<p><p><h2>Preferred Design Procedure</h2>The preferred vibrocompaction design procedure includes evaluating the degree of improvement, the required compaction effort, selecting the appropriate compaction equipment and design parameters (point spacing, vibration frequency, probe penetration and extraction, and duration of compaction). The design values are typically derived from CPT cone resistance values or corrected SPT N‑values ((N₁)₆₀). Vibrocompaction is used to improve sandy sites to minimize total and/or differential settlement, increase bearing capacity, or increase seismic resistance. The design is dependent on the governing application.</p><p>The Federal Highway Administration (FHWA) has a design document that covers both of the preferred design procedures for this technology. The document is summarized below.</p><p><table class='tablepress' id='tablepress-317'><thead><th><center>Publication Title</th><th><center>Publication
Year
</th><th><center>Publication Number</th><th><center>Available for Download</th></thead><tbody><tr><td >Ground Modification Methods, Volume 1</td><td ><center>2016</td><td ><center>FHWA NHI-16-027</td><td ><center>Yes<sup>1</td></tr></tbody></table><br><p class="disclaimer">¹ <a href="https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi16027.pdf&quot; target="_blank" rel="noopener">https://www.fhwa.dot.gov/engineering/geotech/pubs/nhi16027.pdf</a></p>T… necessary degree of improvement is determined through in-situ soil parameters. A soil investigation should determine the gradation of the in-situ soils, the silt and clay content or fines content (F_c), and the existing relative density (D_ro). The relative density can also be determined in the lab using ASTM procedures to determine the void ratio or dry density of the soil in its natural state, loosest state, and densest state.</p><p>It is rarely necessary to achieve a relative density above 85%. Typical performance criteria will require a 60% relative density for construction of a floor slab, 70% relative density for a column footing, and 80% relative density for a mat foundation. The target relative density should be evaluated based on the realistic degree of improvement which can be attained. The achievable degree of improvement depends on the in-situ and backfill soil gradation (can be same material), tributary area per point, and spacing. Most coarse-grained soils with a fines content less than 10% are considered to be compactable, hence viable for improvement by vibrocompaction.</p><p>In order to achieve optimal compaction energy transfer, it is advised to use a compaction process where energy is transferred both along the shaft and the bottom of the probe. Vibratory compaction increases the horizontal stresses in the soil, which permanently changes the stress conditions after compaction and can cause the soil to become overconsolidated. The highest horizontal stresses are expected to be near the compaction points.</p><p>Vibrocompaction is often executed using vibroflotation, Terra-Probe, Vibro-rod, and the vibrating beam installation methods. A number of contractors in the U.S. are capable of this installation. Typical vibrators can generate a centrifugal force of 15 to 50 tons. Modern vibrators can generate a centrifugal force of up to 400 tons, with a maximum displacement amplitude exceeding 1 inch. Typical spacing of probe points ranges from 5 to 15 feet, depending on soil type, initial soil density, final density requirements, and the size of the probe. Typical depths of vibrocompaction range from 10 to 50 feet. However, vibrocompaction has been used up to a depth of 200 feet (Chu et al. 2009). The improvement depth should extend to the bottom of the liquefiable/unstable material and extend laterally to a distance equal to the depth of treatment.</p><p>Vibration frequency is an important parameter for the effectiveness of densification during the penetration and compaction phases. The frequencies used should be based on vibrator mass, length and size of probe, and shear wave velocity of the soil. The duration of compaction should be selected based on soil properties, required degree of densification, and vibration energy transferred to the ground. During insertion and extraction, the vibration frequency should be higher than 30 Hz to minimize shaft resistance along the probe. During the compaction phase, the frequency is usually between 15 and 20 Hz (typical values of soil resonance frequency). The most efficient compaction process is to insert the probe to the required depth as rapidly as possible at a high vibration frequency, followed by compaction of the soil at or close to the resonance frequency of the soil, and then extract the probe at a high vibration frequency. Vibrocompaction projects typically require a certain degree of densification, which can be specified using minimum acceptance criteria after compaction (e.g., average corrected SPT N‑value or average CPT cone resistance).</p><p>The in-situ soil should be evaluated based on which zone its gradation curve falls in, using the figure below. Generally, no backfill is required for Zone A during the densification. Vibrocompaction in Zone B might require a sand backfill while Zone C should utilize a gravel backfill in place of sand. Soils which have a gradation curve in Zone D are not readily improved through vibrocompaction. Figure 1 shows ranges of soil types treated by vibrocompaction.</p><p><figure id='attachment_3306' style='max-width:1006px' class='caption alignnone'><img class="wp-image-3306 size-full" src="https://www.geoinstitute.org/sites/default/files/geotech-tools-uploads/…; alt="Soil gradation plot showing the ranges of soil types that can be treated using vibrocompaction." width="1006" height="542" /><figcaption class='caption-text'> Figure 1. Range of soil types treated by vibrocompaction (Elias et al. 2006).</figcaption></figure></p><p>Backfill material should be evaluated by its suitability number (S_N). A lower suitability number indicates that the soil is easily compacted and the probe can be withdrawn quicker while still achieving acceptable compaction.</p><p>Figures 4-33 and 4-34, provided in Schaefer et al. (2016), can be used to determine the tributary area, per point, necessary to achieve the target relative density based on spacing and soil type. Neither of these figures considers a target relative density higher than 85%. The spacing is typically a square or triangular pattern. The use of a square probe pattern will require 5 to 8 percent more points than a triangular probe pattern. Where time and budget allows, the spacing should be established by a test program at the beginning of the project.</p></p>