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Voids under concrete slabs occur when the soil under the gravel base settles or contracts in size. This contraction can be caused by moisture under the slab, vibration, heavy loads, or poor compaction at the time of construction. One remedy is to remove the concrete by jackhammering and re-compact the soil, add to the gravel base, repour, and refinish the concrete.
A less expensive method of repair is pressure grouting. This method involves drilling through the concrete and pumping a cement-based grout under pressure to fill the void. If necessary, it might also require lifting the slab back to its original level or as close to the original level as possible without doing further damage to the concrete slab. This method normally costs between 1/3 to 2/3 the cost of a conventional replacement without the added noise and dust.
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Once holes are drilled through the slab, injection pipes are driven through the voids and soft soils to residual soils. The grout hose is then connected to the pipe to inject materials. | |
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As grout is pumped through the pipe, it permeates the soft soil and fills the void to stabilize the slab. As pressure starts to rise, the pipe is lifted while the grout strengthens the surrounding soil. |
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When completed, the slab is stabilized and its load-bearing capability is increased. |  |
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Settling towards the middle (cracks wider at base) |
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Settling towards the outside (cracks wider near top) |
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Wall or footing settling (staircase or vertical cracks) |
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Wall bowing (horizontal cracks) |
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Slab settling (uneven crack-bowing) |
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