Curt Peterson 2013-08-23 00:21:46
Soil Movement – The Fix As a forensic civil/geotechnical engineer, I come across many buildings that have experienced significant damages because the soil beneath the structure has moved. There are two primary mechanisms of soil movement. The first mechanism is settlement which can be due to compressible soils, collapsible soils, and/or improperly compacted fill soils. The second mechanism is from expansive soils, which are clayey soils that can expand when wet and shrink when dry. I get numerous calls each month from homeowners or building owners who are experiencing cracking in their buildings related to soil movement. Below are some of the most common repair strategies utilized in Arizona for soil related problems that I encounter. Regrade – The most common contributing factor behind soil movement is poor drainage. In fact, of all of the homes I have evaluated during the past 17 years (which includes thousands of homes) poor drainage is the triggering mechanism at the overwhelming majority of those homes. I often see where landscaping improvements such as curbs, mounds, flatwork, swimming pools, etc. change the original drainage patterns and create poor drainage conditions. These poor drainage conditions prevent rain runoff from exiting the lot and allow the moisture to soak into the soil beneath the foundation. If the soils are expansive, this increased moisture can trigger expansion; if the soils are collapsible, this increased moisture can cause settlement. The repair for poor drainage is to remove or alter the obstacles that are blocking drainage and regrade the soil to direct the water away from the structure and off the property. If improvements prohibit regrading, then a drainage system including below grade pipes and surface collection boxes can be installed. Once these drainage improvements are performed, the moisture will stop migrating beneath the foundation of the home and the cracking on the inside of the home can be cosmetically repaired. Piles – When the soil beneath a foundation has undergone significant settlement and there is a potential for additional soil movement, the use of piles is a potential repair strategy. The principal behind piles is to transfer the building loads from the near-surface problematic soils to deeper, more competent soils. Helical piles are a common type of pile used in Arizona foundation repairs. Helical piles are similar to a corkscrew, are metal, approximately 3 inches in diameter, have a sharpened tip and auger, which is drilled into the soil. A special “L” bracket is attached to the top of the pile and placed under the foundation of the structure. The weight of the structure is transferred to the piles and the structure can be re-leveled and lifted back into a condition comparable to the way it was before soil settlement occurred. I have personally observed structures that were 9 inches out of level, raised 7 inches using piers. In Arizona, the typical pier depth is about 20 feet; however, on a recent project in Gilbert, the piers had to extend to a depth of 75 feet. Grouting – In some cases soil settlement also various types of expandable foam that can also be used in a similar manner. Cutoff Wall – There are instances where surface water cannot be effectively controlled to prevent moisture from migrating beneath the foundation. One recent example was a site where clean sands were encountered at the ground surface down to about 3 feet. At 3 feet, a highly expansive soil was present. Surface moisture would quickly seep through the sands, migrate beneath the foundation, and trigger expansion of the clays below. To prevent this from occurring, I recommended a membrane cutoff wall. A cutoff wall is a vertical moisture barrier placed in the soil that isolates the expansive soil from the moisture source. Cutoff walls are typically installed by excavating a 4-5 feet deep vertical trench adjacent to the foundation of the building and placing a water proof membrane in the trench and backfilling with soil or slurry. In conclusion, if soil movement has occurred beneath a home or structure, there are various ways to address the movement. As discussed above, the most common and effective method is to correct drainage. At times, more aggressive repairs have to be considered. If you have a project with soil related issues, give my office a call to discuss in greater detail. Curt Peterson is the founding engineer of Peterson Geotechnical Group, a forensic geotechnical engineering consulting firm. He holds a bachelor’s and master’s degree in civil and geotechnical engineering and has observed thousands of structures throughout his career. As principal engineer and with over 16 years’ experience in the field of engineering each project is given the up most attention with focus on accurate analysis and meeting the client’s deadlines. The firm provides expert services including deposition and trial testimony and mediation attendance on construction defect lawsuits. For more information call (480)-219-0014 or visit the website, www.pgg-eng.com
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