Chipmunks Have Tunnelled Under My Patio and Retaining Wall

Yes, and it is the reason to take a small population seriously. Chipmunk burrow systems are extensive — many feet of tunnel with multiple entrances and chambers — and when that network runs under a slab, a stoop, a patio or behind a retaining wall, it removes the compacted material those structures bear on. Over a few seasons you get settling, cracking, tilting pavers and, behind retaining walls, bulging where the backfill has been undermined and water now channels through. A single animal rarely matters. An established colony does.

The entrance is the tell. Chipmunk holes are clean, round, about two inches across, and — crucially — have no mound of excavated soil around them, because they carry the spoil away in their cheek pouches to keep the entrance inconspicuous. Ground squirrel and rat burrows usually leave a spoil pile. Mole and vole work is different again: moles push up ridges and conical mounds, voles run at grass level with no digging. A clean two-inch hole with no dirt around it, next to a hard structure, is a chipmunk.

Usually more than you see at once. Chipmunks are territorial and largely solitary, so a burrow system typically holds one adult, but a favorable site supports several adjacent systems — and a patio, retaining wall or rock landscaping offers a lot of adjacent real estate. People commonly report seeing one and then, once they start watching, realize there are five or six working different entrances. Counting entrance holes gives a better estimate than counting sightings.

Cover, food and easy digging, in that combination. Rock landscaping, retaining walls, stacked stone, woodpiles and dense foundation plantings all give protected cover with ready-made voids. Bird feeders are a major draw — spilled seed under a feeder will support a chipmunk population indefinitely, and it is by far the most common attractant we find. Add loose backfill behind a wall or under a patio, which is far easier to tunnel than undisturbed ground, and the site is close to ideal.

Occasionally, and usually by accident rather than intent — chipmunks are not looking to live indoors the way mice are. Where they do cause building problems is by burrowing along foundations, getting into crawlspaces through gaps, and occasionally into basements or garages. The more common indoor consequence is a chipmunk that has got in and cannot get out, which is a different and more frantic problem. Sealing ground-level gaps handles both.

They go dormant rather than truly hibernating — chipmunks spend the winter in the burrow living off cached food, waking periodically to feed. So the visible activity stops and people assume the problem has resolved, but the animals and the burrow network are still there and both return in spring. Winter is actually a reasonable time to do barrier work on a structure, since you are not disturbing active young, though the ground being frozen has practical limits.

Flooding a burrow under a patio or behind a retaining wall is a genuinely bad idea — you are introducing water into exactly the backfill whose stability you are trying to preserve, and washing out fines makes the settling worse. Repellents perform poorly against an animal with an established burrow and a food source, and they need constant reapplication. Neither addresses the tunnel network, which remains as a void whether or not the animal is in it.

A physical barrier plus removing what drew them. Hardware cloth buried along the edge of the structure and turned outward at the base stops re-tunnelling at the vulnerable line. Then take away the food — move or remove bird feeders, or fit catch trays and clean up spilled seed — and reduce the cover by clearing woodpiles and thinning dense plantings against the wall. Removal without the barrier and the food changes means a new chipmunk takes the vacant system within a season.

Sometimes. If there is visible settling, cracking or a wall beginning to bulge, the voids themselves need addressing as well as the animal — that is a landscaping or masonry job rather than a pest one, and we will say so plainly. Where it is caught early and the structure is sound, sealing off re-entry and letting the ground consolidate is usually enough. The point of dealing with it early is precisely to stay in the second category.

When you count more than a couple of entrance holes near a hard structure, or when you see any settling, tilting pavers or movement in a retaining wall. Also worth calling before you build — if you are putting in a patio, stoop or wall on a property that already has chipmunks, incorporating a barrier during construction costs very little and saves the whole problem. Spring and autumn are the practical windows for the work.