Deep Soil Mixing vs Underpinning for Soft Soil

Why this matters

When a settled foundation sits over soft saturated clays, loose silts, or organic peat, conventional helical and push pier solutions either cannot reach competent bearing within practical pier length or generate so much skin friction loss in the weak strata that pier capacity drops below the design load. Two ground-improvement strategies compete with conventional underpinning in this scenario: deep soil mixing (DSM) and rigid inclusions. This article frames the decision between underpinning and DSM for residential and light commercial foundation remediation when soft soil is the controlling problem.

Soft soil identification

The decision starts with geotech data. A soft soil profile that justifies considering DSM typically includes:

• Standard Penetration Test (SPT) N-values under 5 to depths greater than 15 feet (ASTM D1586)
• Cone Penetration Test (CPT) tip resistance qt under 20 tsf to depths greater than 20 feet (ASTM D5778)
• Vane shear strength under 500 psf for soft clay (ASTM D2573)
• Moisture content above the liquid limit, or organic content above 5 percent

A pier system that has to push or screw through 25 feet of N-3 silt before reaching competent bearing will see significant downdrag (negative skin friction) as the soft layer settles around the pier shaft after installation. Downdrag can convert a 50,000 lbf pier into a 25,000 lbf pier within a few months. DSM addresses this by stiffening the soft layer in bulk so it no longer settles around the pier or independently of the foundation.

How deep soil mixing works

A DSM rig (Bauer BG series, Soilmec SR series, or smaller residential rigs from Bencor, Hayward Baker, or Geo-Solutions) drives a mixing tool to the design depth while injecting a cementitious binder slurry. The tool blends the binder with the in-place soil column to form a soil-cement column ranging from 18 inches to 8 feet in diameter. Columns are arranged in a grid (typically 1.5 to 3 column diameters on center) and produce a composite improved-soil block under the foundation.

Binder selection depends on soil type: Type I/II Portland cement for inorganic clays and silts at 200 to 400 kg per cubic meter of treated soil, blended cement with ground granulated blast furnace slag (GGBS) for marine clays, and high-cement-content binder with admixtures for organic soils. Unconfined compressive strength targets at 28 days range from 200 psi for residential soil-bearing improvement to 800 psi for structural support columns.

When to choose underpinning

Underpinning (helical or push piers) is the right tool when:

• Competent bearing exists within reachable pier length (typically 50 feet for residential helicals, 65 feet for push piers with the right reaction)
• The structure is light enough that pier capacity at the engineered torque covers the load with safety factor 2 or higher
• Site access supports staging equipment at the foundation perimeter
• The project is repair of an existing settled structure where bypass of the soft layer is the design intent
• Budget and schedule favor a smaller, more mobile crew

For typical residential repair on soft soils where bearing is reachable within 35 feet, underpinning with helicals at the calculated torque is the conventional and the appropriate choice.

When to choose deep soil mixing

DSM is the better choice when:

• Competent bearing is not reachable within practical pier length, or downdrag would overwhelm pier capacity
• The structure footprint covers a large area and per-pier costs accumulate beyond DSM equivalent
• The foundation will be a new slab over the improved ground (rather than a repair to an existing foundation), where uniform global stiffness matters more than discrete point support
• Liquefaction mitigation is also a design goal (DSM columns provide drainage and stiffness that mitigate cyclic softening)
• The project includes a parking structure, light commercial, or institutional building where rigid-inclusion ground improvement is the prevailing local approach

DSM is rarely the right tool for retrofit residential repair because the equipment is too large for typical residential access, and because the existing foundation has to be lifted off the soil for the mixing tool to reach the target depth.

Hybrid approach

A growing residential and light commercial pattern is rigid inclusions plus a load-transfer platform: smaller-diameter (12 to 18 inch) drilled-displacement piles or Controlled Modulus Columns (CMC, a Menard Geosystems trademark) installed through the soft layer on a grid, capped with a 12 to 18 inch granular load-transfer mat over geogrid (Tensar TX160 or equivalent), with the foundation built on the mat. This bridges the soft soil without lifting the structure off the ground and produces less spoil than conventional DSM.

Hybrid rigid-inclusion-plus-mat designs are commonly used under new foundations on remediated industrial sites and have become a tool that small DSM specialty contractors offer for residential additions on documented soft-soil lots.

Quality assurance differences

Underpinning QA: torque logs (helicals), force gauges (push piers), individual pier load tests on a percentage of installed piers per the engineer (ASTM D1143 for static load testing).

DSM QA: continuous installation parameter logging (rotation, depth, slurry flow, withdrawal rate) on every column, wet-grab samples at every 5th column tested for unconfined compressive strength at 7 and 28 days per ASTM D2166, and post-installation core sampling on 1 to 5 percent of columns per the spec.

Engineering and permitting

Both approaches require a licensed geotechnical engineer's design and the structural engineer's foundation re-analysis. DSM and rigid-inclusion work additionally requires a specialty geotechnical contractor with the equipment fleet; most residential foundation repair contractors will subcontract this work to firms like Hayward Baker, Bencor, Menard, or Keller. Permit pathways vary: jurisdictions familiar with ground improvement treat it as a foundation system; others treat it as soil modification requiring a separate review.

References

• ACI 562-21 Code Requirements for Assessment, Repair, and Rehabilitation of Existing Concrete Structures
• IRC R401 and R403 Foundations (2024 edition)
• FHWA-NHI-13-046 Design Manual for Deep Mixing for Embankment and Foundation Support
• ASTM D2166 Unconfined Compressive Strength of Cohesive Soil
• ASTM D1586 Standard Test Method for Standard Penetration Test (SPT) and Split-Barrel Sampling
• ASTM D1143 Standard Test Methods for Deep Foundations Under Static Axial Compressive Load
• Federation of Piling Specialists (FPS) and Deep Foundations Institute (DFI) Augered Cast-In-Place and Drilled Displacement Pile Manual
• Menard Geosystems Controlled Modulus Columns Technical Data (2024 release)