FLOOD01211

• County: Pueblo
• Community: Pueblo
• Stream Name: Fountain Creek
• Title: Pueblo Flood Control Project
• Date: 1/1/1990
• Prepared For: Pueblo County
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Project

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IV - GEOLOGY AND SOILS DESIGN

4-01. General GeoloEv. Pueblo, Colorado lies on the Great Plains
physiographic province at the confluence of Fountain Creek and the Arkansas
River approximately 30 miles east of the Wet Mountains of the Rocky Mountains.
The city is built on shale, limestone and sandstone of Cretaceous Age and
alluvium of Quaternary Age. The principal structural feature of the Pueblo
area is the Pueblo Anticline, the axis of which is approximately parallel to
Fountain Creek. The strata have a regional dip to the northeast of 70 to 300
feet per mile.

4-02. Fountain Creek GeoloEv. Along Fountain Creek, two formations of
Cretaceous Age are exposed. The Niobrara formation, composed primarily of
shale and limestone, is overlain conformably by the Pierre shale. Both the
Niobrara formation and the Pierre shale are overlain unconformably by
Quaternary Alluvium. Core samples were obtained from borings 44 and 45. Both
of the cores were from the upper chalky shale unit of the Niobrara formation.
These core samples were soft to moderately hard calcareous shale. The
calcareous shale is the foundation for the Sth Street bridge. This shale is
considered an adequate foundation for the present Sth Street bridge.

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4-03. Subsurface Investi~ation and Soil Conditions. A subsurface
investigation was conducted by the Albuquerque District in January 19S4.
Forty-six test borings were advanced using a 6-5/S-inch-diameter hollow stem
auger. The borings were advanced to depths of S to 25 feet. The soils were
continuously sampled, classified and logged. Standard penetration tests were
taken at 5-foot intervals and disturbed samples were obtained for mechanical
analysis, Atterberg limits and moisture contents. Grain-size distribution,
moisture contents, and Atterberg limits were determined for selected samples
obtained during the investigation program. Boring logs and results of
laboratory analysis are presented in Appendix B.

4-04. Soil Conditions. In evaluating the soils data, the project was divided
-into three areaS with similar soils occurring in each. The first area is in
the northern part of the proposed project. The southern limit is the 4th
Street bridge. Sixteen of the borings were located in this area. Eight of
these borings were located along the center line of the proposed levee. The
material along the center line is predominantly'coarse, silty sand with some
gravel. The percentage of gravel increased with depth. Silts are generally
nonplastic. Sandy clay and clayey sand were encountered in one of the
borings. The soils are generally very loose to medium dense. The remainder
of the test borings in the region were along the river near the proposed
channel excavation. The near-surface soils are generally poorly graded fine
sands, silty sands, and sandy silts. The percentage of silt and clay-size
particles generally decreased with depth. The deeper soils are sand and gravel
interbedded with some silty layers. The soils are very loose to loose and
slightly moist to moist above the water table. Two core borings were advanced
to depths of 31 and 34 feet near river bank abutments of the Sth Street
bridge. These boring locations are shown on Plate 21.

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4-05. The
the south
this area.

second area is bounded on the north by the 4th Street bridge and on
by the AT&SF railroad bridge. Fifteen test borings were located in
Six of the borings were located along the center line of the

IV-l