a few feet to as much as 20 and are dependent on the amount of in-filling, either natural or man-made, which has occurred since the mine was abandoned.

Shafts, though small in area, can be particularly dangerous because in-filling of the shaft is often incomplete and potential for subsidence remains. When mines were closed in the past , the shafts were often filled in a haphazard manner. A shaft might become plugged part way down by fallen timbers or other debris so that the part of the shaft above the plug was filled while the part below remained open., In such cases all would appear to be solid at the surface. Over time however any plug will eventually loosen. When it finally gives way the fill material in the upper part of the shaft may drop and a new pit or opening will appear on the surface. An example of a circular subsidence pit over a probable shaft occurs at Locality 18 on the Golden Extent of Mining Map above and is shown in Figure B.

Figure B.

Subsidence Troughs Over Tunnel and Slope Entries

Tunnel and slope entries usually have only a shallow cover near the mine portal. After abandonment, the roof of the tunnel or slope is sometimes intentionally brought down or simply collapses owing to lack of maintenance. The result is a trough-like feature. These kinds of troughs range in length from 10 to as much as 80, in width from five feet to 15 and in depth from one foot to 10. They present little hazard because subsidence usually is complete throughout the length of the toughs. What hazards are present, are found in the uncollapsed sections of the tunnels and slopes and these hazards are minimal due to the small size of the opening relative to the thickness of the overlying bedrock.

Subsidence Trenches and Elongate Pits Over Stopes Extended to the Surface

When mining clay from underground the operators commonly drove their stopes upward to within a few feet of the surface. The collapse of the remaining overburden into the open stopes has left deep trenches and elongate pits over the lengths of many of the mines. This type of subsidence is the most common of all and is particularly prevalent on the east side of the Dakota hogback from the I-70 highway cut, north to Coal Creek Canyon.

The pits and trenches on the Dakota hogback range in length from 10 to as much as 100 and have widths which range from five feet to 40.

Figure C. Part of the North Golden Clay Mine (Location 16). Most of this mine was worked by underground methods. The subsidence trench in this portion of the mine has been enlarged by surface mining of clay pillars left between collapsed stopes.

Subsidence Troughs Over Near-Surface Stopes

A number of shallow, elongate depressions are found associated with some of the abandoned coal mines. They range in length from 20 to 40, in width from five to 20 and in depth from 1 to 5 feet. Their origin is believed to be due to subsidence over old coal mine stopes because they are located over the mapped mine location and because they overlie the trend of the basal Laramie.

Subsidence Over Reclaimed Land

This type of subsidence is not the result of collapse of underground openings but is the product of differential settlement over open pit clay mines which have been cosmetically back-filled. The open trenches and pits of this type of mine have sometimes been filled to the level and contour of the surrounding land surface to the extent that it looks pleasing to the eye. However, the fill is not as compacted as the enclosing bedrock, which in many clay mines consists of fairly hard sandstone. The difference in compactability of the two materials is of little importance as long as little or no load is placed on the fill material. When structures such as buildings, roads and bridges are built, and loads are applied, the fill will compact more than the sandstone and the structure will undergo a bending movement at the interface between the two materials. The result can be significant damage to the integrity of the structure.

This type of subsidence has already occurred in the family housing section of the Colorado School of Mines. A group of apartment buildings were built over a worked-out clay mine which had been reclaimed by filling the trenches left between sandstone ribs. The fill was thought to be well-compacted and where there was doubt concrete caissons were sunk through the fill to solid sandstone. Despite these precautions, differential compaction did occur and some of the buildings were damaged. Streets and sidewalks in the area have been warped and broken by subsidence and these effects can still be seen.

Figure D. Damaged sidewalks in the family housing area of the Colorado School of Mines. Much of the street in the left part of the photo is over a filled clay pit.

Figure E. Condemned apartment building in the family housing area of the Colorado School of Mines. Note the distortion in the window frame.

All of the geologic information contained on this page was taken directly from Coal and Clay Mine Hazard Study and Estimated Unmined Coal Resources, Jefferson County, Colorado, by Amuedo and Ivey, 1978