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<. if' Ii) Ilc ~..( ;'!.t t: f; ~' l '-- VIII - GEOLOGY, SOILS M;D CONSTRUCTION t~TERIP~S 8-01. Geology of project area. The proposed Local Protection Project for Las Animas, Colorado, and vicinity is located in Southeastern Colorado near the \leatern margin of t:,e Great Plains Physiograhic Province. Las Animas is located on the south bank of the Arkansas River approximately tHO miles west of the confluence of the Arkansas and Purgatoire Rivers. The valley section consists of valley fill and alluvia:. fans composed of silty sands, clays and some reworked gravels. There is a light clay loam topsoil and a heavy clay loam subsoil to a ma>;imum depth of approximately ten feet in over- bank areas adjacent to the present river channel. The depth of clay, sand and gravel in the channel is quite variable but probably seldom exceeds fifty feet. Bedrock is exposed in the present Arkansas River channel at Bents Old Fort about 15 miles to the west of the proj~~t area. The primary strata under- lying valley alluvium consists of Upper Cretaceous rocks of the Benton Group. Th"se are dark gray to balck shales with some interbedded thin limstones. Ground water levels within the project area will generally conform to the. w~ter level in the Arkansas River. 8-02. Topo~raphy of project area. The project work area is located within the broad, relatively flat Arkansas River valley. Tne river channel is incised 6 to S feet below valley level and has a slope of about 5.0 feet per 10ile through the project reach. The levee alignments, except for tiebacks, generally _parallel the river channel. Vegetation within the proposed work uree varies from partially clear river channel to cultivated crops, grass, brush, weeds, and dense growths of cotton"ood and tamarisk. 8-03. Method and scope of subsurface investigations. Subsurface and foundation investigations for the levee alignment, railroad bridge and borrow areas consisted of 85 auger holes. Auger holes All-l through All-12 were survey stage investigations drilled with a 6-inch hand auger to obtain disturbed samples for laboratory classification. Depths of the survey stage investigation varied from 7 to 14 feet and were generally limited by th" ground water table. Subsurface investigations in 1967 consisted of 73 augher holes; AIl-13 through c'Ill-S4, \lith depths varying from 2.3 feet to 38.2 feet. Thirth-seven holes, noted in table 1, Appendix A, were drilled \lith a 6-inch hand augher to depths varyin3 from 2.3 to 10.0 feet. Disturbed samples were obtained for laboratory classification. Thirty-six holes, noted in table 1, Appendix A, "ere drilled 'tvi th a truck-mounted, hollm-J-s tern, 7-inch continuous fligh t aUGer and dis- turbed samples ",pre obtained through the hollm, ste"l "ith a scanuard pene- tration spoon. THO :luger holes, Al:I-72 and AH-73, Here continued into the silale foundation to depths of 30.6 and 30.5 feet and shale samples obtained by Den:lscn barrel and core barrel. Ground ,;ster table, if any, "as determineu in mos t ;10128. Locations and vis uP 1 logs of R 11 augher holes near the levee aL gmnents are shown in plan and profile on pertinent plates. Auger holes anu visual loss for the survey stage investigation and borrm'l are.s investigation are shmn1 on plate 1. 18 , ".,:tr!,,~ ~, ">;-'.