I-10 Twin Peaks Traffic Interchange, Environmental Assessment

Affected Environment and Environmental Impacts October 2005
closed basin of internal drainage. The Fort Lowell Formation, which is generally
300 to 400 feet thick, grades from silty gravel near the edges of the basin to silty
sand and clayey silt in the central part of the basin. In most of the basin, the Fort
Lowell Formation was deposited in fans by streams that spread out from the
canyons in the surrounding mountains. The Tinaja beds, which vary from less than
one foot to more than 2,000 feet thick and have up to three subunits, consist of
gravel and sand (upper bed) that grade into a very thick sequence of gypsiferous
clayey silt and mudstone in the center of the basin (lower bed). The Fort Lowell
formation is early and middle Pleistocene in age, while the Tinaja beds range in
age from Miocene to Pliocene. Tilting, accompanied by minor faulting, ended the
sedimentation of the Fort Lowell Formation and initiated erosion and the early
stages of the present drainage system, including the deposition of young
Quaternary alluvium along the Santa Cruz River.
Localized Surficial Geology
Since the project site is located within or close to the range of influence of the
meander migration of the Santa Cruz River, it can be expected that the near surface
soil deposits would exhibit gradational characteristics ranging from fluvialdeposited
silty and clayey soils to sandy and gravelly deposits. Below these
surficial deposits, dense layers of sand and gravel of the Fort Lowell Formation
with varying quantities of silt and clay would be encountered. The base of the Fort
Lowell Formation is expected to be at approximately 2,000 feet, or approximately
130 feet below existing grade, and so the very dense Upper Tinaja unit is unlikely
to be encountered on this project.
Seismic Conditions
Based on ADOT seismic acceleration maps (Euge, Kenneth, and Schell 1992) and
AASHTO Standard Specifications for Highway Bridges, seismic loading is not
incorporated in the design of the bridges. Furthermore, there are no special seismic
design requirements for the foundations and abutments of bridges in this category.
Mineral Resources
Sand and gravel mining operations have operated and continue to operate within
the study area. An existing sand and gravel mining operation (I-10 Avra Valley
Mining and Development) is located on the west side of the Santa Cruz River
south of Avra Valley Road, which is north of the project area. Although no longer
a mining operation, a former sand and gravel mine was located in the northern
portion of the project area between the Santa Cruz River and I-10. This area is now
used for industrial operations. An active sand and gravel mining operation (Rinker
Materials) is located in the southern project area immediately south of the south
access road. In addition to sand and gravel mining, Arizona Block and Brick, in
the southern portion of the project area mines and processes raw material on-site
into adobe block.
Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista
4-7
Project No.: NH-010-D (AIW)
TRACS No.: 10 PM 236 H5838 01D