Design and construction for ground improvement - Guidelines, Part 2: Preconsolidation using vertical drains
2004 Edition

Per IS 15284 Part 2 (2004), vertical drains should be constructed using sand wicks filled into preformed pipes made from HDPE or permeable woven fabrics, including natural fabrics. The sand must be well-graded, ranging from coarse to medium grains, with permeability between 10⁻³ to 10⁻⁵ cm/s, and should be saturated prior to installation. Furthermore, vertical drains must connect at the surface to a permeable sand blanket, at least 400 mm thick, with a minimum embedded length of 150 mm of the prefabricated drain within this blanket to facilitate efficient pore water discharge and expedite consolidation.

The spacing and depth of vertical drains are determined by extending drains through the full thickness of compressible soil layers responsible for settlement, ensuring effective pore pressure dissipation. Drain spacing is typically arranged in equilateral triangular or square patterns, influenced by drain diameter, soil permeability, stratification, and desired consolidation time. Spacing usually ranges from 1.5 to 3 meters. The design approach involves soil profiling, selecting drain diameter, calculating spacing based on soil properties, and applying consolidation theories like Barron’s or Hansbo’s methods to optimize consolidation duration and settlement.

IS 15284 Part 2 categorizes installation methods into displacement and non-displacement types. Displacement methods, such as driven or vibratory closed-end mandrels, laterally push soil aside causing significant disturbance and smear zones, temporarily reducing shear strength and are unsuitable for clay soils with sensitivity above 4-6. Non-displacement methods, including jetting and continuous flight auger techniques, remove soil during boring, resulting in moderate to minimal disturbance and smaller smear zones but usually require more supervision and complex procedures. The extent of soil disturbance influences the effective drain radius considered in design.

The standard prescribes calculating the overall degree of consolidation using the formula U = 1 - (1 - U_z)(1 - U_r), combining vertical (U_z) and radial (U_r) consolidation components. Vertical time factor T_v is calculated as C_v · t / H², where C_v is vertical consolidation coefficient, t is time, and H is drainage path length. Radial time factor T_r is computed as C_r · t / R², with C_r as radial consolidation coefficient and R as half the drain spacing. Tables provided (Table 2 and Table 3) assist in determining degrees of consolidation corresponding to these time factors and radius ratios. Coefficient C_r may be obtained from horizontal consolidation tests or estimated via permeability ratios.

During staged preloading, a minimum factor of safety (FoS) of 1.25 must be maintained against slip or bearing capacity failures at every loading stage. Each stage should achieve at least 90% consolidation before proceeding to the next. Shear strength is considered to improve proportionally with the degree of consolidation; this enhanced strength is used for stability assessments before further loading. The staged approach ensures safe progression, especially for soft soils or when high final loads are involved, verifying consolidation status and strength improvement at each interval.