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The Case of the Recurring Foundation CracksBackgroundIn June 1994, an owner purchased a 30 year old house in the Toronto area. The house is a single-storey solid masonry home that is supported on a 240 mm concrete block basement foundation. The basement is heated, and the concrete block wall is uninsulated. At the time of the purchase, an horizontal gap was noticed between the outer wythe of the brick wall, and the concrete block foundation wall at the south west corner of the building. A small vertical crack was also observed on the exterior of the block foundation wall. This crack extended from the corner of an above grade window, down below grade to an undetermined depth. No cracking of the interior finished surface was noted at this time. The rest of the foundation wall exhibited no signs of cracking. By September of 1994, the horizontal gap and the crack were much larger. The gap was found to be 10 mm in width, approximately 100 mm long, and extended about 50 mm into the wall. (Photograph 1.) Cracking of the interior drywall finish was also observed. In November, a drainage subcontractor excavated along 3 metres of the foundation wall to the drain level. The subcontractor reported that there was no foundation drain at this location; therefore, a perimeter drain was installed. There were no signs of entry of bulk water into the basement before or after the drain was installed. The cracks in the foundation wall, which extended from the window to the footing level approximately 1.4 m below grade, were grouted and the excavation was backfilled. (Photograph 2) The interior drywall was not repaired so that the home-owner could monitor further cracking. In April 1995, no further cracking of the interior drywall was observed. No further cracking of the exterior of the foundation wall was observed. (Photograph 3: Note the settlement of the soil in the vicinity of the wall and the resulting uneven surface of the paving stones). Furthermore, the horizontal gap between the outer wythe brick and the foundation wall no longer existed. File closed? Not quite. In August 1995, the home-owner again reported more cracking; an even larger gap, about 12 mm wide and 250 mm long, had opened up between the brick and the block foundation. While the epoxy-grouted cracks in the block wall remained unchanged, a new crack had appeared between the two grouted cracks shown in Photograph 3. As well, the interior drywall was now more extensively cracked. Cracks extended along the basement ceiling where the ceiling intersects the west and south walls. What was going on? The drain subcontractor volunteered to come back, and excavate the area again. The drain was found to be installed properly. The soil was found to be a clay with some silt, of glacial till origin. Displaced fence posts and the tilting of unheated slab-on grade buildings in the area indicate that the soil is likely frost susceptible. The sides of the excavation stood vertically for a few days. Upon excavation, a closed step-crack which followed the mortar joints was observed in the block wall. This crack ran from the centre of the window diagonally towards the south-west corner of the building as depicted in Figure 1. |