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Case Study: Effectiveness of Vibroflotation on Silty Sand

Vibroflotation Compaction at Thermalito Afterbay (Harder et. al., 1984)


The construction of Thermalito Afterbay Dam in Northern California was completed in 1967.  In August of 1975 an earthquake of magnitude 5.7 revealed an active fault that had not been previously detected.  As a result of the rupture of the Cleveland Hill Fault during the 1975 earthquake, the Department of Water Resources evaluated the embankments resistance to liquefaction under a 6.5 magnitude earthquake.  Their analysis predicted that the silty sand layers in the foundation of the embankment would liquefy entirely under these seismic conditions and would result in failure of the dam.  In 1979 storage restriction of the reservoir was implemented in order to reduce risk of failure until the seismic evaluation was completed.  The need to restore the dam to full operation was evident and therefore numerous remedial methods for stabilization of the dam were considered at Thermalito Bay.

Site Characterisitics

The embankment was 8 miles long with a maximum height of 39 feet.  The foundation consisted of several layers of different soils including clay, silt, sand and gravel.  The surface layer throughout most of the embankment was composed of a clay and silt layer several feet thick.  The silty sand layers which were targeted for densification contained a median of 15 percent fines, with 30 percent of the samples containing more than 20 percent fines.  The groundwater table downstream of the dam was found at a depth of 5 feet to 10 feet.

Statement of Problem

The seismic evaluation performed predicted that the silty sand layers of the embankment foundation would liquefy entirely under an earthquake of magnitude 6.5.  Densification of these silty sand layers was necessary to mitigate liquefaction risks.  A primary concern when selecting the remedial method to be used at the site was assuring that the clay embankment would not be at risk of large settlements or heaves during treatment.

Solution and Design

A vibroflotation testing program using 100 horsepower vibroflot was performed on the embankment in order to determine the effectiveness of vibroflotation in densifying silty sand.  Two worksites were selected to assure a range of conditions were represented in the testing program.  However, this vibroflot was not used to penetrate the clay and silt surface layer.  Instead pre-drilling with a 24 inch bucket auger was used to create holes that reached the silty sand layers.  These holes were then backfilled with sand before the vibroflot was inserted.  An equilateral triangular spacing scheme was utilized with spacings ranging from 6.5 feet to 9.5 feet.  Table 2 summarizing the vibroflotation test program and variables can be found below.

Table 2: Thermalito Bay worksites table (Harder et. al., 1984)

Table 2: Thermalito Bay worksites table (Harder et. al., 1984)


Standard and cone penetration tests were performed before and after the vibroflotation testing program was performed at the embankment.  SPT blow counts and CPT resistance at both sites showed generally no change before and after compaction.  SPT and CPT results for worksite 2 can be seen below and demonstrate the ineffectiveness of vibroflotation testing program.

Figure 17: Thermalito Bay CPT and SPT results (Harder et. al., 1984)

Figure 20: Thermalito Bay CPT and SPT results (Harder et. al., 1984)


Based on the results of the vibroflotation testing program at Thermalito Afterbay we can conclude that vibroflotation is not an effective method for the densification of silty sands below a cohesive soil cap.  The failure of vibroflotation as a technique in this case is most likely due to the relatively high fines content of 15 percent in the silty sand layer.  Generally, vibroflotation is ideal for sands with less than 10 percent fines content.

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