Vibration Technology Of Deep Compaction Of Sandy Foundations

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The method of deep vibro-compaction of water-saturated sandy foundations was proposed and developed by VNIIGS by P. D. Lobasov. The essence of this method is that dynamic excitation is transmitted to the water-saturated sandy mass by a spatial rod structure rigidly connected to a vibro-exciter operating at a frequency of 20–25 Hz.

P. D. Lobasov counts that during vibrations in the compacted mass of water-saturated sand, a long-term volumetric dynamic excitation is created, which ensures the reduction of sand density simultaneously throughout the entire height of the layer. This eliminates the harmful effect of the surcharge on liquefaction, which improves the conditions for rearrangement and helps achieve high density throughout the entire volume of compacted soil.

The very first experimental works conducted by P. D. Lobasov and V. M. Anisimov showed the high efficiency of the proposed method. After compaction, the internal friction angle of the sand usually reached about 45°, and the density index was 0.8–0.9. Improving the engineering properties of the sand increased the bearing capacity of the foundations.

Currently, for deep compaction of sandy foundations, the VNIIGS VUUP-6 vibro-installation is used, which consists of a VPP-2M (V-401) vibro-driver and the compactor itself. The compactor is a lattice-type tube structure with a diameter of 130 mm and a length of 7 m, with horizontal ribs welded to it every 400–450 mm. The diameter of the compactor is 1000 mm, and its height is 7 m. The total mass of the vibro-installation is 3400 kg. With the help of the VUUP-6 vibro-installation, sand can be compacted to a depth of 6 m. It should be noted that the depth of the compacted layer can be significantly increased (up to 12–15 m) when using the VNIIGS VSh-1 vibrator as a vibro-exciter (for technical characteristics of vibrators, see § 13). Deep compaction of sand by a vibro-compactor includes the following main technological operations (P. D. Lobasov, 1969):

• Lifting: Lifting, moving, and installing the vibro-compactor with a truck crane to the installation site.
• Submersion: Submerging the vibro-compactor into the ground to the specified depth.
• Compaction: Vibro-compaction of the sand throughout the entire depth.
• Extraction: Extracting the vibro-compactor from the compacted sand.
• Repositioning: Repositioning the vibro-installation to a new vertical axis of compaction.

After installing the vibro-compactor vertically on the ground surface, the vibrator is turned on. The compactor is submerged into the liquefying ground to a depth 1 m less than the depth of the underlying soil layer to be compacted. The submerged compactor is extracted back to the surface at a specific rate (extraction speed 0.2 m/min) and then submerged again until a stable vibro-compaction mode is achieved, which causes shaking and compaction of the sand. The evaluation of results is carried out based on the indicators of the achieved density, both by sounding results and by soil samples taken from pits.

The deep compaction method was successfully used at a number of projects during the preparation of sandy foundations for constructed facilities (B. M. Zubkov, E. D. Kovalevsky, V. M. Anisimov, 1983). VNIIGS developed the “Instruction for Deep Compaction of Water-Saturated Sandy Foundations” VSN 182-81.