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 1st 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

 

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.

 

How to do a Foundation Repair by Foundation Engineering of NorCal

Due to the magnitude of the differential settlement and the vulnerability to future settlement, stabilization and lifting are required as follows: Install helical piles or push piers for the underpinning and lifting of the foundation in the moderate to severe areas. Typically, the helical piles or push piers cost around $2,000 each for installation.

 

Only licensed foundation stabilization contractors who specialize in this work should be contacted for construction.

 

  • Costs for this solution will be approximately:
    • $8,000 helical piles or push piers (if 4 are required)
    • $2,000 exterior concrete removal and replacement
    • $1,500 plan check & permits
    • $2,000 structural engineering
    • $2,000 geotechnical engineering
    • $1,500 inspection
    • $2,000 stucco & drywall repair after lift
    • Total Costs: $19,000
    • Please note, these are rough order of magnitude estimated costs. Actual costs may vary.

 

 

Special care should be taken to address the drainage issues along the exterior of the home that do not have hardscape (such as the areas with pavers). New concrete slabs should be installed within 5ft of the structure around the entire structure’s perimeter. The new concrete slab to house interface should be properly sealed to prevent water seeping through. New slabs should be sloped away from the structure at 3/8 inch per 12 inch. Linear trench drains and drain lines should be installed at the new slab to soil interface to move collected water off the new slab into the drain. A licensed landscape contractor or licensed landscape architect may be consulted for final design recommendations. Additional roof gutter installation, roof gutter assessment/maintenance, and downspout subterranean drain lines assessment/maintenance, around the perimeter is necessary to avoid large sheet flow water off the roof in proximity of the foundation. Surface topography should be re-graded so as to ensure surface runoff flows away from each side of the house. A new drip irrigation line should be installed under the slab to keep the current moisture content in the soil consistent throughout the seasons.

 

After the underpinning and lifting is achieved, exterior stucco cracks may be locally repaired by a qualified handyman or licensed contractor specializing in this work. At the exterior repairs to cracked stucco, use either an injected epoxy compound to help prevent water intrusion or remove/replace stucco 6 inches beyond the crack to ensure material continuity (the same welded-wire mesh as existing should be used in the replacement material).

 

After the underpinning and lifting is achieved, at the roof framing ridge, if rafter members have separated beyond ¼inch from the ridge board, a Simpson Strong Tie connection (such as a LRU26Z, LSU26, THA213, etc.) should be installed to ensure framing member continuity in a gravity or seismic event. Contractor shall take special precautions to ensure the Simpson Strong Tie connection is properly connected so additional pulling of the roof rafter away from the ridge does not occur. Additional strengthening shall include a 2×6 horizontal member to connect at mid-height of the rafters as shown in the sketch below on each side of the roof ridge board.

Example signs of moderate to severe settlement for Foundation Engineering of NorCal

Example signs of moderate to severe settlement

 

  • The floors around the perimeter of the living room addition slope downward to the perimeter north-west (towards the backyard). Max slopes in these rooms are at 7/8inch vertical in 8ft horizontal.
    • Around the other regions of the house, the downward slopes are considered minor.

 

Sketch #1: Floor Elevation Survey by Monometer

 

 

  • Exterior stucco cracking occurred at various locations along the windows and doors along the backyard wall.

Photo 2A & 2B: Side yard exterior wall stucco cracking

 

 

Photo 3A & 3B: Front yard exterior wall stucco cracking

 

Photo 4A & 4B: Side yard exterior door stucco cracking

 

Photo 5: Side yard exterior stucco bulging/cracking

 

Photo 6A & 6B: Side yard exterior stucco cracking at addition interface

Corrosion Protection For Foundation Repair with Helical Piles or Push Piers

The following are notes to myself that may be of interest to contractors.

 

Sections

 

  1. 6 Reasons for Corrosion Protection

 

  1. How much Corrosion Protection should be provided and what options are available when site specific conditions are unavailable?

     

 

 

 

 

 

  1. 6 Reasons for Corrosion Protection

 

There are 6 reasons why corrosion protection must be provided on all projects (unless this firm receives an environmental or geotechnical engineering statement that corrosion protection may be relaxed and the building official approves the statement):

 

#1: The 2016 California Building Code requires it for any site conditions with “possible” deleterious action.

 

Because a Civil/Structural Engineers design is subjected to the building department having jurisdiction, there may be other local requirements in local codes. However, we can reasonably check the CBC for general requirements throughout the state. The following excerpt is from the 2016 California Building Code:

 

1810.3.2.5 Protection of materials

“Where boring records or site conditions indicate possible deleterious action on the materials used in deep foundation elements because of soil constituents, changing water levels or other factors, the elements shall be adequately protected by materials, methods or processes approved by the building official. Protective materials shall be applied to the elements so as not to be rendered ineffective by installation. The effectiveness of such protective measures for the particular purpose shall have been thoroughly established by satisfactory service records or other evidence.”

 

The City of Los Angeles Research Report #26163 for SafeBase Push Piers requires corrosion protection in its product approval conditions. It states:

 

“Corrosion resistance and longevity of the foundation support system shall be addressed by the registered design professional on a job specific basis.”

 

#2: The Civil/Structural Engineering practice’s “Standard Level of Care” reflects an appropriate concern and requirement for the longevity of materials used in design.

 

Civil/Structural Engineers shall perform their services consistent with the professional skill and care ordinarily provided by engineers practicing in the same or similar locality under the same or similar circumstances. This “Standard Level of Care” is what a jury of peers would reasonably do in a similar situation. That is, amongst most other civil/structural engineers, would they also require corrosion protection in the absence of information of the soil conditions? The answer from our perspective is yes because engineers error on the side of conservativism for public safety.

 

 

#3: Despite the contractor’s lifetime (or less) warranty on the materials, a Civil/Structural Engineer is required to fulfill his/her professional obligations to the building occupants/owners.

 

The Civil/Structural Engineer is hired as a consultant to ensure the appropriateness of the design to perform its intended function over time. Because a contractor and/or material supplier may no longer provide services of any kind at any future time, the specified material should be adequately protected against corrosion.

 

 

#4: Civil/Structural Engineer’s professional liability insurance does not cover gross or willful negligence.

 

A building owner may hire an independent Civil/Structural Engineering consultant to perform a peer review of the engineering design immediately after the design is completed (prior to construction). The peer reviewer may notice that the material specification does not include corrosion protection when deleterious actions may be possible in unknown soil conditions. The project’s Civil/Structural Engineer would then be found to be grossly or willfully negligent in not specifying corrosion protection. Because professional liability insurance does not cover gross or willful negligence, the Civil/Structural Engineer’s personal assets would then be exposed to forfeiture in a lawsuit.

 

 

#5: The underpinning industry’s best practices is to use steel that has corrosion protection.

 

Most manufacturers of helical piles and push piers provide steel that has corrosion protection as a standard on every project. To deviate from the norm, would be unwise and creates bad optics in a courtroom.

 

 

#6: Material that detaches from the push pier or bracket during corrosion can enter subterranean aquifers causing harmful chemicals to drinking water.

 

Corrosion protection will greatly limit the potential for continuous deterioration of the steel during corrosion. Without protection, steel contaminants may pass through the soil (vertically or horizontally) in the water movement through underground strata layers.

 

 

For all of the above reasons, corrosion protection must be provided on all projects that do not have an environmental or geotechnical engineering statement that corrosion protection may be relaxed and the building official approves the statement.

 

 

 

  1. How much Corrosion Protection should be provided and what options are available when site specific conditions are unavailable?

 

In Table 1 of the City of Los Angeles Research Report #26163, the following footnotes are provided:



As stated in the footnotes, the engineer must determine “actual” capacity and “expected corrosion loss” (footnote 1) and “actual corrosion loss” (footnote 3) from site specific conditions.

 

Because the 0.036inch lost thickness is an estimated calculated value from a formula based on certain assumed site conditions, it was provided by the authors as a “for reference only” value. Typically, “for reference only” implies that it may or may not be appropriate for use on any actual project, unless the project’s soil conditions falls within the same criteria used in the calculation of that value.

 

The question then arises what did Section 3.9 of ICC AC358 assume and does the assumed cover all of the best and worst possible sites? Above ground or underground? Disturbed soils and undisturbed soils?

 

The ICC AC358 provides this equation for bare steel thickness lost with respect to time:


where t = 50 years for the calculated result of 0.036inches

ICC AC358 does not reference the research that this equation is borrowed from. It turns out that ICC AC358 has retrieved this equation from another published source (Federal Highway Administration NHI-00-044, 2000 based on National Bureau of Standards “Circular 579” authored by Romanoff in 1957) for disturbed soils. This document developed the above equation to estimate loss on after removing various steel materials from various disturbed soils, soils with varying degrees of corrosion situations from mild to severe.

 

The disturbed soils region for retrofit underpinning construction against concrete footings is usually within the upper 3ft to 5ft of soil where oxygen is trapped during soil excavation and replacement. This would effectively be the region of the bracket and sleeve which are subjected to ongoing corrosion due to the presence of the trapped oxygen in the replaced soil. {For the steel shafts that are embedded below this upper disturbed region, the corrosion potential is negligible (as concluded in a later study) as there would not be trapped oxygen.} Thus, the engineer needs to take into account corrosion in the upper disturbed regions.

 

ICC AC358 Section 1.2.2 gives the limitations of the soil’s severity for which the equation is applicable. Also, ICC AC358 Section 1.2.2 states the types of severe soil conditions that are not acceptable for installation:

 

1.2.2 …corrosion situations as defined by the following:

(1) soil resistivity less than 1,000ohm-cm;

(2) soil pH less than 5.5;

(3) soils with high organic content;

(4) soil sulfate concentrations greater than 1,000ppm;

(5) soils located in landfills, or

(6) soil containing mine waste.

 

The following table is provided for reference as an example from an actual underpinning retrofit project’s soils report that concluded the soil was very corrosive. It shows the descriptions for the corrosion parameter’s thresholds:


In the above example, the soils conditions are considered highly severe.

 

The ICC AC358 equation for estimating loss in thickness is based on an empirical study that has typical site conditions (mild to severe soils). So, if an actual job site conditions are typical and within the ICC AC358 section 1.2.2 limits, the calculated estimated loss in thickness (sacrificial thickness) is a valid method for designing for corrosive conditions across 50year life of the structure.

 

Near the end of Section 3.9 of ICC AC358, the following was provided:

 

“Corrosion loss shall be accounted for regardless of whether devices are below or above ground or embedded in concrete.”

 

All of this engineering firms designs uses the reduced thickness value for all projects in the upper disturbed regions and lower undisturbed regions.

 

However, the appropriateness of unprotected steel is still in question as there may be atypical highly severe site conditions, such as that indicated in the example above, that do not fall within the limits of ICC AC358 section 1.2.2 and thus the equation for estimating loss in thickness is not applicable for the undisturbed soil region. It seems that a greater loss is to be expected upon atypical highly severe soil conditions.

 

Section 6.8 requires that a site specific study must be conducted to determine corrosive conditions:

6.8 …a site-specific foundation and soils investigation report is required for proper application of these products. The foundation and soils investigation report shall address corrosive properties of the soil to ensure that a potential pile corrosion situation does not exist.

 

Section 6.1 reiterates the evaluation reports must include the aforementioned limits from 1.2.2:

6.1 The device or system shall not be used in conditions that are indicative of a potential pile corrosion situation as defined by soil resistivity less than 1,000 ohm-cm, pH less than 5.5, soils with high organic content, sulfate concentrations greater than 1,000 ppm, landfills, or mine waste.

 

The ICC AC358, specifically states that any soil condition beyond these limits should not use steel piles, even piles that are powder or zinc coated. This is not the code, but it further highlights the need to know if there are atypical highly severe soil conditions.

 

For some projects, the building owners elects to not engage a geotechnical engineer to perform a soils study. In those cases, we have no site specific information on corrosion situations.

 

Therefore, in the absence of site specific information, to be in compliance with the 2016 California Building Code’s 1810.3.2.5 corrosion protection requirement, this firm accepts either of the following approaches:

 

  1. Design for all conditions, from mild to severe to highly severe (assuming there are atypical highly severe soil conditions outside the limits of ICC AC358 section 1.2.2 that would create deleterious actions so that we can be in compliance with the code in the case that there are, upon later discovery, soil conditions that would create deleterious actions). One might reasonably use 2 or more of the following common protective measures in combination,, to limit the effects of highly severe conditions:
    1. Sacrificial thickness loss in steel material (already used on every project’s design)
    2. Sacrificial anodes / cathodic protection
    3. Galvanization
    4. Powder coating
    5. Bonded coating
    6. Polyethylene encasement

     

  2. Instead of replacing the soil in the upper 3ft to 5ft of excavated soil, specify the contractor pour a non-air entrained concrete around the bracket and sleeve so as to surround bracket and sleeve from future oxygen.

 

  1. Require a site specific soils corrosion test to determine if the soil conditions fall within the parameters of ICC AC358 Section 1.2.2. If this is the case, the common protective measures listed above may be used individually if the soil condition are within the limits of ICC AC358 Section 1.2.2.

     

  2. Review soils reports for properties in the immediate vicinity of the subject site to determine if any nearby sites have highly severe corrosive conditions.

     

  3. Prohibit the use of any steel, protected or not, to be in compliance with ICC AC358 Section 6.1 and 6.8.