Design and construction for ground improvement - Guidelines, Part 1: Stone columns
2003 Edition

According to the standard, soils suitable for stone column ground improvement typically have an undrained shear strength ranging from 7 to 50 kPa, including loose sands and silty or clayey sands. Soils that are not recommended include sensitive clays and silts with a sensitivity of 4 or more, as well as soils lacking an adequate bearing layer beneath weak zones to support the column base. Ensuring these conditions allows effective load transfer and stability of the stone columns.

Load capacity estimation involves calculating the ultimate load based on soil parameters and column characteristics. The total capacity accounts for the lateral soil resistance preventing bulging, the supplementary support provided by surcharge loads, and the bearing contribution from soil between columns. The limiting axial stress is computed considering soil shear strength, effective stress, and friction angle of the column material. This stress is then applied over the column cross-section and divided by a safety factor, commonly 2, to obtain the safe load. Ideally, these estimates are validated with load tests on trial columns.

The standard recommends three primary installation methods: (1) Non-displacement method, which involves creating boreholes stabilized by casing or bentonite slurry before placing and compacting stone fill; (2) Displacement method, where a closed-end mandrel is driven into the ground and stone is discharged as the mandrel is withdrawn, compacted internally; and (3) Vibro-replacement method, employing vibratory probes to form and compact columns via either wet or dry processes, depending on soil and groundwater conditions. Each method has specific procedural precautions to ensure borehole stability and column integrity.

The standard identifies failure modes that depend on soil type and column geometry. Bulging failure occurs when the column length exceeds four times its diameter, causing lateral expansion under load. Shear failure is typical for shorter columns resting on rigid bases, where the column or surrounding soil can shear off. In soft or very soft clay and silt layers, localized bulging may occur, especially in the presence of organic materials. The design must consider these failure modes using soil stratification data and apply appropriate safety factors to prevent such failures.

Field verification includes conducting load tests on individual stone columns and groups of columns to assess load-settlement behavior and ultimate capacity. Soil investigations before and after installation involve borehole drilling, cone penetration tests, pressuremeter tests, standard penetration tests, and vane shear tests, supplemented by laboratory analyses of soil samples. Settlement monitoring under incremental loading is essential to confirm design adequacy. These procedures ensure that the constructed columns meet the required performance criteria under working loads.