Expansive Soils
Description
Section titled “Description”Swelling soils and swelling bedrock contain clay which causes the material to increase in volume when exposed to moisture and shrink as it dries. They are also commonly known as expansive, shrinking and swelling, bentonitic, heaving, or unstable soils and bedrock. In general, the term refers to both soil and bedrock contents although the occurrence of the two materials may occur concurrently or separately. The difference between the materials is that swelling soil contains clay, while swelling bedrock contains claystone. In this profile, the term is used to refer to both materials, as they are both relevant to the planning area.
The clay materials in swelling soils are capable of absorbing large quantities of water and expanding 10 percent or more as the clay becomes wet. The force of expansion is capable of exerting pressures of 15,000 pounds per square foot or greater on foundations, slabs, and other confining structures. The amount of swelling (or potential volume of expansion) is linked to five main factors: the type of mineral content, the concentration of swelling clay, the density of the materials, moisture changes in the environment, and the restraining pressure exerted by materials on top of the swelling soil. Each of these factors impact how much swelling a particular area will experience, but may be modified, for better or worse, by development actions in the area.
In Colorado, swelling soils expand and contract naturally during seasonal wetting (winter and spring) and drying (summer and fall) conditions and in their natural, undeveloped state they cause little damage.
However, exposure to additional water sources, such as lawn and garden irrigation or precipitation drainage from houses, and reduced evaporation properties caused by the development of roads, sidewalks, buildings and parking lots, may cause the swelling soils to expand more than they would if they remained undeveloped. In addition, the re-grading of development areas may expose more swelling soil to moisture than the natural state, causing a more widespread swelling event.
In Jefferson County, there are also areas of steeply dipping bedrock or heaving bedrock along the foothills. In these areas, sedimentary bedrock layers are steeply upturned and tilted to form the distinctive hogback features. This causes bedrock to swell unevenly in a linear pattern, instead of the uniform pattern more common to flatter areas of swelling soils, and subjects structures to extreme amounts of both vertical and lateral stress. In Jefferson County, areas of potential dipping and heaving bedrock are identified as a geologic hazard and construction in those areas is heavily restricted.
Swelling soils are one of the nation’s most prevalent causes of damage to buildings. According to the 2018 State Hazard Mitigation Plan, annual losses nationwide are estimated in the range of $2 billion. In Colorado, the cost is estimated at $16 million annually. Potential damages include severe structural damage; cracked driveways, sidewalks, and basement floors; heaving of roads and highway structures; condemnation of buildings; and disruption of pipelines and other utilities. Destructive forces may be upward, horizontal, or both. Buildings designed with lightly loaded foundations and floor systems often incur the greatest damage and costly repairs from expansive soils. Building in and on swelling soils can be done successfully, although more expensively, as long as appropriate construction design and mitigation measures are followed. In some cases, avoidance may be the best mitigation policy.
Geographic Extent
Section titled “Geographic Extent”The extent of swelling soils across Jefferson County is primarily contained in the developed portion of the County at the base of the foothills in the northeast portion of the planning area. In fact, the swelling soils neatly follow the rise of the Rocky Mountains along the western and southern portions of the County. The extent of dipping bedrock in the planning area neatly abuts the extent of the mostly horizontal plains of swelling soil on the east, and the fall of the hogback formations on the west. The figures below demonstrate the mapped geologic hazard layers utilized by the planning area for development.
Figure 4-20 Jefferson County Expansive Soils
Figure 4-21 Jefferson County Dipping Bedrock and Subsidence
Previous Occurrences
Damage of varying degrees of severity occurs on an ongoing and seasonal basis. The frequency of damage from expansive soils is associated with the cycles of drought and heavy rainfall and also reflects changes in moisture content based on typical seasonal patterns. Building codes and structure ages also contribute to overall damages, as newer structures are usually built with more resistant techniques or as development restrictions in vulnerable areas minimize expansion and exposure. Published data summarizing damages specific to Jefferson County is not available, but it is acknowledged that a certain degree of damage to property and infrastructure occurs annually, as noted above.
Since the last plan update, the most significant areas that intersect Golden and Morrison remain largely undeveloped; however, growth in western Arvada, western Lakewood, and unincorporated areas along Highway 93 and CO-470 since the last update exposes new development to this hazard. It is important to note that recent development east of Highway 93 in West Arvada and north of Golden was not reflected in the 2015 plan. It is reflected in this plan and shows increased exposure for these areas.
The mapped extent of the hazards clearly impacts approximately 50% of the planning area. However, when considering the geographic impact on the planning area, it is important to note that the entire southern portion of the County is occupied by Pike National Forest, and therefore has a minimal impact on this hazard mitigation plan as development in the area is highly regulated outside of County authority. Of the actively developed and monitored lands in the County, more than 75% is subject to swelling soils or dipping bedrock hazards.
Based on this information, the geographic extent rating for swelling soils is extensive. Probability of Future Occurrences
The planning area has extensive development regulations to minimize the damages incurred by dipping bedrock and other geologic hazards in the County. As such, while previous occurrences are certainly commonly known, it is reasonable to assume that damages and future occurrences should be decreasing.
Since records of specific occurrences are not available to the planning process, it is difficult to estimate the probability of future occurrences. The hazards occur seasonally and annually, which should theoretically equate to a highly likely rating. However, mitigation efforts in place in the County since 1995 should prevent the likelihood of the hazard having damaging impacts. Due to the extensiveness of swelling soils in the County the probability rating for this hazard is considered as likely.
Magnitude and Severity
Section titled “Magnitude and Severity”Information from the event of record is used to calculate a magnitude and severity rating for comparison with other hazards, and to assist in assessing the overall impact of the hazard on the planning area. In some cases, the event of record represents an anticipated worst-case scenario, and in others, it is a reflection of common occurrence. For this hazard, there is no specific event of record, and the extensive mitigation efforts taken since the initial identification of the hazard nearly thirty years ago are taken into account with the magnitude and severity ratings. Therefore, this hazard will be evaluated for potential worst-case scenarios possible under current regulatory standards. Such an event could potentially damage entire neighborhoods, including roads, sidewalks, properties, and utility pipes. Even minor damages on such a scale would quickly incur enormous costs. While critical infrastructure services are not directly vulnerable to the hazard, structures experience the same risks identified for private and commercial properties: if they are built on swelling soil without adequate or appropriate building mitigation, they are vulnerable to damage. In worst case scenarios, this could include loss of communication lines or severe damages to structures rendering them uninhabitable. If this occurred to a hospital or jail, for instance, it could have significant social repercussions, in addition to the incurred costs. Injuries, illnesses and deaths associated with the hazard would be unique and minimal, and probably incurred as secondary hazards resulting from damages to infrastructure. Overall, though the fiscal damage may be extensive, the overall severity and impacts of the hazard are readily mitigated, reducing the overall impacts.
Based on these factors, the magnitude and severity rating for swelling soils is considered limited.
Climate Change Considerations
Section titled “Climate Change Considerations”Changing climate conditions are expected to affect soil resources in many ways. During hot, dry years annual grasses that stabilize and protect topsoil often fail to germinate or do not grow well. This leaves soil surfaces highly vulnerable to erosion from wind and precipitation runoff. Without the availability of nutrient- rich topsoil, crops struggle to survive and flourish. As discussed previously, higher rates of erosion can have a profound effect on agricultural production and on the economies of rural areas of the county.
Many soils and rocks have the potential to swell or expand based on a combination of its mineralogy and water content. The actual swelling of expansive soils will be caused by a change in the environment (e.g. water content, stress, chemistry, or temperature) in which the material exists. Since the 1950s, snow precipitation and duration of snowpack have both decreased while rising temperatures have increase rate of water evaporating into the air and earlier runoff, creating drier soil conditions in Colorado (EPA 2016). More extremes in climate conditions (e.g. wet-dry conditions), could potentially exacerbate the swelling of expansive soil issues in the future.
Vulnerability Assessment
Section titled “Vulnerability Assessment”General Property
Section titled “General Property”Similar to the subsidence hazard, the majority of the hazard’s significance is drawn from the exposure of existing development to this hazard. As identified in the hazard profile and noted above, extensive areas of the planning region east of the foothills are characterized to some extent by swelling soils. Older construction may not be resistant to the swelling soil conditions and, therefore, may experience expensive and potentially extensive damages. This includes heaving sidewalks, structural damage to walls and basements, the need to replace windows and doors, or dangers and damages caused by ruptured pipelines. Newer construction may have included mitigation techniques to avoid most damage from the hazard, but the dangers continue if mitigation actions are not supported by homeowners. For example, the maintenance of grading away from foundations and the use of appropriate landscaping near structures must be continued to prevent an overabundance of water in vulnerable soils near structures.
While continued public education efforts may help increase compliance for landscaping and interior finishing mitigation actions, physical reconstruction of foundations is probably not feasible in all but the most heavily impacted of existing development. Therefore, damages may be expected into the future for existing structures.
GIS was used to create a risk assessment for geological hazards in Jefferson County. Dipping bedrock (i.e. heaving bedrock) hazard data was overlaid on Jefferson County parcel and assessor’s data. For the purposes of the analysis, if the hazard zone intersects an improved parcel center, its improved value is included and parcel is counted in Results are sorted by occupancy type and by jurisdiction to demonstrate how the hazard’s risk varies across the planning area.
This analysis outlines the potential exposure of improvements built on dipping bedrock for existing development in the planning area. This represents only a tiny portion of the swelling-soil related building exposure, as a swelling soils GIS layer was not available. However, the exposure to the dipping bedrock alone identifies that there could be potential for damage from this hazard. The table indicates that Golden, Lakewood, Morrison, Arvada and the unincorporated areas east of the foothills have the greatest exposure to this hazard. In this analysis, improved values (typically structures and buildings) are assumed to be potentially exposed, but not necessarily ‘at risk.’ This analysis does not take into account site- specific mitigation measures that may be in place, thus estimating losses for dipping bedrock is difficult.
Compared to 2016, in general exposure of buildings to dipping bedrock increased for all jurisdictions, likely due to development outward for all jurisdictions. Residential property exposure to dipping bedrock increased for parcels for Arvada, Golden, Lakewood, and Unincorporated. For example, residential property improved parcels exposed to dipping bedrock increased from 22 to 203, nearly a 10-fold increase.
Table 4-33 Exposure of Buildings to Dipping Bedrock
Section titled “Table 4-33 Exposure of Buildings to Dipping Bedrock”| Jurisdiction | Property Type | Improved Parcels | Building Parcels | Total Value | Population |
|---|---|---|---|---|---|
| Arvada | Commercial | 2 | 2 | $3,589,478 | |
| Arvada | Industrial | 2 | 2 | $8,225,885 | |
| Arvada | Residential | 203 | 205 | $128,134,394 | 508 |
| Arvada | Total | 207 | 209 | $139,949,757 | 508 |
| Golden | Agriculture | 1 | 1 | $70,874 | |
| Golden | Commercial | 78 | 99 | $231,482,526 | |
| Golden | Exempt | 15 | 31 | $208,931,936 | |
| Golden | Industrial | 86 | 89 | $131,473,603 | |
| Golden | Mixed Use | 9 | 17 | $125,961,762 | |
| Golden | Residential | 1,786 | 2,562 | $1,247,910,033 | 5,739 |
| Golden | Total | 1,975 | 2,799 | $1,945,830,734 | 5,739 |
| Lakewood | Commercial | 2 | 15 | $8,684,812 | |
| Lakewood | Exempt | 2 | 3 | $106,386 | |
| Lakewood | Industrial | 1 | 4 | $176,850 | |
| Lakewood | Residential | 1,391 | 1,391 | $975,570,386 | 3,158 |
| Lakewood | Total | 1,396 | 1,413 | $984,538,434 | 3,158 |
| Morrison | Commercial | 1 | 1 | $1,681,678 | |
| Morrison | Exempt | 3 | 3 | $17,958,698 | |
| Morrison | Industrial | 1 | 1 | $181,443 | |
| Morrison | Total | 5 | 5 | $19,821,819 | 0 |
| Unincorporated | Agriculture | 9 | 9 | $1,131,166 | |
| Unincorporated | Commercial | 203 | 242 | $584,914,140 | |
| Unincorporated | Exempt | 45 | 48 | $358,498,400 | |
| Unincorporated | Industrial | 183 | 190 | $286,860,023 | |
| Unincorporated | Mixed Use | 65 | 74 | $118,221,706 | |
| Unincorporated | Residential | 20,393 | 20,696 | $9,911,186,736 | 52,775 |
| Unincorporated | Total | 20,898 | 21,259 | $11,260,812,171 | 52,775 |
| Grand Total | 24,481 | 25,685 | $14,350,952,913 | 62,180 |
Source: Jefferson County GIS and Assessor’s Data
People
Section titled “People”There are no reported injuries or deaths to these soil hazards in Jefferson County, and direct impacts on people are likely to be very minimal.
Critical Facilities and Infrastructure
Section titled “Critical Facilities and Infrastructure”Existing critical facilities impacted by dipping bedrock and other swelling soil hazards are of particular concern, as the damages caused to these structures may impact the ability of the planning area to provide critical services to the population. Schools built on the area may pose a danger to occupants if the buildings are severely damaged in an event. If building integrity is compromised, it may also reduce the sheltering capacity or public health distribution capacity of the County, as schools are often used for these functions.
includes the results of a GIS overlay of critical facilities on the dipping bedrock areas. Critical facilities exposed to dipping bedrock increased in number for all jurisdictions compared to the 2016 plan. The unincorporated jurisdiction has the most critical facilities at risk and a majority of those are communication and transportation FEMA lifelines. A number of schools and fire stations in the planning area are potentially exposed. This analysis does not take into account site-specific mitigation measures that may be in place.
Table 4-34 Critical Facilities in Dipping Bedrock Zones in Jefferson County
Section titled “Table 4-34 Critical Facilities in Dipping Bedrock Zones in Jefferson County”| Jurisdiction | FEMA Lifeline | Critical Facility Type | Count |
|---|---|---|---|
| Arvada | Communications | Land Mobile Private Towers | 1 |
| Arvada | Communications | Microwave Service Towers | 2 |
| Arvada | Energy | Electric Substation | 2 |
| Arvada | Energy | Power Plant | 2 |
| Arvada | Hazardous Material | Tier II | 1 |
| Arvada | Transportation | Bridge | 2 |
| Total | 10 | ||
| Golden | Communications | Land Mobile Private Towers | 9 |
| Golden | Communications | Microwave Service Towers | 9 |
| Golden | Food, Water, Shelter | Water Facility | 1 |
| Golden | Hazardous Material | Household Hazardous Waste | 1 |
| Golden | Hazardous Material | Tier II | 1 |
| Golden | Health and Medical | Nursing Home | 2 |
| Golden | Safety and Security | EOC | 1 |
| Golden | Safety and Security | Government Facility | 6 |
| Golden | Safety and Security | Law Enforcement | 1 |
| Golden | Safety and Security | School | 2 |
| Golden | Transportation | Bridge | 6 |
| Total | 39 | ||
| Lakewood | Communications | Land Mobile Private Towers | 1 |
| Lakewood | Energy | Electric Substation | 2 |
| Lakewood | Food, Water, Shelter | Wastewater Plant | 1 |
| Lakewood | Transportation | Bridge | 7 |
| Total | 11 | ||
| Morrison | Communications | Land Mobile Private Towers | 1 |
| Morrison | Safety and Security | Fire Station | 1 |
| Morrison | Transportation | Bridge | 2 |
| Total | 4 | ||
| Unincorporated | Communications | Land Mobile Private Towers | 31 |
| Unincorporated | Communications | Microwave Service Towers | 29 |
| Unincorporated | Energy | Electric Substation | 6 |
| Unincorporated | Energy | Power Plant | 1 |
| Unincorporated | Hazardous Material | Tier II | 13 |
| Unincorporated | Health and Medical | Nursing Home | 9 |
| Unincorporated | Safety and Security | Fire Station | 2 |
| Jurisdiction | FEMA Lifeline | Critical Facility Type | Count |
|---|---|---|---|
| Safety and Security | Government Facility | 1 | |
| Safety and Security | Law Enforcement | 1 | |
| Safety and Security | School | 17 | |
| Transportation | Bridge | 45 | |
| Transportation | Government Facility | 4 | |
| Total | Total | Total | 159 |
Source: HIFLD and CERC
Economy
Section titled “Economy”The economic cost of this hazard is typically minor in the short term, although over time they can add up to significant impacts.
Historical, Cultural, and Natural Resources
Section titled “Historical, Cultural, and Natural Resources”Collapsible and expansive soils are a natural environmental process. Nonetheless they have the potential to alter the landscape and can cause damages to historic and cultural resources.
Future Development
Section titled “Future Development”The most effective mitigation actions for expansive soil are complete avoidance or non-conflicting use, or correct engineering design (which includes foundation design, adequate drainage, landscaping, and appropriate interior finishing.) While some areas are devoted to non-conflicting use permits, in particular the areas which are included in the dipping bedrock zones, so much of the Colorado basin is covered in swelling soils that complete avoidance is not possible.
Land use planning regulations in place should temper the risk of swelling soil impacts on future development. Continued efforts to regulate building in areas of high or moderate swelling potential increase the number of structures and infrastructure built with swelling-adaptive methods, which in turn reduces the amount of damage incurred each year on the property. Continued education on the hazard, particularly with landscaping and maintenance concerns, will be needed to reduce the impacts of the hazard on development. As existing development deteriorates and requires either renovation or reconstruction, mitigation methods should be implemented to bring the developments up to contemporary mitigation standards.
Since the last plan update, the most significant areas that intersect Golden and Morrison remain largely undeveloped; however, growth in western Arvada, unincorporated areas along Highway 93, and in Lakewood exposes new development to this hazard. It is important to note that recent development east of Highway 93 in West Arvada and north of Golden was not reflected in the 2015 parcel and associated databases. It is reflected in this plan and shows increased exposure for these areas.
Overall Hazard Significance
Section titled “Overall Hazard Significance”Swelling soil in Jefferson County has, historically, exerted significant impacts on the County, particularly during the large growth expansion experienced between 1970 and 1995. In response to the growing hazard, Jefferson County formed and convened an Expansive Soils Task Force in the spring of 1994 and implemented development regulations by 1995. As a result, the impacts of the hazards in the planning area have been extensively mitigated, either by restricting where development is permitted or by heavily regulating the type of construction permitted in certain areas to adequately address the hazard. The geographic extent of the hazard is considered extensive. The probability of future occurrences is considered likely and the magnitude/severity for the event of record is limited. In addition, the HMPC considers the hazard to have a low overall impact on the jurisdiction. This equates to an overall impact rating of medium. In many ways, the swelling soils hazard is an excellent example for demonstrating the effectiveness of how mitigation efforts may reduce the vulnerabilities and risks of a previously high- concern hazard. Sound planning and engineering practices should keep the impact to future development low, however the potential for damages exist in older residential areas.