Innovative Work Saves $2.91 Million for Arizona DOT

Posted by Naresh Samtani on Jun 1, 2022

The Arizona Department of Transportation (ADOT) is designing improvements for Interstate 10 (I10), between Ina Road and Ruthrauff Road in Tucson, Arizona. The improvements include 3.6 miles of eight lanes of mainline Portland Cement Concrete Pavement (PCCP) pavement and a total of four new I10 mainline bridges over the Rillito Creek and Cañada Del Oro (CDO) Wash which are major regional ephemeral streams (i.e., streams that do not experience flows except during flood events).

Naresh Samtani of NCS GeoResources, LLC (NCSG) was retained as a staff member by SCE Engineering (SCE), a local multi-disciplinary firm in Tucson, Arizona, to perform two types of specialized analyses that NCS Consultants, LLC (NCS) had successfully implemented for Pima County in Arizona and other projects but upon ADOT’s request were performed for the first time on an ADOT project. The Phoenix, Arizona, office of Kimley-Horn and Associates (KHA), a national firm, was the prime designer/consultant for ADOT and SCE was a sub-consultant to KHA.

Infiltration Analysis
In ephemeral streams, agencies and designers commonly use the assumption that the stream bed becomes fully-buoyant (i.e., the entire stream bed is saturated and submerged) during a transient flood event due to which the pore-water pressure (PWP) profile is hydrostatic. This assumption means that the effective stress (i.e., total stress minus PWP) can be up to approximately one-half of the total stress for the typical cohesionless soils found in ephemeral stream beds. Since the geotechnical tip and side resistances are a function of the effective stress such an assumption results in deep foundations that are almost 30% to 40% longer for a given diameter. In 2010, a protocol to evaluate the actual buoyancy and PWP profiles was developed and implemented by Naresh Samtani and Edward Nowatzki of NCS for Pima County projects. This protocol was further refined in 2020 in a peer-reviewed journal paper by Naresh Samtani. Using this updated protocol, Naresh Samtani, as a staff member of SCE and with support from other staff members of SCE, performed site-specific infiltration analyses to determine the actual buoyancy effects and PWP profiles during transient flood events in the two ephemeral streams (Rillito Creek and CDO Wash). The results allowed for a significant reduction in drilled shaft diameters and/or lengths. KHA’s bridge designer estimated that such foundation modifications led to a total of $1.13 million savings in foundation costs for the four I10 bridges.

Pavement Subgrade Vertical Stress Analysis
Remediation of poor subgrade is often specified to a depth of 3-feet below the bottom of the pavement section (or top of subgrade) regardless of the pavement type and traffic loads. This is based on the assumption that the depth of significant influence (DOSI) of vertical stresses due to wheel loads extends to a depth of 3-feet.  The validity of this assumption for the case of Portland Cement Concrete Pavement (PCCP) was evaluated by Naresh Samtani, as a staff member of SCE and with support from other staff members of SCE, by performing a site-specific three-dimensional (3-D) finite element analyses. The DOSI of vertical stresses is a function of the stiffness (R-value) of the subgrade, pavement thickness and properties, wheel loads, and axle configurations. It was found that for the 10-inch thick PCCP designed by ADOT for the I10 project, the DOSI could be much smaller than 3-feet which means that remediation measures can be much shallower than 3-feet. The results allowed for the elimination of most of the subgrade overexacavation and/or recompaction under the PCCP pavement, which would have typically been required by ADOT. Use of this project-specific approach resulted in savings of $1.78 million for ADOT as documented by KHA’s project manager and roadway designer. The modification of the subgrade overexcavation and/or recompaction requirements also had the added benefit and value of mitigating: (a) conflicts with utilities, and (b) disturbance of cultural resources.

The combined savings resulting from these two analyses is $2.91 million which represents a cost-benefit factor of over 20, relative to the cost of the analyses.

These are just the most recent examples of cutting-edge technology applications and cost-reductions that Naresh Samtani can provide for your projects. Contact us to find out how NCSG can help your project in a similar manner and enhance your team and work products.

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