A mechanically stabilized earth wall rarely fails on the day it is finished. The bulges, panel separations, and settlement that show up months or years later usually trace back to decisions made before the first block or panel was set: an undersized drain, backfill with too many fines, or inadequate field density testing. Partnering with Earth Engineering Inc. for thorough geotechnical oversight and in-situ soil analysis catches these issues early, making the difference between a routine maintenance item and an expensive reconstruction.
What Actually Holds an MSE Wall Up
An MSE wall is a composite gravity structure. Layers of steel strips, welded wire mats or geogrid are laid between compacted lifts of select fill, and friction between the soil and the reinforcement locks the mass together so it behaves like a single block resisting the earth pressure behind it.
The facing panels carry very little load. That surprises owners, but it explains why a wall can look perfect from the street while the reinforced zone behind it is already losing capacity. Failure modes are usually grouped four ways:
- Internal failure: reinforcement pullout, rupture, or corrosion inside the reinforced mass.
- External failure: sliding, overturning or bearing capacity loss of the whole reinforced mass.
- Compound failure: a slip surface that passes partly through and partly behind the reinforced zone.
- Facing failure: connection breakage, panel spalling, or bulging between connection points.
Water Does More Damage Than Any Other Single Factor
Saturated soil conditions can significantly affect the stability, performance, and long-term durability of an MSE retaining wall. The degree of impact depends on whether saturation occurs within the reinforced soil mass, in the retained soils behind the wall, or in the foundation soils beneath the wall. From a geotechnical engineering standpoint, the primary concerns are increased hydrostatic and seepage forces, reduced effective stress, decreased reinforcement pullout resistance, and reduced foundation stability. Importantly, an MSE wall can be engineered to function under saturated conditions, but the design must specifically account for groundwater and drainage.
Most of the drainage problems we investigate fall into a short list:
- Clogged Drainage Systems – Fine soil particles can clog drainage pipes, granular drainage layers, or geotextiles, restricting water flow and causing groundwater to accumulate.
- Inadequate Surface Drainage – Improper grading or lack of surface water controls allows stormwater to infiltrate the reinforced fill, increasing moisture levels and pore-water pressures.
- Groundwater Seepage – Groundwater entering from behind or beneath the wall can create seepage forces, potentially resulting in soil migration, internal erosion, or instability.
- Saturated Foundation Soils – Poor drainage beneath the wall can contribute to reduced bearing resistance, settlement, and global stability concerns, depending on foundation soil conditions.
- Inadequate Drainage Outlets – Drainage systems without proper outlets or with blocked discharge points may allow water to accumulate rather than drain away.
Construction Phase Inspection and Compaction Shortcuts
For Mechanically Stabilized Earth retaining wall construction inspection in Philadelphia, focus should be placed on the following:
- Subgrade approval: Have the geotechnical engineer evaluate the bearing soils before placement of the leveling pad and reinforced fill.
- Backfill compliance: Verify material properties and perform moisture-density and field compaction testing at the frequency required by the specifications.
- Reinforcement installation: Inspect each reinforcement layer before it is covered, including length, spacing, orientation, damage, and connections.
- Subsurface drainage: Inspect drainage components before concealment, particularly where saturated soils or groundwater seepage are present.
Verify that unsuitable soils have been removed and the exposed subgrade is firm, stable, and consistent with the geotechnical recommendations. Evaluate soft or saturated areas before leveling pad installation. The leveling pad provides a level, stable surface for initial placement and alignment of MSE wall facing panels or blocks. It is typically constructed of compacted crushed aggregate. Inspection for standing water, seepage, erosion, and softening of the underlying foundation soils should be included.
Specifications typically call for the placement of backfill to be inspected and compacted to 95 percent of maximum dry density per ASTM D-698 (AASHTO T-99), placed in loose lifts of 8 to 10 inches. There are occasions where this testing is not done, which can result in undercompacted soil in the reinforced zone. Other factors include inadequate testing frequency, which can be attributed to part-time inspection during construction or not performing enough tests to properly characterize the compaction.
Inadequate inspection and testing during construction are significant contributing factors in wall failures.
Foundation Soils and Global Stability
The reinforced block can be flawless, and the wall can still rotate if what sits beneath it is soft. Under-wall foundation failures typically stem from three recurring subsurface problems: compressible soils, organic silts, uncontrolled historic fill, and solution features in soluble carbonate bedrock. In regions with karst terrain, sinkholes can develop beneath a wall with little to no warning, compromising the entire structure regardless of above-ground workmanship.
Global stability analyses are essential for evaluating the overall stability of an MSE retaining wall, including the reinforced soil mass, retained soils, and underlying foundation soils. Unlike internal and external stability evaluations, global stability analyses consider potential deep-seated failure surfaces that may extend beneath or behind the entire wall system.
Global stability analyses are frequently omitted from MSE wall design packages because the wall designer’s contractual scope is often limited to the internal and external stability of the reinforced soil mass, rather than the overall stability of the entire system. This is an established industry practice, but it can create a significant gap in engineering responsibility and lead to wall failure if the global stability evaluation is not performed. As a geotechnical engineering consulting and design firm, EEI performs the entire MSE wall design, including global stability.
Geotechnical Testing That Identifies Problems Before Construction
A geotechnical investigation for an MSE wall identifies potential issues that are important for the design of the wall. When conducting geotechnical engineering and testing, field testing is performed, and the soil strength parameters can be determined. Laboratory testing of retrieved samples can complement the field testing for estimating design parameters. Some of the methods consist of:
- Standard penetration testing (ASTM D1586) with split-spoon sampling
- Cone penetration testing (CPT).
- Field vane shear for undrained strength in soft clays and organic silts.
- Test pits to log existing conditions and retrieve samples for laboratory testing and identify fill, buried debris, and perched water that other methods mayunder-represent.
- Electrochemical screening for corrosivity.
- Piezometers where groundwater is encountered.
Warning Signs Worth Acting On
- Horizontal bulging
- Joint openings or fines washing out through panel gaps.
- Cracked or separated coping, and guardrail posts leaning outward.
- Standing water or persistent seepage at the wall face.
- Pavement cracking parallel to the top of the wall.
Any of these conditions warrant evaluation of the wall. Frequently Asked Questions
Can a Bulging MSE Wall Be Repaired Without Full Reconstruction?
Yes, in many cases. Soil nails or ground anchors plus drainage retrofits and toe buttressing can stabilize a wall that is moving slowly, provided monitoring data shows the movement is not accelerating toward a global failure.
Schedule a Geotechnical Evaluation of Your Wall
Earth Engineering performs subsurface investigations, in-situ testing, field density verification, and laboratory analysis with our own staff. Contact our team to discuss a new wall design, a construction-phase testing program, or an assessment of a structure that has already started to move.