Jun 192017
 

Manitou Springs occupies a narrow valley where Fountain Creek emerges from the foothills northeast of Pikes Peak and west of Colorado Springs. The valley slopes are composed of interbedded resistant sandstone and conglomerates (i.e., gravelly sandstone), and weaker mudstones and shale. The outcropping sandstone is most prevalent on the steeper slopes on the north side of the valley.

During the wet spring of 1995, rockfall and landslides incidents increased throughout Colorado, some resulting in fatalities. In Manitou Springs, a fortunate set of circumstances occurred before the Memorial Day holiday weekend when local residents observed the movements of a large, dangerous block of rock before it actually could fall. The observation set into motion an emergency declaration by the town, resulting in a compulsory evacuation of homes located below the rocky slope, the closing of the road in the area, and an immediate rock stabilization project. During this emergency situation, the Colorado Geological Survey was asked to provide expert assistance to help stabilize the rock. The emergency evacuation decree remained in effect until the rock was stabilized and the area subsequently declared safe.

The ledge of jointed sandstone along with several large displaced blocks is seen at the center of the image. Photo credit Jon White.

The ledge of jointed sandstone along with several large displaced blocks is seen at the center of the image. Photo credit Jon White.

A prominent 12-foot-thick ledge of strongly-jointed sandstone forms the rim of this slope. Two essentially vertical and intersecting joint sets produce large orthogonal sandstone blocks that are being undermined by the more easily weathered mudstone beds below the ledge. The blocks begin to topple as the underlying rock that supports them erodes, creating dangerous overhangs. At the time of discovery, this particular block had moved 5.5 feet from the back face of the sandstone ledge and tilted precariously over the next sandstone ledge below. Had the 70-ton block fallen, it would have certainly crushed a home below.

A precarious rock above Manitou Springs started to move in 1995 after a period of wet weather. As an emergency measure, high-strength steel cables were wrapped around the rock and anchored to the surrounding ledge to arrest the movement. Photo credit Jon White.

A precarious rock above Manitou Springs started to move in 1995 after a period of wet weather. As an emergency measure, high-strength steel cables were wrapped around the rock and anchored to the surrounding ledge to arrest the movement. Photo credit Jon White.

The extremely unstable, tilted, rock could not be removed due to the proximity of homes directly below, so high-strength steel cables were wrapped around the rock and anchored to the surrounding ledge. Once the block was safely restrained, additional cables were physically attached to the top of the block at anchor points that were cemented into drill holes to provide an additional level of support for the block and safety for the homes below.

After the rock was stabilized, additional cables were physically attached to the top of the rock block and secured to surrounding stable rock. Photo credit Jon White.

After the rock was stabilized, additional cables were physically attached to the top of the rock block and secured to surrounding stable rock. Photo credit Jon White.

May 162017
 

The Association of American State Geologists announced that their annual John C. Frye Memorial Award for 2017 is granted to the CGS and the staff members who authored the report The West Salt Creek Landslide: A Catastrophic Rockslide and Rock/Debris Avalanche in Mesa County, Colorado (CGS Bulletin-55). Utilizing a rich field data set, the report includes a comprehensive review of the geologic history of the area and presents a detailed timeline of the events surrounding the “the longest landslide in Colorado’s historical record.”

White, Jonathan L., Matthew L. Morgan, and Karen A. Berry. “Bulletin 55 - The West Salt Creek Landslide: A Catastrophic Rockslide and Rock/Debris Avalanche in Mesa County.” Bulletins. Golden, CO: Colorado Geological Survey, 2015. Bulletin 55.

White, Jonathan L., Matthew L. Morgan, and Karen A. Berry. “Bulletin 55 – The West Salt Creek Landslide: A Catastrophic Rockslide and Rock/Debris Avalanche in Mesa County.” Bulletins. Golden, CO: Colorado Geological Survey, 2015. Bulletin 55.

History of the Award:

Environmental geology has steadily risen in prominence over recent decades, and to support the growth of this important field, the Frye Award was established in 1989 by GSA and AASG. It recognizes work on environmental geology issues such as water resources, engineering geology, and hazards.

John C. Frye joined the US Geological Survey in 1938, he went to the Kansas Geological Survey in 1942, he was its Director from 1945 to 1954, he was Chief of the Illinois State Geological Survey until 1974, and was Geological Society of America Executive Director until his retirement in 1982, shortly before his death. John was active in Association of American State Geologists and on national committees, and was influential in the growth of environmental geology.

The Award is given each year to a nominated environmental geology publication published in the current year or one of the three preceding calendar years either by GSA or by a state geological survey. A shared $1000 prize and a certificate to each author is presented at the AASG Mid-Year meeting, held Tuesday morning at the GSA annual meeting.


Citation: White, Jonathan L., Matthew L. Morgan, and Karen A. Berry. Bulletin 55 – The West Salt Creek Landslide: A Catastrophic Rockslide and Rock/Debris Avalanche in Mesa County. Bulletins. Golden, CO: Colorado Geological Survey, 2015. Bulletin 55.
May 032017
 

The CGS recently installed the first of five new seismic recording stations that will collect information on seismic events around the state and the region. The CGS seismic network acts in conjunction with those maintained by the University of Colorado and Colorado State University, the Incorporated Research Institutions for Seismology (IRIS), and the US Geological Survey‘s National Earthquake Information Center (NEIC) — to provide near real-time earthquake detection. The addition of our monitoring capacity, the wider network allows the geoscience research community to better understand background seismicity in Colorado and better discriminate between natural and induced seismic events that may occur in the region.

The CGS already operates four other stations with Streckeisen STS-2 Broadband Sensors (capable of sensing ground motions over the frequency band 0.01 Hz (100 sec) to 15 Hz). They were part of a national consortium — USARRAY — that was a portable seismic network migrating around to different locations in the US several years ago. State-level organizations were allowed to ‘adopt’ some of the stations that were deployed within each state. The CGS purchased the four stations in 2010 — they are included on the map below as red boxes.

The set-up for a typical recording station includes the seismometer and its associated data recorder, a power system, and a communications system. The install site is carefully chosen for its relative acoustic silence — such that human-caused (road and air-traffic) and natural (wind, animal) noise levels are minimal at the relevant frequencies. The CGS cooperates with the Colorado State Land Board and the Colorado State Parks system in locating optimal sites for the stations in the CGS network. The particular station illustrated here is our Briggsdale Seismic Station #T25A-1 near Greeley, Colorado.

The physical installation of an isolated off-grid seismometer station includes the excavation of a pit for the seismometer ‘vault’ to sit in, a trench for cabling from the seismometer to the recording and power equipment, and a photo-voltaic (solar) power and data transmission tower. Adequate fencing to isolate the installation from noise and physical disturbance — in this particular case, grazing cattle — is important.

Images from the installation of our fifth seismometer station near Briggsdale, Colorado.

A seismometer is a device that can sense a wide range of ground motions or vibrations. Environmental considerations require that its underground installation be both level and thermally insulated. A sub-surface concrete pad is prepared with a glass plate embedded on the top to provide a perfectly flat platform for the seismometer to sit on. After precise leveling, the seismometer is then connected to a data recording system that is installed some distance away in a weather-proof console — again to keep possible vibrations from the tower at a minimum. The data recording box includes an A-to-D (Analog-to-Digital) converter that digitizes the signal and prepares it for transmission via the communications system.

The communications system consists of a modem and a GPS transceiver. Once recorded by the seismometer, a seismic trace is converted to a digital signal, processed and sent via the modem to a local cell tower where it is relayed first to IRIS and then on to the NEIC for correlation and display. The GPS provides a standard clock signal for data synchronization, an important factor in coordinating each individual seismic station with the wider network of stations. The IRIS website provides current near-real-time data for the Briggsdale station as well as all other stations in the network.

The power system includes a deep-cycle marine battery, and a photovoltaic panel for recharging along with a voltage inverter/charge controller to ensure a stable power supply for the data recording and communications system.

The map includes our seismic stations (totaling five as of 2017) along with others available across Colorado:

Feb 062017
 

With all the precipitation in the Rockies this year (we’re at +153% normal snowpack at the moment), we thought we would re-release a publication that highlights at least one important aspect of Colorado snowfall — that is, the significant danger of avalanches. The Snowy Torrents: Avalanche Accidents in the United States 1980-86, compiled and written by Nick Logan and Dale Atkins and illustrated with Larry Scott’s fine pencil drawings, was first published in 1996. We still have a few hard-copies available and, because of that, yes, we do charge for the PDF download. However, Larry went back and re-made the PDF from the original publication file, producing a file that is far better than the rather poor digital scan we had offered previously.

The volume details 146 oft-times harrowing stories surrounding avalanches, the lives they claim, survivors and witnesses, along with assessments as to what happened, why it happened, and what could have been done to prevent loss of life and/or property. The authors are never judgmental, and their clear-eyed accounts contain a wealth of wisdom that will add to the knowledge-base of any winter backcountry enthusiast.


Citation: Logan, Nick, and Dale Atkins. SP-39 The Snowy Torrents: Avalanche Accidents in the United States, 1980–86. Special Publications 39. Denver, CO: Colorado Geological Survey, Department of Natural Resources, 1996.
Feb 012017
 

By Jill Carlson

On March 23, 2003, a large avalanche occurred about one mile west of the Town of Silver Plume. The avalanche brought trees, rock, soil and snow to the valley floor, knocked down overhead utility lines, blocked the I-70 frontage road, damaged the town’s water treatment plant (WTP), and dammed Clear Creek. The dam was breached using explosives before the plant’s electric pump motors were flooded. With damage to the WTP’s chlorine contact tank and building, Silver Plume residents had to boil their tap water for over a month.

The avalanche occurred three days after near-record snowfall. It was triggered by additional snow loading in the starting zone caused by a change in wind direction, and began in a known avalanche path above timberline on Pendleton Mountain. Its unusually large volume and velocity caused it to unexpectedly reach the valley floor, along a path not previously identified as an avalanche chute. Rick Gaubatz, the Town’s water commissioner, counted 110 rings in a spruce tree that was found in the avalanche debris at the damaged WTP, indicating that an avalanche of similar magnitude had not occurred in the immediate area in at least 110 years.

Avalanche debris in the runout zone taken by Xcel Energy from a helicopter on the morning after the avalanche occurred, 24 March, 2003.

Avalanche debris in the runout zone taken by Xcel Energy from a helicopter on the morning after the avalanche occurred, 24 March 2003.

Continue reading »

Jan 302017
 

Introduction

The earth’s surface can subside because of underground mining when rock is removed at depth. Although subsidence can occur due to hard rock mining, this article only considers the effects of coal mining.

When coal is extracted underground, gravity and the weight of the overlying rock may cause the layers of rock to shift and sink downward into the void left by the removal of the coal. Ultimately, this process can affect the surface, causing the ground to sag and crack and holes to form. Merely an inch of differential subsidence beneath a residential structure can cause several thousand dollars worth of damage.

Subsidence can happen suddenly and without warning. Detailed investigations of an undermined area are needed before development occurs to resolve the magnitude of the subsidence hazard and to determine if safe construction is possible. While investigations after development can determine the extent of undermining and potential subsidence, often, existing buildings cannot be protected against subsidence hazards. The cost of remedial measures is often extremely high. Continue reading »

Jan 162017
 

On solid ground — that’s how many of us think of good old, stable earth. So it’s disconcerting when the ground moves out from under us in any way.

Because of our environment, history, and geology, Colorado has conditions where ground movements can costs millions of dollars in annual property damage from repair and remediation, litigation, required investigations, and mitigation. There has been recent attention to swelling clay soils and heaving claystone bedrock, and the media has helped publicize these problems, which are predominant along the Front Range. But that’s only half the story. Geologic hazards in Colorado also include ground that sinks. Ground subsidence and soil settlement pose significant hazards in Colorado in many areas throughout the state. A variety of causes, some human-made and others inherent to the geology and geomorphology of Colorado, cause these sinking problems. Continue reading »

Jan 122017
 

At the end of the 19th and beginning of the 20th Century, some of the first settlers of the plateau region of western Colorado along the Colorado River, and the Uncompahgre and Paonia river basins, looked to fruit crops for their livelihood. The semi-arid but moderate climate was well suited for fruit orchards once irrigation canal systems could be constructed.

But serious problems occurred when certain lands were first broken out for agriculture and wetted by irrigation. They sank, so much in places (up to four feet!) that irrigation-canal flow directions were reversed, ponding occurred, and whole orchards, newly planted with fruit trees imported by rail and wagon at considerable expense, were lost. While not understood, fruit growers and agriculturists began to recognize the hazards of sinking ground. Horticulturists with the Colorado Agricultural College and Experimental Station (the predecessor of Colorado State University) made one of the first references to collapsible soil in their 1910 publication, Fruit-Growing in Arid Regions: An Account of Approved Fruit-Growing Practices in the Inter-Mountain Country of Western United States (pdf download). They warned about sinking ground and in their chapter, Preparation of Land for Planting, made one of the first recommendations for mitigation of the hazard. They stated that when breaking out new land for fruit orchards, the fields should be flood irrigated for a suitable time to induce soil collapse, before final grading of the orchard field, irrigation channels excavation, and planting the fruit tree seedlings. Continue reading »

Jan 102017
 

Many areas of Colorado are underlain by bedrock that is composed of evaporite minerals. Indicative of the word evaporite, these minerals were deposited during the cyclic evaporation of shallow seas that existed in central Colorado millions of years ago. As the water continued to evaporate, the remaining solution became hyperconcentrated with salts: minerals such as gypsum, anhydrite, and halite (rock salt). These minerals precipitate out of solution and accumulate in shallow nearshore basins on the bottom of the sea floor. Depending on the paleoevironment, thinly interbedded fine sandstone, mudstone, and black shales can also occur in the evaporite. Mostly Late Paleozoic and Mesozoic rock formations contain evaporite beds in Colorado. Some are thin and discontinuous — only minor beds within a rock formation. Others are massive, with evaporitic minerals many hundreds of feet thick.

Evaporitic bedrock locations in Colorado. [Gypsum Mines from Mineral Resources of Colorado, 1968, P. 191; Geology Modified From Tweto, 1979]

Evaporitic bedrock locations in Colorado. [Gypsum Mines from MI-07 Mineral and Water Resources of Colorado, 1968, P. 191; Geology modified from Tweto, 1979]

Millions of years of burial, plastic deformation, mountain building, and erosion have forced the evaporite beds to the shallow subsurface and/or ground surface today. Evaporite minerals in Colorado are a valuable mining resource. Historic mining occurred throughout the state where thin gypsum beds were exposed. Active mining continues in the massive deposits near the town of Gypsum. Continue reading »

Apr 222016
 

Subsidence experts visiting the Netherlands. Deltares hosted 15 international subsidence experts to discuss subsidence problems worldwide at the annual meeting of UNESCO Land Subsidence working group. Gilles Erkens, subsidence expert Deltares showed the impact of subsidence in the Netherlands during a field trip.

Land subsidence is causing more and more damage every year. It scarcely registers on the radar of many countries. Even so, the impact on coastal cities and peat areas is increasingly apparent. Levels of flood damage are rising and the risk of casualties is following. Land subsidence can also lead to major economic losses such as structural damage and high maintenance costs for roads, railways, dikes, pipelines and buildings. The total bill worldwide mounts up to many billions of dollars annually. It can only rise further in the future with population growth and the intensification of economic activities in delta areas.