Water-Reactive Polymer Grout Seals Leaking Diaphragm Walls in Deep Excavations

Deep excavations in crowded cities depend on diaphragm walls, the massive concrete barriers that hold back earth and groundwater while builders carve out basements, metro stations, and foundations tens of meters below street level. Yet these structures carry a persistent weakness: the joints between individual wall panels. When groundwater finds a path through a poorly sealed joint, it can erode soil along the wall, enlarge seepage channels, and rapidly destabilize both the excavation and neighboring buildings. A new study published in Case Studies in Construction Materials offers a detailed laboratory and field demonstration that a low-viscosity, water-reactive polyurethane grout can transform these vulnerable interfaces into some of the strongest and driest parts of the wall system.

The research team, led by Wenwu Zhang and Xiaodong Yang, set out to answer a question that has lingered in geotechnical engineering: exactly how does permeation grouting strengthen the hidden boundary between soil and concrete? Traditional repair methods for leaking wall joints demand heavy equipment, long construction windows, and considerable cost, often paralyzing work on an active site. Cement-based grouts, the workhorse of the industry, struggle in fine-grained silty and clayey soils because their cement particles are too large to penetrate narrow pores and fractures. Chemical grouts, by contrast, are particle-free solutions that can seep into fine pore structures, and permeation grouting has already proven effective at reinforcing the interface between two different materials without any excavation.

At the heart of the study is a permeable polyurethane developed independently by the research group. It is produced by mixing two yellow liquid components, a polymeric MDI-based isocyanate and a blend of polyether polyols with a phosphate catalyst, in equal mass ratios. Once injected, the hydroxyl groups of the polyols react with the isocyanate groups in an exothermic polymerization that rapidly builds a three-dimensional polymer network, while some isocyanate groups trimerize into stable isocyanurate rings. Because the material reacts with water, groundwater itself becomes part of the curing chemistry. The grout has a viscosity of just 10 millipascal-seconds at room temperature and a compressive strength of 2.5 megapascals, properties that allow it to permeate soil rather than merely split it apart.

To isolate the mechanics of the soil-concrete interface, the team built a controlled laboratory program using silty clay from central China, in which 81 percent of particles fall between 0.002 and 0.075 millimeters. Concrete blocks of strength grade C30, the same grade commonly used in diaphragm wall construction, served as the substrate. The researchers machined parallel grooves into some blocks with a diamond-tipped milling tool to create three quantified roughness levels, from a smooth as-cast surface to a highly rough profile with a joint roughness coefficient of 23.49, and verified groove geometry with profilometric measurements to within a tenth of a millimeter. Soil was compacted onto the concrete surfaces, and a custom constant-pressure grouting system injected the polyurethane at 0.2 megapascals, a pressure chosen to keep the process in the permeation regime without fracturing the soil.

The direct shear tests, run at vertical stresses of 200 to 600 kilopascals and moisture contents of 5, 13, and 21 percent, produced strikingly consistent patterns. Both peak and residual shear strengths rose almost linearly with vertical stress and with surface roughness, with regression coefficients above 0.9. At the roughest interface, peak shear strength reached 1.39 times the smooth-interface value, and residual strength climbed to 1.51 times. Moisture content told a more nuanced story: strength increased from 5 percent to a maximum at 13 percent moisture, then fell sharply at 21 percent. The explanation lies in the chemistry of curing. At low moisture, the exothermic reaction overheats and stalls, leaving voids between particles; at high moisture, water blocks polymer penetration and dilutes the reaction, producing a porous, weak foam. At 13 percent, the polymer cures into a dense, well-bonded network.

Microstructural analysis revealed what is happening at the scale of individual grains. Scanning electron microscopy showed that untreated silt consists of loose, single grains separated by large pores, while grouted soil displays particles cemented together by polymer that fills the gaps and binds them into a coherent mass. Mercury intrusion porosimetry quantified the transformation: cumulative mercury intake dropped from 0.4140 to 0.1296 milliliters per gram, a reduction of about 57 percent, meaning total porosity fell to roughly 43 percent of its original value. The pore structure of the grouted soil became markedly more homogeneous, eliminating the large, connected pores that would otherwise serve as preferential seepage paths. Crucially, the researchers identified a cohesive transition layer at the soil-concrete boundary, a polymer-rich zone that bonds the two materials together and provides both chemical adhesion and mechanical interlocking.

These microscopic findings explain the macroscopic failure behavior. The team found that shear failure at grouted interfaces falls into two modes: failure within the soil matrix itself, and failure along the interface. By measuring the area of soil left adhering to the concrete after shearing, they showed that increasing roughness and moisture shifts the failure surface away from the interface and into the reinforced soil, meaning the bond has become stronger than the ground it holds. Residual cohesion after large shear displacement remained below 25 percent of peak cohesion, but the residual friction angle stayed high because the rough concrete surface and polymer-filled soil continue to interlock even after the cohesive layer ruptures.

The decisive test came in the field. On a project in Hangzhou, Zhejiang Province, where 1000-millimeter-thick diaphragm walls retain a 20-meter-deep excavation supported by concrete and steel struts, the team injected the polyurethane grout at 0.3 to 0.5 megapascals into two joints in the northern section of the wall. As excavation progressed layer by layer, the grouted joints remained tightly sealed at 5, 10, and 15 meters of depth, while adjacent ungrouted joints showed slight seepage. White solidified material was visible where the grout had expanded and hardened on contact with water, forming a foamed consolidated mass that filled cracks and permanently blocked flow. By the time excavation was complete, neither grouted joint had leaked at any stage.

The Hangzhou validation was qualitative, based on systematic inspection during each excavation stage rather than instrumented monitoring, and the laboratory model used a planar interface that simplifies the complex geometry of real wall joints. The authors acknowledge these limits and call for large-scale joint tests, real-time monitoring, and numerical models to bridge laboratory mechanisms to field-scale design. Still, the consistency between the predicted mechanisms and the observed performance provides strong evidence that pore-filling, interfacial bonding, and moisture-sensitive curing operate together at engineering scale. For cities racing to build downward, the message is compelling: a portable grouting rig, a two-component polymer, and a clear understanding of soil moisture may be enough to turn the weakest link in an underground wall into a watertight, load-bearing one.

Subject of Research: Permeable polyurethane grouting reinforcement of soil-concrete interfaces in diaphragm wall joints for seepage control in deep excavations

Article Title: Interfacial enhancement of diaphragm walls through permeable polyurethane polymer grouting reinforcement: Laboratory test and field application case

Article References: Zhang, W., Yang, X., Ding, L., Diao, Y., & Ji, X. (2026). Interfacial enhancement of diaphragm walls through permeable polyurethane polymer grouting reinforcement: Laboratory test and field application case. Case Studies in Construction Materials, 25, Article e06548. https://doi.org/10.1016/j.cscm.2026.e06548

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06548

Keywords: diaphragm walls, polyurethane grouting, permeation grouting, soil-concrete interface, shear strength, seepage control, deep excavation, mercury intrusion porosimetry, scanning electron microscopy, geotechnical engineering, field application, groundwater leakage