PLATE 4 LANDSLIDE-HAZARD MAP FOR THE ST. … · 2015. 12. 15. · 13 12 06 01 02 01 09 04 15 10 14...
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GEORGE–HURRICANE METROPOLITAN AREA Basemap consists of National Agricultural Imagery Program natural color aerial photography. Universal Transverse Mercator Projection, zone 12. North American Datum 1983. Old US-91 Bloomington Hills North Little Valley § ¨ ¦ 15 § ¨ ¦ 15 § ¨ ¦ 15 § ¨ ¦ 15 Middleton . d R r e v i R Sun River St. George Blvd. . t S f f u l B r e v i R EXPLANATION Green Valley Sunbrook ! ( 8 ! ( 18 37 N 113 15' W ! ( 18 e i x i D . r D t n i u q a n o T . r D . r D Bloomington Man O War Rd. . d R m a h g i r B § ¨ ¦ 15 r e v i R . d R n i g r i V r e v i R Stone Cliff Knolls Ledges Diamond Valley Riverside . r D 0 5 4 1 1580 South Washington Fields t s a E 0 0 7 4 h t u o S § ¨ ¦ 15 ! ( 9 Creek Reservoir Sand Hollow Reservoir ! ( 9 ! ( 59 ! ( 9 ! ( 17 R 16 W R 15 W R 17 W R 16 W T 42 S T 43 S 113 22' 30" W 113 30' W 113 37' 30" W R 15 W R 14 W T 42 S T 41 S T 41 S T 40 S R 14 W R 13 W R 13 W R 12 W 37 7' 30" N 37 15' N R 17 W R 16 W R 16 W R 15 W R 15 W R 14 W R 14 W R 13 W T 41 S T 40 S T 42 S T 41 S T 42 S T 43 S 37 15' N 37 7' 30" N 37 N 113 22' 30" W 113 30' W 113 37' 30" W by DISCUSSION USING THIS MAP MAP LIMITATIONS t r o F e c r a e P h s a W 1 0 1 2 0.5 Miles 1 0 1 2 0.5 Kilometers Scale 1:24,000 Approximate mean declination, 2007 14 o H T R O N E U R T H T R O N C I T E N G A M UTAH Map Location VH H M D E R S N I A T N U O M w o n S n o y n a C Santa Clara Heights d n a S w o l l o H h s a W H T U O S S L L I H t s e W k c a l B e g d i R l l i H b b e W s l l i H y t i C e c i r P n o t e l d d i M k c a l B e g d i R b o n i h S e b i K e m o D n o t g n i h s a W R E N R A W E G D I R R E N R A W Y E L L A V D N A S N I A T N U O M n o t g n i h s a W k c a l B e g d i R l l i M k e e r C e m o D g r u b s i r r a H k e e r C d o o w n o t t o C l i a u Q k e e r C s d e e L k e e r C e n i v e p a r G h s a W h s A k e e r C S F F I L C E N A C I R R U H n i k r e V a L k e e r C n i g r i V r e v i R E N A C I R R U H S F F I L C d l u o G h s a W e l l i v n i k t A h s a W E N A C I R R U H S F F I L C N I A T N U O M K E E R C E L T T I L Very High: Existing landslides (Category A). High: Areas where Category B geologic units crop out on slopes greater than 15 percent (8.5 ). Moderate C: Areas where Category C geologic units crop out on slopes greater than 20 percent (11.3 ). M L c B Moderate B: Areas where Category B geologic units crop out on slopes less than 15 percent (8.5 ). Low: Areas where Category D geologic units crop out on slopes greater than 30 percent (16.7 ). o o o o SYMBOLS Municipality boundary Interstate Highway State Highway Major local surface street Other road The Landslide-Hazard Map is based on limited geological, geotechnical, and hydrological data; site-specific studies are required to produce more detailed geotechnical information. The map also depends on the quality of those data, which varies throughout the study area. The mapped boundaries of the landslide-hazard categories are approximate and subject to change with additional information. The landslide hazard at any particular site may be different than shown because of geological variations within a map unit, gradational and approximate map-unit boundaries, and the generalized map scale. Small, localized areas of higher or lower landslide hazard may exist within any given map area, but their identification is precluded because of limitations of map scale. This map is not intended for use at scales other than the published scale, and is designed for use in general planning to indicate the need for site- specific studies. The Landslide-Hazard Map shows areas of relative landslide hazard, and provides a basis for requiring site- specific hazard studies. Site-specific studies can resolve uncertainties inherent in generalized geologic-hazard mapping and help ensure safety by identifying the need for hazard mitigation. The Landslide-Hazard Map identifies areas based on previous landslide history, material characteristics, and slope where site-specific, slope-stability conditions (material strength, orientation of bedding or fractures, ground-water conditions, erosion or undercutting) should be evaluated prior to development. The level of investigation needed at a given site depends on the relative hazard and the nature of the proposed development (structure size and placement, required cutting and filling, and changes in ground-water conditions). A valid landslide-hazard evaluation must address all pertinent conditions that could affect, or be affected by, the proposed development, including earthquake ground shaking. This can only be accomplished through the proper identification and interpretation of site-specific geologic conditions and processes (Hylland, 1996). Such conditions in areas near to the site that may affect the site must also be considered. Landslide-hazard studies must be interdisciplinary in nature and performed by qualified, experienced geotechnical engineers and engineering geologists working as a team. Utah Geological Survey Circular 92 Guidelines for Evaluating Landslide Hazards in Utah (Hylland, 1996) presents minimum standards for performing landslide-hazard evaluations in Utah. Circular 92 outlines a phased approach to slope-stability studies beginning with a geologic evaluation and progressing through reconnaissance and detailed geotechnical-engineering evaluations as necessary based on the results of the previous phase. Blake and others (2002) and Black and others (1999) provide additional guidance for evaluating landslide hazards. Local jurisdictions may adopt more stringent requirements for slope-stability evaluations, as they deem necessary, to meet local needs and conditions. For example, the City of St. George requires studies on all slopes greater than 15 percent that lie within designated Hillside Development Overlay Zones, and that requirement takes precedence over the recommendations in this report. The UGS recommends that the following site-specific investigations be conducted for each of the landslide-hazard categories. William R. Lund, Tyler R. Knudsen, Garrett S. Vice, and Lucas M. Shaw 2008 PLATE 4 Landslide is a general term that refers to the gradual to rapid movement of a mass of rocks, debris, or earth down a slope under the force of gravity. Landslides can be both damaging and deadly. The U.S. Geological Survey estimates that in the United States, landslides on average cause $1-2 billion in damages and more than 25 deaths annually. In the St. George – Hurricane metropolitan area, landslides have disrupted transportation routes, houses and commercial sites, and public utilities. Rock and soil units susceptible to slope failure underlie parts of the study area, as evidenced by the number of landslides identified there. As defined for this study, landslides are either rotational or translational. Rotational slides have curved, concave rupture surfaces, which may be either shallow or deep seated, along which the slide mass may move with little internal disruption. Translational slides move along planar or gently undulating shear surfaces and typically slide out over the original ground surface. Translational slides commonly utilize discontinuities such as bedding planes, joints, or faults as a surface of rupture. Both rotational and translational slides may range from very slow to rapid. For additional information about the landslide hazard in the St. George – Hurricane metropolitan area, refer to the Landslide-Hazard text document in this report. HAZARD REDUCTION As with most geologic hazards, early recognition and avoidance is the most effective way to mitigate landslide hazards. However, avoidance may not always be a viable or cost-effective hazard-reduction option, especially for existing developments, and other engineering techniques are available to reduce potential landslide hazards. Techniques for mitigating landslide hazard include care in site grading, with proper compaction of fills and engineering of cut- and-fill slopes; paying careful attention to site drainage and dewatering of shallow or perched ground water where landslides may be a hazard; constructing retaining structures at the toe of slopes; and mechanical stabilization using tieback or other means that penetrate the landslide mass, pinning it to underlying stable material. Other techniques used to reduce landslide hazards include bridging, weighting, or buttressing slopes with compacted earth fills and installation of landslide warning systems (Keller and Blodgett, 2006). However, some geologic units, for example the Petrified Forest Member of the Chinle Formation, may be too weak to buttress, and may continue to move upslope of the buttress (Francis Ashland, UGS Geologic Hazards Program, written communication, 2007) Where development is proposed in areas identified on the Landslide-Hazard Map as having a potential for slope failure, a phased site-specific study should be performed early in the project design phase. A site-specific investigation can establish whether the necessary conditions for failure are present at a site. If the conditions for slope failure do exist, the consultant should provide appropriate design recommendations. Although this product represents the work of professional scientists, the Utah Department of Natural Resources, Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use. The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under any circumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims by users of this product. For use at 1:24,000 scale only. The Utah Geological Survey does not guarantee accuracy or completeness of data. Landslide Susceptibility of Geologic Units A Susceptibility Category Geologic Units 1 Comments B C D Existing landslides Petrified Forest Mbr., Chinle Fm. Woods Ranch Mbr., Toroweap Fm.; Harrisburg Mbr., Kaibab Fm.; Shnabkaib Mbr. and red members, Moenkopi Fm.; Whitmore Point Mbr., Moenave Fm.; Temple Cap Fm.; Carmel Fm.; Cretaceous bentonitic beds; Iron Springs Fm. Remaining bedrock and unconsolidated geologic units in the study area. Existing slope failures are considered the most likely units in which landsliding may initiate. This rock unit consists chiefly of bentonitic clay, which is highly expansive and has low shear strength especially when wet. This unit includes the greatest number of landslides mapped in the study area. These bedrock units contain varying amounts of gypsum, shale, claystone, mud- stone, siltstone, or a combination of the above that imparts weak shear strength characteristics to the units, at least locally, and makes them susceptible to slope failure. These units contain the second greatest number of landslides in the study area. These geologic units either contain a higher percentage of stronger rock types, crop out on slopes too low to generate slope failures, or generate failures that are too small to map at 1:24,000-scale. As a result, they exhibit few or no mapped landslides. Some landslides identified within these units are likely the result of failures in underlying, weaker geologic units. 1 Refer to UGS 1:24,000-scale geologic maps (see SOURCES OF DATA and REFERENCES in accompanying text) for a description of map units. VH Map Unit Hazard Category Recommended Site-Specific Study H M L Very High High Moderate Low Detailed engineering geologic and geotechnical-engineering evaluation necessary. Predevelopment stabilization recommended for historical and geologically young (late Pleistocene) landslides. Detailed engineering geologic and geotechnical-engineering evaluation necessary. Geologic evaluation and reconnaissance geotechnical-engineering evaluation necessary; detailed engineering geologic and geotechnical-engineering evaluation may be necessary. Geologic evaluation and reconnaissance geotechnical-engineering evaluation necessary, detailed geotechnical-engineering evaluation generally not necessary. UTAH GEOLOGICAL SURVEY a division of Utah Department of Natural Resources Plate 4 Utah Geological Survey Special Study 127 Geologic Hazards and Adverse Construction Conditions, St. George−Hurricane Metropolitan Area, Washington County, Utah
Transcript of PLATE 4 LANDSLIDE-HAZARD MAP FOR THE ST. … · 2015. 12. 15. · 13 12 06 01 02 01 09 04 15 10 14...
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LANDSLIDE-HAZARD MAP FOR THE ST. GEORGE–HURRICANE METROPOLITAN AREA
Basemap consists of National Agricultural Imagery Programnatural color aerial photography.
Universal Transverse Mercator Projection, zone 12.North American Datum 1983.
OldUS-91
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R 16 W R 15 W
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T 42 ST 43 S
113 22' 30" W113 30' W113 37' 30" W R 15 W R 14 W
T 42 ST 41 S
T 41 ST 40 S
R 14 W R 13 W
R 13 W R 12 W
37 7' 30" N
37 15' N
R 17 W R 16 W
R 16 W R 15 W
R 15 W R 14 W
R 14 W R 13 W
T 41 ST 40 S
T 42 ST 41 S
T 42 ST 43 S
37 15' N
37 7' 30" N
37 N
113 22' 30" W
113 30' W
113 37' 30" W
by
DISCUSSION
USING THIS MAP
MAP LIMITATIONS
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1 0 1 20.5Kilometers
Scale 1:24,000
Approximate meandeclination, 2007
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Very High: Existing landslides (Category A).
High: Areas where Category B geologic units crop out on slopes greater than 15 percent (8.5 ).
Moderate C: Areas where Category C geologic units crop out on slopes greater than 20 percent (11.3 ).
M
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BModerate B: Areas where Category B geologic units crop out on slopes less than 15 percent (8.5 ).
Low: Areas where Category D geologic units crop out on slopes greater than 30 percent (16.7 ).
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SYMBOLS
Municipality boundary
Interstate Highway
State Highway
Major local surface street
Other road
The Landslide-Hazard Map is based on limited geological, geotechnical, and hydrological data; site-specific studies are required to produce more detailedgeotechnical information. The map also depends on the quality of those data, which varies throughout the study area. The mapped boundaries of thelandslide-hazard categories are approximate and subject to change with additional information. The landslide hazard at any particular site may be differentthan shown because of geological variations within a map unit, gradational and approximate map-unit boundaries, and the generalized map scale. Small,localized areas of higher or lower landslide hazard may exist within any given map area, but their identification is precluded because of limitations of mapscale. This map is not intended for use at scales other than the published scale, and is designed for use in general planning to indicate the need for site-specific studies.
The Landslide-Hazard Map shows areas of relative landslide hazard, and provides a basis for requiring site-specific hazard studies. Site-specific studies can resolve uncertainties inherent in generalized geologic-hazardmapping and help ensure safety by identifying the need for hazard mitigation.
The Landslide-Hazard Map identifies areas based on previous landslide history, material characteristics, andslope where site-specific, slope-stability conditions (material strength, orientation of bedding or fractures,ground-water conditions, erosion or undercutting) should be evaluated prior to development. The level ofinvestigation needed at a given site depends on the relative hazard and the nature of the proposed development(structure size and placement, required cutting and filling, and changes in ground-water conditions). A validlandslide-hazard evaluation must address all pertinent conditions that could affect, or be affected by, theproposed development, including earthquake ground shaking. This can only be accomplished through theproper identification and interpretation of site-specific geologic conditions and processes (Hylland, 1996). Suchconditions in areas near to the site that may affect the site must also be considered.
Landslide-hazard studies must be interdisciplinary in nature and performed by qualified, experiencedgeotechnical engineers and engineering geologists working as a team. Utah Geological Survey Circular 92Guidelines for Evaluating Landslide Hazards in Utah (Hylland, 1996) presents minimum standards forperforming landslide-hazard evaluations in Utah. Circular 92 outlines a phased approach to slope-stabilitystudies beginning with a geologic evaluation and progressing through reconnaissance and detailedgeotechnical-engineering evaluations as necessary based on the results of the previous phase. Blake andothers (2002) and Black and others (1999) provide additional guidance for evaluating landslide hazards. Localjurisdictions may adopt more stringent requirements for slope-stability evaluations, as they deem necessary, tomeet local needs and conditions. For example, the City of St. George requires studies on all slopes greater than15 percent that lie within designated Hillside Development Overlay Zones, and that requirement takesprecedence over the recommendations in this report. The UGS recommends that the following site-specificinvestigations be conducted for each of the landslide-hazard categories.
William R. Lund, Tyler R. Knudsen, Garrett S. Vice, and Lucas M. Shaw2008
PLATE 4
Landslide is a general term that refers to the gradual to rapid movement of a mass of rocks, debris, or earth down a slope under the force of gravity.Landslides can be both damaging and deadly. The U.S. Geological Survey estimates that in the United States, landslides on average cause $1-2 billionin damages and more than 25 deaths annually. In the St. George – Hurricane metropolitan area, landslides have disrupted transportation routes, housesand commercial sites, and public utilities. Rock and soil units susceptible to slope failure underlie parts of the study area, as evidenced by the number oflandslides identified there. As defined for this study, landslides are either rotational or translational. Rotational slides have curved, concave rupturesurfaces, which may be either shallow or deep seated, along which the slide mass may move with little internal disruption. Translational slides movealong planar or gently undulating shear surfaces and typically slide out over the original ground surface. Translational slides commonly utilizediscontinuities such as bedding planes, joints, or faults as a surface of rupture. Both rotational and translational slides may range from very slow torapid.
For additional information about the landslide hazard in the St. George – Hurricane metropolitan area, refer to the Landslide-Hazard text document in thisreport.
HAZARD REDUCTIONAs with most geologic hazards, early recognition and avoidance is the most effective way to mitigate landslide hazards. However, avoidance may notalways be a viable or cost-effective hazard-reduction option, especially for existing developments, and other engineering techniques are available to reducepotential landslide hazards. Techniques for mitigating landslide hazard include care in site grading, with proper compaction of fills and engineering of cut-and-fill slopes; paying careful attention to site drainage and dewatering of shallow or perched ground water where landslides may be a hazard; constructingretaining structures at the toe of slopes; and mechanical stabilization using tieback or other means that penetrate the landslide mass, pinning it to underlyingstable material. Other techniques used to reduce landslide hazards include bridging, weighting, or buttressing slopes with compacted earth fills andinstallation of landslide warning systems (Keller and Blodgett, 2006). However, some geologic units, for example the Petrified Forest Member of the ChinleFormation, may be too weak to buttress, and may continue to move upslope of the buttress (Francis Ashland, UGS Geologic Hazards Program, writtencommunication, 2007)
Where development is proposed in areas identified on the Landslide-Hazard Map as having a potential for slope failure, a phased site-specific study shouldbe performed early in the project design phase. A site-specific investigation can establish whether the necessary conditions for failure are present at a site.If the conditions for slope failure do exist, the consultant should provide appropriate design recommendations.
Although this product represents the work of professional scientists, the Utah Department of Natural Resources,Utah Geological Survey, makes no warranty, expressed or implied, regarding its suitability for a particular use.The Utah Department of Natural Resources, Utah Geological Survey, shall not be liable under anycircumstances for any direct, indirect, special, incidental, or consequential damages with respect to claims byusers of this product.
For use at 1:24,000 scale only. The Utah Geological Survey does not guarantee accuracy or completeness ofdata.
Landslide Susceptibility of Geologic Units
A
SusceptibilityCategory Geologic Units1 Comments
B
C
D
Existing landslides
Petrified Forest Mbr., Chinle Fm.
Woods Ranch Mbr., Toroweap Fm.; Harrisburg Mbr., Kaibab Fm.; Shnabkaib Mbr. and red members, Moenkopi Fm.; Whitmore Point Mbr., Moenave Fm.; Temple Cap Fm.; Carmel Fm.; Cretaceous bentonitic beds; Iron Springs Fm.
Remaining bedrock and unconsolidated geologic units in the study area.
Existing slope failures are considered the most likely units in which landsliding may initiate.This rock unit consists chiefly of bentonitic clay, which is highly expansive and has low shear strength especially when wet. This unit includes the greatest number of landslides mapped in the study area.
These bedrock units contain varying amounts of gypsum, shale, claystone, mud-stone, siltstone, or a combination of the above that imparts weak shear strength characteristics to the units, at least locally, and makes them susceptible to slope failure. These units contain the second greatest number of landslides in the study area.These geologic units either contain a higher percentage of stronger rock types, crop out on slopes too low to generate slope failures, or generate failures that are too small to map at 1:24,000-scale. As a result, they exhibit few or no mapped landslides. Some landslides identified within these units are likely the result of failures in underlying, weaker geologic units.
1Refer to UGS 1:24,000-scale geologic maps (see SOURCES OF DATA and REFERENCES in accompanying text) for a description of map units.
VH
Map Unit Hazard Category Recommended Site-Specific Study
H
M
L
Very High
High
Moderate
Low
Detailed engineering geologic and geotechnical-engineering evaluation necessary. Predevelopmentstabilization recommended for historical and geologically young (late Pleistocene) landslides.Detailed engineering geologic and geotechnical-engineering evaluation necessary.Geologic evaluation and reconnaissance geotechnical-engineering evaluation necessary; detailedengineering geologic and geotechnical-engineering evaluation may be necessary.Geologic evaluation and reconnaissance geotechnical-engineering evaluation necessary, detailedgeotechnical-engineering evaluation generally not necessary.
UTAH GEOLOGICAL SURVEYa division of
Utah Department of Natural Resources
Plate 4
Utah Geological Survey Special Study 127
Geologic Hazards and Adverse Construction Conditions,
St. George−Hurricane Metropolitan Area, Washington County, Utah
