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Why does interlock sink and heave in Cambridge? The base, the ground and the freeze-thaw cycle

Interlock in Cambridge sinks and heaves because of what is under the pavers, not the pavers themselves. Frost heave needs three things together, a fine-grained soil, a supply of water and a freezing plane, and a thin base over slow-draining subsoil gives it all three every winter. Settlement is the other cause: a base that was never compacted, or had its fines washed out, and simply dropped.

Last updated October 24, 2026 · Reviewed by the Cambridge Interlocking team

Why does interlock heave in winter?

The National Research Council of Canada's Canadian Building Digest on ground freezing and frost heaving sets out the conditions plainly. Heave needs a freezing plane in the soil, a fine-grained soil through which moisture can move, and a supply of water. When all three are present, water is drawn toward the freezing front and forms ice lenses that lift whatever sits above them. The digest notes that coarse sands and clean gravels do not heave, while fine sands and silts are very susceptible, and clays are also very susceptible, though they normally heave slowly and often with tremendous pressure.

A paver driveway built on a thin layer of sand over the graded subsoil of a 1990s or 2000s Preston or Hespeler subdivision is a textbook case. The subsoil is fine-grained and drains slowly, water collects in it under the pavers, and the frost reaches it every winter. The surface comes up in January and February, then settles unevenly as the ice lenses melt in spring. After fifteen or twenty cycles the result is the wave at the garage apron and the ridge along the edge that the contractor we book for Cambridge sees on most re-levelling calls.

Heave or settlement? How to tell which you have

The two look alike from the kitchen window and they are different problems. A driveway that is high in late winter and lower by June is heaving. One that is low all year, with the same dip in August as in February, has settled. Many Cambridge driveways do both: heave at the apron where water collects, settlement in the tire tracks and over a utility trench that was backfilled and never compacted.

• Settlement is downward and permanent. The base or the subgrade consolidated under load, often because it was placed in one thick lift and never compacted, or because water running through the joints washed the fines out of it. Pavers over a repaired gas or water service sink this way as the trench fill settles over a few years.
• Frost heave is upward in winter and partly back down in spring, but it rarely returns to level. Each cycle loosens the base and opens the joints a little more, so the surface ratchets out of shape over the years.

The repair for each starts underneath, but what goes back in differs. A heaving section needs fabric and drainage as much as it needs depth; a settled one mostly needs the base rebuilt and compacted properly. The repair and re-levelling page goes through both.

How thick should the base be under interlock in Cambridge?

The construction standard the trade works to is the Concrete Masonry and Hardscapes Association's technical bulletin PAV-TEC-002 on construction of interlocking concrete pavements. It calls for a minimum compacted base of 100 mm (4 in) for sidewalks, patios and pedestrian areas over well-drained soils, and at least 150 mm (6 in) for residential driveways over well-drained soils, and it says in so many words that in colder climates, continually wet or weak soils will require that bases be thicker. Cambridge is a colder climate and much of its subsoil is neither well drained nor strong, so those figures are floors, not targets. The same document specifies bedding sand screeded to a nominal 25 mm (1 in) uncompacted and never used to correct an uneven base, base compacted to at least 98 per cent Proctor density, and joints kept between 2 and 5 mm.

The bulletin also says when to consider a geotextile: where soil stays saturated part of the year, where freeze-thaw occurs, and over clay and moist silty subgrades. The fabric is a separation layer that stops fine subsoil migrating up into the stone base and turning it, over years, into something that holds water and heaves. In Cambridge that describes most lots.

Where the water goes decides whether it comes back

The NRC digest's practical advice is that controlling soil temperature is rarely economical, so the levers are soil type and water. Replacing fine soil with coarse granular material is how heave is usually prevented, and careful attention to drainage reduces it, though drainage alone rarely removes it entirely. For a driveway that means a base deep enough and open-graded enough that water passes through it rather than sitting in it, a separation fabric so the base stays clean, and somewhere for the water to go: a perforated drain under vehicle areas on clay, a trench drain at an apron that falls toward the garage, and a surface pitched so water leaves rather than pools. The City of Cambridge's by-law inquiries page adds the rule that yards must be graded to prevent ponding of storm water and that drainage cannot be directed at a neighbouring property or onto a walkway or sidewalk, which a badly pitched driveway or patio does.

The City's Property Standards By-law 26-021 also requires every parking area, driveway, walkway and step to be graded appropriately and kept in good repair, defined as free from holes and loose, broken or damaged materials. A badly heaved walkway is a trip hazard first and a by-law matter second.

Can it be fixed without redoing the whole driveway?

Often, yes. If the rest of the surface is sound and the cause is local, a washed-out apron, a settled utility trench or a failed edge restraint, that section is lifted, the base corrected and the same pavers relaid and blended in. What tips the job toward a full lift-and-relay is finding that the whole base is a few centimetres of sand over clay, because then the next dip is already forming a metre from the repair. The contractor we book for Cambridge tells you which you have once the first pavers are up. Either way the original pavers are usually reused.

What this means for your quote

Whether the job is a new driveway or a re-levelling, the quote should state the excavation depth, the base depth and material in millimetres, the compaction, whether geotextile and drainage are included, and what happens if the excavation finds something different from what was expected. A per-square-foot price with none of that behind it is not a lower price; it is a thinner base, and on Cambridge ground a thinner base is the fifteen-year-old wavy driveway this site exists to fix. The driveway installation page sets out what the contractor specifies as standard, the Cambridge interlocking cost guide explains what moves the price, and you can describe what your surface is doing here to arrange a site visit.

Sources

• Canadian Building Digest CBD-26, Ground Freezing and Frost Heaving - National Research Council Canada
• PAV-TEC-002-22, Construction of Interlocking Concrete Pavements - CMHA (formerly ICPI)
• Common by-law inquiries - City of Cambridge
• Property Standards By-law 26-021 - City of Cambridge