Guideline & Diagrams for How an Engineer Determines if a Foundation Repair is Required

Guideline & Diagrams for How an Engineer Determines if a Foundation Repair is Required

Note: this is just a guideline and there is no way to know if your house needs a foundation repair unless we perform an on site evaluation to determine the unique conditions present. Call us for an appointment.

Assessments of Suspected Deficiencies

Cracks in walls, sloping floors, doors out of alignment, are common signs of differential settlement of the foundation. In general, differential settlement occurs when soil under a footing compresses. The cause of soil compression could be either a) moistening of the subgrade soil by water infiltration, b) drying of clay-like soils, c) crumbling of decomposing organics in a subterranean layer, d) inadequate or non-uniform compaction of the soil prior to construction of the foundation, e) additional loading applied to an existing foundation through structural additions/expansions/modifications, or f) hillside slope creep due to an adjacent slope slowly shifting.

• Inadequate drainage of surface water away from the home often contributes to subgrade soil saturation. Inadequate drainage occurs when the topography is flat, topography slopes toward the house, irrigation volume is unnecessarily high, torrential rains create ponding conditions, water has no escape route (bathtub scenario), or drains are blocked. The adjacent yards/planter areas are flat or drains towards the house. For this particular house, water may have saturated the soil adjacent to this foundation due to inadequate surface drainage during irrigation, past torrential rains, or an interior water leak. Proper drainage is important to keep water away from perimeter foundations to prevent foundation movement. Water percolates through the top soil to saturate the subgrade soil under the foundation. In such conditions, the soil under the foundation becomes more compressible.
  See Figure 2 below for additional information.
  
• Clayey soils have a high expansion and contraction rate. Upon dying, these soils shrink which leads to differential settlement. Upon moistening, these soils expand. In either case, differential settlement will occur.
  

Figure 1: Clay Soil Expansion-Contraction Spectrum

• If organic material (roots, fossils, etc.) decomposes, the surrounding soil will collapse into the previous volume. It is difficult to ascertain the probability that this has occurred.
  
• If the soil was not compacted properly or uniformly prior to construction of the original house footings, short term (2 weeks) and long term (5 years) settlement is expected when the load of the house is applied to the soil. It is not possible to assess compaction under the house, once the house is constructed above. However, most likely, the soil settlement the house due to short and long term settlement of the house was uniform and equally distributed such that differential settlement did not occur within the 1^st 5 years. Most likely, non-uniform compaction was not a contributing cause to differential settlement at the original house.
  

Figure 2: Settlement Over Time

• If your house has had structural additions to the back as noted above, most likely, the soils under the addition’s foundations were not properly compacted and differential settlement occurred as noted in cause “d” (above). Inadequate compaction in this area also may have led to the further vulnerability of future differential settlement as noted in cause “a” and “b” (above).
  
• A hillside condition may occur at your property. This may be a cause of settlement.
  

The impact to the exterior of a wall from local differential settlement along the face of a wall can be seen in the Figure 3 below.

Figure 3: Expansion-Contraction Impact to House Walls

There is moderate levels of settlement at the living room. Accordingly, there may have been a moderate impact to the structural system. The downward settlement of the foundation caused the backyard north-western stud wall to be pulled downward. A similar impact to the house’s structural system from local differential settlement can be seen in the Figure 4 below.

Figure 4: Expansion-Contraction Impact to House Foundations and Framing

As the stud wall moves downward the diagonally sloping roof rafters would have had to accommodate the vertical movement of the wall. In this scenario, the rafters are pulled outward away from the ridge beam. When this occurs the toe-nails connecting the rafter to the ridge board are experiencing tension with the outward movement of the rafter. As the nails are withdrawn, their capacity to resist gravity and seismic forces are significantly diminished. Accordingly, the structural system is weakened by the loss of strength of the connections of the roof rafters.

As noted below in Figure 5, with the observations above, there is moderate stress levels in the structural system. Even with moderate foundation settlement, the structure is vulnerable to major structural damage due to gravity forces, seismic forces, and/or continued settlement. Accordingly, we are recommending stabilization of the foundations.

Figure 5: Settlement Stress Spectrum

Note: This is for conventionally framed house attics/roofs. For pre-fabricated truss framed attics/roofs, add 1/4″ to each of the settlement numbers for an accurate reading of the spectrum.

Conclusions

Significant displacement of localized structural systems may have occurred due to foundation settlement caused by the saturation of the subgrade expansive soil from poor drainage. A Foundation Repair is required.