Diminishing Returns of Passive-Zone Improvement for a Braced Excavation in Coastal Soft Soil

Ground improvement can substantially reduce excavation-induced deformation in coastal soft soil, but increasing treatment depth does not necessarily produce a proportional mechanical benefit. This study investigates a 7.0 m-deep braced excavation using a two-dimensional Hardening Soil finite-element model established from the project geotechnical investigation and test data. The calculated wall-displacement profile is compared with field measurements to assess the model’s representation of the observed deformation pattern. Parametric analyses are then conducted for retaining-pile embedment, soft-soil strength, base-improvement depth, cutoff-wall geometry, and surface surcharge. Base improvement produces pronounced deformation and stability benefits up to the 5 m treatment case (d/H ≈ 0.71). For a fixed treatment footprint, increasing the treatment depth from 5 to 7 m increases the treated volume by approximately 40%, but reduces the three maximum deformation measures by only 2.2–3.4% relative to the 5 m case. The strength-reduction safety factor increases from 1.893 to 1.896. The response-specific deformation coefficients and the marginal stability efficiency confirm the sharp reduction in incremental benefit over the 5–7 m interval. The results identify a case-specific transition toward diminishing mechanical returns and indicate that deformation control should prioritize effective toe restraint, targeted passive-zone strengthening, reliable support engagement, and construction-surcharge management.