Preferred Design Procedure

There is not an FHWA method for the design of mechanical soil mixing method.

Mechanical stabilization of subgrades and base courses includes the following approaches:

• Using thick granular layers
• Using geosynthetics (geotextiles and geogrids)
• Blending two or more soils
• Compaction

This document is focused only on the design guidance for blending two or more soils. Detailed design guidance for using thick granular layers are discussed in the Beneficial Reuse of Waste Materials Design Guidance. Detailed design guidance for using geosynthetics to stabilize subgrades and base courses are discussed in the Geosynthetic Reinforced Construction Platforms Design Guidance, Geosynthetic Reinforcement in Pavement Systems Design Guidance, and Geosynthetic Separation Layers in Pavements Design Guidance. Details for the design of compaction are discussed in the Traditional Compaction Design Guidance.

Table 1 presents typical inputs and outputs for design and analysis procedures of mechanical stabilization of subgrades.

Table 1. Typical inputs and outputs for design and analysis procedures.

Performance Criteria/Indicators

Serviceability
Rutting of initial lift
Fatigue life
Vertical displacement
Drainage

Subsurface Conditions

Permeability/Hydraulic Conductivity
Groundwater elevations
Sensitivity
Soil Classification
Effective Internal friction angle, ϕ'
Young's Modulus
Gradation of base course
Modulus of subgrade reaction (k)
Undrained Shear Strength, s_u

Loading Conditions

Traffic load
Equipment load
Load magnitude
Frequency of load
Consolidation potential
Water pressure

Material Characteristics

Type of geosynthetic
Manufacturing process of geosynthetic
Manufacturing properties of geosynthetic (e.g. grab strength, tear strength, puncture)
Hydraulic properties of geosynthetic (e.g. permittivity, apparent opening size)
Gravel characteristics (gradation, permeability, resilient modulus, thickness, density)
Durability of granular layer
Durability of geosynthetic properties (e.g. Uv)
Lightweight fill characteristics (e.g. density, permeability)
Recycled material characteristics (e.g. stiffness modulus, compressibility)

Construction Techniques

Installation procedures
Compaction

Geometry

Placement location
Aggregate depth
Asphalt concrete thickness
Subgrade/base course thickness
Geosynthetic seam