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 Case Study: Densifying Sands Near Existing Structures

Vibroflotation for Ground Improvement (Sreekantiah, 1993)


The Mangalore Chemicals and Fertilizers Company Limited is situated on the West Coast of India in Mangalore, Karnataka.  The company desired to install new machinery at their existing location.  In order to install the new machinery, the very loose granular soil which comprised the site had to be improved in order to achieve adequate bearing capacity as well as to assure that settlements of the soil where the new machinery was to be installed would be within permissible limits.

The company is located on a site that originally consisted of agricultural land.  In 1966, the ground level elevation, which was originally 6 feet above sea level, was raised through dredging.  The soil used for dredging was recovered during the construction of the New Mangalore Port on the West Coast of India.

 Figure 10: Mangalore location (Sreekantiah, 1993)

Figure 13: Mangalore location (Sreekantiah, 1993)

Site Characteristics

The subsurface soil on the site was comprised of the soil that had been dredged in 1966.  The profile consisted of two layers: a clean sand layer underlain by a clay layer with consistency that varied from soft to stiff throughout the site.

Standard and cone penetration tests were performed throughout the site in order to the assess the subsurface soil conditions.  The clay layer was determined to be a marine clay with varying consistency.  The upper sand layer consisted of a stratum varying from fine sand to sandy gravel with N values ranging from 1 to 16.  Rock was found at depths ranging from 92 feet to 98 feet below ground elevation.

Statement of Problem

The Mangalore Chemicals and Fertilizers Company site posed a great challenge.  The use of a shallow foundation to support the machines that were to be installed proved unfeasible.  However, the use of traditional driven piles was also not feasible because construction disturbance posed a risk of damage to the nearby existing machinery at the site.

Solution and Design

Bored compaction piles were one of the alternatives considered for increasing bearing capacity of the subsurface soil. However, difficulties were encountered during construction of the bored piles.  Additionally, the load tests performed on the bored compaction piles proved unsuccessful. The expected applied loads from the machinery proposed to be installed on the site ranged from 229 pounds per square inch to 236 pounds per square inch.  By virtue of the limiting site conditions, ground improvement by vibroflotation was determined to be the best course of action.

A thorough analysis of the subsurface soils was conducted in order to determine whether vibroflotation would be a feasible improvement technique.  The upper sand layer was determined to contain less than 10 percent fines making it ideal for improvement by vibroflotation.  An area of 79 feet by 20 feet was selected to be compacted to a desired depth of 23 feet below ground elevation.

CEM-INDIA performed the work, electing a wet process using a standard vibroflot with a 15.75 inch diameter, 26.5 feet length and a weight of 6750 pounds.  A square grid spacing scheme of 6.5 feet by 6.5 feet centre to centre was implemented.  The vibroflot was inserted at intervals of 6.5 feet to the desired depth and was removed in 9.6 inch intervals.  The criteria provided for acceptance in the contract with CEM-INDIA was a maximum acceptable settlement of 0.47 inches for a 6.2 feet diameter plate under a pressure of 65 pounds per square inch.  Additionally, a static cone penetration test resistance of 22 pounds per square inch  had to be achieved post compaction.


After compaction was completed, the site was tested for the previously stated criteria.  Five of the eight load tests performed on the improved area resulted in a settlement of less than 0.47 inches, while two load tests resulted in a settlement of less than 0.60 inches and one resulted in a settlement of 1.6 inches.  The static cone penetration test resistance of 2175 pounds per square inch was generally achieved and demonstrated that between depths of 6.5 feet and 23 feet penetration resistance achieved a threefold to fivefold increase post compaction, as can be seen below in Figure 14.  Raft foundations were selected for use on the improved site area.  Foundation settlement was monitored after the installation of the new machinery and was determined to be within permissible limits.

Figure 11: Mangalore cpt results (Sreekantiah, 1993)

Figure 14: Mangalore CPT results (Sreekantiah, 1993)


The installation of new machinery on an existing site posed challenges.  The subsurface soil which was comprised of granular soil underlain by a clay layer made the use of shallow foundations unfeasible.  However, the risk of potential damage to nearby existing machinery due to disturbance caused by construction made traditional driven piles also unfeasible.  Vibroflotation proved to be a successful technique for improving the ground on site.  It densified the soil which resulted in an increased bearing capacity as well as reduced settlement to permissible limits.  Vibroflotation was the best technical and economical solution for improving the loose granular soil without posing a great risk to existing structures and machinery near the site.  

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