Design and Verification of Blast Densification for Highway Embankments on Liquefiable Sands

This proposal summarizes the progress and describes the scope of work in year 3 of the three year effort to develop a quantifiable design methodology and attendant verification approach for improving loose sands deposits underlying highway embankments in seismically active regions. The project involves a major field project where blast densification of loose sands is undertaken periodically to improve the loose sands beneath a large fill. Originally envisioned as a two year project the field work was delayed because of the recession of 2009 as it slowed the owner's plans for development of the site. The production blasting now is scheduled for early 2011, delayed from its original time of August 2009. The delay has allowed Northwestern to conduct a detailed laboratory evaluation of the liquefiable soils from the site, and as a result, the focus of the field work has changed somewhat since first envisioned, as will subsequently be explained. The proposed work is a joint effort between GeoSyntec Consultants, the engineering consultant for the project, and Northwestern University. Matching funds are derived from GeoSyntec's analytical and field efforts for the project, as well as from the blasting contractor at the site. Transportation systems included many embankments for highways and railroads. In seismically sensitive areas west of the Rocky Mountains and over broad areas of the eastern and central US - estimated to cover as much as 40% of the continental US, earthquake engineering for highway facilities is very important (FHWA 1997). A key design issue for such facilities is whether or not liquefaction - or the loss of shear strength of sands - will occur during an earthquake. If such a possibility exists, then one must either relocate the embankment or improve the potentially liquefiable soil to the point where the improved soil will not liquefy under the expected earthquake. Loose sands are the soils most susceptible to liquefaction. Two questions arise during design: (i) will liquefaction occur under a given earthquake loading, and (ii) what are the consequences of liquefaction? The most egregious effect is a flow failure of the embankment through the liquefied soil. Because highway and railroad embankments traverse large areas, the costs of mitigating the effects of the liquefiable soils are large, as are costs related to realignment, if this option is followed in design. To improve the ground over large areas, densification of loose sands by controlled blasting is an economical approach. Design methods for this approach are empirical, as will be described later. A few case studies have shown that loose sands apparently compress almost immediately after blasting, but when common verification tests are conducted, such as the cone penetration test (CPT) and the standard penetration test (SPT), the outcomes provide rather counterintuitive results. If taken soon after the blast, the CPT tip resistance decreases, and at times never increases to levels above the pre-blast level, at least for the amount of time the studies included. At the same time, the ground surface settles almost immediately after blasting, implying that loose sands in the subsurface have increased density. However, the lack of increase in penetration resistance suggests that the strength and stiffness of the soil apparently does not. This leads to questions about future performance. Have the loose sands really been improved to the point where liquefaction is not a possibility? One simple, yet common approach for evaluating liquefaction potential (answer to question (i) above) is to compare SPT N-values at a site with those needed to prevent liquefaction for a given magnitude of earthquake. Similar relations have been proposed for CPT results. Because there is much evidence to show the relation between penetration resistance and liquefaction susceptibility, it is not clear why such field measures of resistance do not increase after blasting has apparently increased the density of the loose soils. Consequently there is a need to understand the fundamental mechanics behind the effects of blasting on the constitutive responses of the soil and to develop reliable and meaningful ways to provide quality control of the process. Also, since the design is large empirical, a rational way to define how much improvement is required to meet the design objective required.

    Language

    • English

    Project

    • Status: Completed
    • Funding: $406687.08
    • Contract Numbers:

      610 4742000 60022750

    • Sponsor Organizations:

      Infrastructure Technology Institute (ITI)

      Northwestern University
      L260 Technological Institute, 2145 Sheridan Road
      Evanston, IL  United States  60208-3109
    • Principal Investigators:

      Finno, Richard

    • Start Date: 20090901
    • Expected Completion Date: 0
    • Actual Completion Date: 20110831
    • Source Data: RiP Project 20236

    Subject/Index Terms

    Filing Info

    • Accession Number: 01468387
    • Record Type: Research project
    • Source Agency: Infrastructure Technology Institute (ITI)
    • Contract Numbers: 610 4742000 60022750
    • Files: UTC, RiP
    • Created Date: Jan 3 2013 3:50PM