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APPENDIX G.4 GEOTECHNICAL 1. GENERAL. the Geology

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APPENDIX G.4 GEOTECHNICAL 1. GENERAL. the Geology APPENDIX G.4 GEOTECHNICAL 1. GENERAL. The geology, physiography, and soil description of existing conditions for the project area for the Chacon Creek GI Watershed Study is provided in this Geotechnical Appendix. Lake Casa Blanca Dam information including description of the dam, specifications, performance history, inspection history, recommendations from the most recent inspection, rehabilitation measures, Webb County maintenance of the dam, and the Hazard Classification of the dam are also provided in this section for the Chacon Creek GI Watershed Study. Lake Casa Blanca Dam is situated upstream of the Chacon Creek Study area. The publications U.S. Army Corps of Engineers, 1978, Casa Blanca Dam, Phase I Inspection Report, and the National Dam Safety Program, Texas Commission on Environmental Quality (TCEQ), 2005, Dam Safety Team Dam Inspection Report along with inquires with Warren Samuelson and Debra Rankin from TCEQ were used to compile the Casa Blanca Dam assessment. 2. GENERAL GEOLOGIC CHARACTERISTICS of the LAREDO AREA. 2.1. Geology and Physiography. The geologic units in Webb County can be characterized as a series of northeast-southwest trending interbedded sand and shale sequences that were deposited in fluvial-deltaic and nearshore marine depositional environments, separated by regional transgressive marine shales. These units were part of a highly destructive, wave-dominated delta system in what is now South Texas during the Eocene (Ricoy and Brown, 1977). The geologic units are Tertiary and Quaternary in age and crop out in northeast-southwest trending belts oriented approximately parallel to the present shoreline of the Gulf of Mexico. These outcrop belts reflect the position of the coast during the geologic time during which deposition occurred (Barnes, 1953). The northwestern part of the county is part of the Rio Grande Embayment, a regional subsurface geologic feature that consists primarily of relatively flat-lying, thick- to-massive sandstones interbedded with thinner shale sequences and few major structural features such as faulting. Most of the sandstone beds were deposited in environments where the primary accumulation of sand has been reworked into barrier bars or strand plains oriented parallel to the depositional strike, with minor accumulations concentrated in dip-trending channels. In areas downdip to the southeast, the geologic units are buried under increasingly thick sequences of Tertiary sediments that are influenced by major structural fault zones and salt diapirs (domes) of the Texas Gulf Coastal Plain and that contain increasing thicknesses of shales (Ewing, 1991). These depositional units have been further modified by subsurface structural elements that developed during the Tertiary period and include the development of syndepositional normal faults (also referred to as “growth faults”) and modified by salt tectonism that formed the salt diapirs (Ewing, 1991). The Wilcox fault zone in southeastern Webb County was the first growth-fault system that developed parallel to the present-day coast (Ewing, 1991). The Pescadito Dome in central Webb County is a deep-seated salt diapir that has pushed up and eroded the overlying formations so that the Laredo Formation is exposed at land surface and surrounded by rocks of the Yegua Formation. Another salt diapir, the Moca salt diapir in northeastern Webb County, is associated with the Moca Oil Field. Average regional dips for the Carrizo Sand through the Catahoula Tuff range from 46 to 88 feet per mile (ft/mi), respectively, from northwestern to southeastern Webb County. The geologic units that form aquifers in Webb County gradually thicken to the southeast toward the Gulf of Mexico along a dip trend. The greatest thickening of the units is in southeastern Webb County in the area influenced by the Wilcox fault zone. The geologic units thicken to the east, and the shale content increases downdip. The increasing shale content is an indication that the sediments downdip were deposited farther out in the basin in a prodelta or nearshore marine environment. The development of the Reklaw Formation, a transgressive marine shale that is equivalent to the base of the Bigford Formation 1 and in Webb County is present only in the subsurface. Farther to the south, the geologic units also become more shaley and dip to the southeast toward the Gulf of Mexico, but the units dip more steeply than the same units to the north. Similar to its configuration in the Reklaw Formation which increases in thickness toward the Gulf of Mexico in southeastern Webb County. Along the strike sections, most of the formations are relatively uniform in thickness. The thinnest section of formations is in northwestern Webb County. The formations in this area are dominated by sandstones interbedded with shales and lignite. The presence of lignite is an indication that the rocks were deposited in shallow water under reducing conditions in an area such as a lagoon. The formations are at deeper altitudes in the southern part of the county and gradually become thinner and shallower in a northerly direction. Farther down dip in the middle part of the county, the formations are slightly thicker than in the area. The formations in the area are still dominated by sandstones. The formations have greater shale content in the area of the Wilcox fault zone. The subsurface formations are much thicker than their equivalents in the northwestern part of the county, lower in altitude than those units to the northeast, and contain a higher percentage of shale. This Interior Coastal Plains comprise alternating belts of resistant uncemented sands among weaker shales that erode into long, sandy ridges. At least two major down-to-the coast fault systems trend nearly parallel to the coastline. Chaparral brush and sparse grasses dominate in the area of Laredo. (Source Walter Geology Library at http://www.lib.utexas.edu/geo/physiography.html) 2.2. Overburden. According to the United States Department of Agriculture publication “Soil Survey of Webb County (1985), soils in or adjacent to the study area include the Tela sandy clay loam, Copita fine sandy loam, Verick fine sandy loam, and Lagloria silt loam. Tela sandy clay loam (SC, CL) typically has a dark grayish brown surface layer, mildly alkaline sandy clay loam about 14 inches thick. The upper part of the subsoil, from 14 to 19 inches, is grayish brown, mildly alkaline sandy clay loam. The middle part of the subsoil, from 19 to 40 inches, is grayish brown, calcareous, moderately alkaline sandy clay loam. The lower part of the subsoil, from 40 to 45 inches, is light brownish gray, calcareous, moderately alkaline sandy clay loam. The underlying layer to a depth of 63 inches is light brownish gray, calcareous, moderately alkaline loam. This soil is well drained, has high available water capacity, slow surface runoff, moderate permeability, and a deep rooting zone. Water erosion is a moderate hazard. Copita fine sandy loam (SC, SM, CL) typically has a surface layer of brown fine sandy loam about 9 inches thick. The upper part of the subsoil, from 9 to 24 inches, is yellowish brown sandy clay loam. The lower part of the subsoil, from 24 to 37 inches, is light yellowish brown sandy clay loam. The underlying layer to a depth of 60 inches is pale yellow sandstone that is weakly cemented in the upper part and strongly cemented in the lower part. The soil is calcareous and moderately alkaline throughout. This soil is well drained, has low available water capacity, moderate surface runoff, moderate permeability, and a moderately deep rooting zone. Water erosion is a moderate hazard. Verick fine sandy loam (SC, SM) typically has a yellowish brown surface layer, is a calcareous moderately alkaline fine sandy loam about 9 inches thick. The subsoil, from 9 to 15 inches, is light yellowish brown, calcareous, moderately alkaline fine sandy loam. The underlying layer is light yellowish brown weakly cemented sandstone to a depth of 60 inches or more. This soil is well drained, has very low available water capacity, medium surface runoff, moderate permeability, and a shallow rooting zone. Water erosion is a moderate hazard. 2 Lagloria silt loam (CL, ML) typically is a pale brown silt loam surface layer about 19 inches thick. The subsoil, which extends to a depth of 42 inches, is pale brown loam. The underlying layer to a depth of 63 inches is light yellowish brown loam. The soil is calcareous and moderately alkaline throughout. This soil is well drained, has medium available water capacity, slow surface runoff, moderate permeability, and a deep rooting zone. Water erosion is a slight hazard. (Source: Soil Survey of Webb County, 1985). The soils along the Chacon Creek in the study area are erosive. A trapezoidal grass lined channel may be a design option on the reach of Chacon Creek between the Tex-Mex Railroad Bridge and the Highway 359 Bridge. The proposed channel will require flat channel slopes and erosion protection for the soil on the channel slopes to prevent damage from erosion. Soil on the north bank of Chacon Creek near Cortez St. (14 September 2006 Site Visit) 2.3. Seismicity. The closest recorded earthquake occurred in 1975 approximately 280 miles northwest of Laredo. This quake was of low intensity and its epicenter was located near Study Butte in Big Bend National Park, Brewster County, Texas. The United States Seismic Risk Map shows Laredo to be in Zone 0, an area where no damage is expected from earthquakes. USGS National Seismic Hazard Maps (below) shows that Webb County has the possibility of a peek acceleration range of 2% to 4% g with 2% probability of exceedance in 50 years. 3 3. CASA BLANCA DAM. 3.1. General Description. Lake Casa Blanca Dam is located approximately three miles northeast of Laredo, Texas and is situated upstream of the Chacon Creek Study area.
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