Why Is My Yard Sinking Behind My Seawall?

A low spot that keeps coming back, a crack opening in the patio near the cap, or a sudden hole in the lawn behind the wall: these are some of the most common reasons waterfront owners call for help. The ground behind a seawall is supposed to stay put. When it sinks, the usual explanation is that soil is leaving, and the path it takes is almost always through or under the wall.

This article explains how that happens, what you can safely do in the first days, and what to avoid. It cannot tell you whether your wall is stable. That requires an on-site evaluation by a marine contractor or licensed professional engineer.

Soil is leaving, and water is carrying it

The soil behind a wall, called backfill, is constantly wetted and drained by rain, irrigation, groundwater and tides. Water moving out toward the open water will carry fine particles with it wherever it finds an opening larger than those particles. Over time that removes material from behind the wall and leaves voids.

A well-built wall is designed so water can leave but soil cannot. Weep holes, joints and drains let water out. Filter fabric or graded stone behind them holds the soil back. Sinking ground usually means one of those barriers has failed somewhere.

DiagramSimplified illustration — not an engineering drawing

How soil escapes from behind a wall

Through separated joints and cracks

Concrete panel seawalls are built from slabs that are not designed to be watertight. Key Colony Beach's seawall manual describes how age, settling, structural movement or an insufficient berm at the toe can make slabs move apart. Backfill then migrates through the openings into the water while salt water enters behind the wall. The manual notes that the uneven water pressure this creates can be critical during heavy rain and low tide, and that backfill in the water at the joints is most visible at low tide.

Cracks do the same thing on a smaller scale. New York Sea Grant notes that soil can leak through holes, cracks and seams that widen as an aging structure settles.

Through failed or missing filter fabric

Filter fabric is a geotextile placed behind joints, weep holes or the whole wall face. The U.S. Army Corps of Engineers' property owner's guide calls filtering one of the most important design details, and probably the most neglected. It explains that a properly designed filter lets water pass while soil particles bridge across the openings and stop moving. Without a filter, fine soil washes out through the gaps, a process engineers call piping.

Fabric can fail in several ways described in that guide:

• Tearing. When panels or stones shift, the fabric behind them can rip, letting retained soil escape.
• Sunlight. Untreated synthetic fabric exposed to ultraviolet light can deteriorate in less than one season, which is why fabric should be UV-stabilized even where it sits behind a structure.
• Never installed. New York Sea Grant notes that some older structures were built with no filter material at all.

Through weep holes without filters

Weep holes relieve water pressure, but they are also openings. New York Sea Grant advises that weep holes should always have filter material at their landward end to prevent soil from being lost through them. A weep hole discharging muddy water or leaving a fan of sediment below it is a sign worth noting.

Beneath the toe

If waves, currents or boat wakes scour away the berm that supports the bottom of the wall, soil can escape underneath it. New York Sea Grant lists spaces or hollows forming beneath a structure as a sign of this. Toe loss usually comes with other symptoms, such as the wall tilting outward or the cap rotating. See signs of bulkhead failure.

From above: overtopping and runoff

Not all soil loss is through the wall. New York Sea Grant notes that hollows forming just behind a structure, with ground that is always wet or littered with debris after heavy waves, can mean the wall is being overtopped. Concentrated runoff from downspouts or irrigation aimed at the wall can also wash soil toward the openings. Key Colony Beach recommends adjusting sprinklers near seawalls and redirecting yard and roof drainage away from the wall.

Why the surface can be misleading

Voids usually start below grade, near the opening where soil is escaping. Turf roots, pavers and a crust of compacted soil can bridge over a growing cavity for a while. The first visible sign may be a small hole or a slight dip that is much smaller than the void beneath it.

That is why a hole behind a wall deserves more caution than a hole in the middle of a yard. It is also why repeated refilling is a warning in itself. If you have filled the same spot more than once, soil is still leaving.

Other causes that can look similar

Not every depression near a wall comes from soil escaping through it. Ask a professional to consider:

• Tieback or anchor problems. Key Colony Beach lists backfill settlement, together with a deteriorating cap and wavy or sagging panels, among the symptoms of tie rod failure.
• Settlement of fill. Loose or poorly compacted fill, or buried organic material that decays, can settle on its own.
• Leaking pipes. Broken irrigation, drainage or utility lines can wash out soil locally.

What to do right away

1. Keep loads away. Keep vehicles, mowers, equipment, stockpiled materials and foot traffic away from the area and from the wall edge nearby.
2. Mark and protect the area. Use stakes and tape or a temporary barrier so no one steps into it, especially children and visitors.
3. Document it. Photograph the hole with a tape measure for scale. Record its width, depth and distance from the wall, and the date and tide stage.
4. Look at the water side at low tide, if safe. From a dock or boat, check the face of the wall opposite the hole for open joints, cracks, weep holes leaking soil, or a scoured toe. Photograph what you find.
5. Reduce water flowing toward it. Turn off nearby irrigation and redirect downspouts away from the wall.
6. Call a professional. Contact a marine contractor, and a licensed professional engineer if the wall is moving or the hole is large or growing. Share your photos and measurements.

What not to do

• Do not keep refilling it with soil. New York Sea Grant's guidance is to determine and correct the cause first, then refill voids with clean fill. Soil added without a repair usually follows the same path out.
• Do not seal joints or weep holes with concrete or solid material. The Corps guide warns that without water passages, high water pressure can develop behind a structure and topple or displace it. Repairs need to pass water and hold soil.
• Do not drill new weep holes. New York Sea Grant cautions owners against drilling a wall themselves, because improper drilling can weaken it.
• Do not probe or dig near the wall. Digging can remove support and damage buried tie rods or fabric.
• Do not flush the hole with a hose. Adding water speeds up the washout.

How professionals typically address it

The right repair depends on the cause, the wall type and its overall condition. Published guidance describes several common approaches:

These descriptions come from Key Colony Beach's seawall manual and New York Sea Grant's maintenance guidance. Some of the work, such as excavating behind the wall, placing material in the water or structural repair, commonly requires permits and engineered plans. Our guides to soil stabilization behind seawalls and seawall drainage go further.

If you are not sure how serious your situation is, the bulkhead health check walks through what you have observed and helps you prepare for a conversation with a professional.