Q&A on New Research on Deep Mixing Method of Soil-Cement Materials

In October 2025, the Deep Foundations Institute (DFI) published a significant, multi-year research study conducted by GEI’s George Onorato, PE, and Giovanni Bonita, PhD, PE, PG, PENG that focused on how overburden stress present during curing affects the strength and behavior of deep mixing method (DMM) soil-cement materials.

George and Giovanni received two multi-year grants through DFI, and GEI Consultants provided matching in-kind grant funding and covered the cost of fabricating specialized testing equipment used in this research.

The study involved collaboration with numerous industry and academic partners and has been recognized internationally as an important contribution to the ground improvement industry.

We asked George and Giovanni about the history of this effort, why the research is valuable, and the insights and applications it offers to several industries using DMM.

The clarity from this research is significant. Can you discuss the history of this effort to improve the quality of deep mixing?

Since the early 1980s, researchers in Asia, Europe, and North America have investigated whether in-situ overburden stresses during curing increase DMM soil-cement strength. However, the effect has been difficult to quantify because mechanisms impacting the behavior were not well understood by early researchers, contractors, and practitioners, making the DMM behavior difficult to interpret and apply.

This DFI study advances historical research efforts by assessing multiple independent datasets and advanced analytical methods. These datasets demonstrate that curing-stress-related strength gain is systematic and strongly soil-type dependent. The observed magnitude is large enough to influence design assumptions and acceptance decisions, indicating that conventional quality control (QC) methods typically under-represent true in-situ engineering properties.

What was unique about this new research and what kind of guidance and direction does it provide regarding this methodology?

This research was unique in its scale, breadth, and industry involvement: bench testing, specialized field-modified oedometer testing, innovative in-situ instrumentation tests, and independent coring data provided from real projects were evaluated together using a consistent analytical framework. Overall, the study provides practical technical guidance to adjust typical QC laboratory Unconfined Compressive Strength (UCS) results based on soil type and expected in-situ stress with depth. Specifically, it presents empirical relationships and soil-type-dependent coefficients to adjust or estimate in-situ UCS as a function of in-situ overburden stresses during curing.

Deep mixing has applications across many industries. Can you discuss some of those and why this research could be important to those industries?

DMM is used for ground improvement and environmental applications in transportation, infrastructure, energy, water, and the building markets, including earthquake mitigation, settlement control, embankment stabilization, excavation support design, slope stabilization, dam/levee stabilization, seepage control, and containment/stabilization of contaminated media.

Across these sectors, the recurring challenge is confidence that the QC reflects in-situ engineering properties; this research helps close that gap by quantifying stress-related UCS gains, especially in fine-grained soils.

How might this contribute to reducing costs on projects where deep mixing is necessary?

The study results indicate that, under most circumstances, in-situ material properties are better than those determined through conventional laboratory UCS testing. This implies some projects may be using more binder than necessary to meet performance targets.

By providing a defensible method to account for in-situ overburden stresses during curing, the work supports binder optimization (often on the order of meaningful percentage reductions) while maintaining required performance.

It also helps reduce costly outcomes tied to rework, contingency, and contractor risk pricing driven by marginal or conservative QC interpretations. On large DMM programs, even modest binder reductions and avoided rework can save millions of dollars on a single project and have sustainability impacts.

What implications does the research have for sustainability within deep mixing projects?

Because cement/binder production and delivery are major drivers of both cost and embodied carbon, the most direct sustainability benefit is the ability to optimize binder dosage while still meeting performance requirements.

By improving confidence in more representative in-situ performance (accounting for in-situ overburden stresses during curing), this research supports reducing unnecessary binder tonnage on projects where conventional QC may under-represent in-situ strength.

It also reduces the environmental footprint associated with delivery—fewer truck trips, lower diesel fuel consumption, and reduced transportation-related emissions.

What is the bottom-line impact for the industries using deep mixing?

The bottom-line impact of the study is that it improves our understanding of in-situ engineering properties, the curing conditions and materials that affect those properties, and our confidence to better interpret QC data-which, in turn, improves efficiency and decision-making across design, construction, and acceptance.

The research provides a more reliable basis for estimating in-situ material behavior (strength) and a defensible method to interpret and adjust QC UCS results for soil type and in-situ overburden stresses during curing, reducing the frequency of the “false marginal” outcomes that drive rework, contingency, and contractor risk pricing.

In parallel, the work supports binder optimization, lowering both cost and embodied carbon. Importantly, the findings are gaining international recognition and being incorporated into evolving practice guidance, specifications, and design standards through organizations such as DFI and ASCE, helping translate the research into more consistent industry practice.