Expansive+Soils+Failures+Overview

Expansive Soils Failures Overview //Devon Saunders, PSU MAE/BAE Structural 2013 // toc
 * Multiple Locations **

Introduction
Expansive soil failures exhibit damage ranging from minimal cracking in a slab to large cracking of a monolithic wall from floor to ceiling. Regions with hot and dry to cold and rainy seasons that have a high content of sandy clay to clay like soils are susceptible to this type of damage. Multiple locations and cases ranging from light structures at Ghazi University in Ankara, several residential homes in Sudan, a courtyard house in Australia, and an elementary school in western Pennsylvania have demonstrated failures due to of these types of soils. These particular occurrences used investigative techniques, sampling, and documentation for the range of failures to come to the conclusion that all are primarily due to expansive soils.

Expansive Soil Sampling, Testing, and Properties
Soils that exhibit swelling when introduced to water will fall into one of five categories as introduced in "Investigating Field Behavior of Expansive Clay Soils". Sampled soils may be graded depending on their material content, suction compression index, and change in surface heave. These classifications and values when put into simplified terms explain an expansive soil based on its material composition and the amount of water it is able to absorb or react with. Landscapes found in arid and semi-arid regions where drying and wetting are common are the types of territories where expansive soils are likely to be found.

There are certain techniques that should be followed when performing an analysis for expansive soils on site. Whether it be for pre-construction purposes or to determine the causes of failure; the sampling, data collection, and testing of the soil can prevent future collapses. ASTM D 5298, the filter paper method, outlines the finding of values from sampled soil for use in calculating the soil heave. Sampled soil can be tested for change in sunction(D h) and change in water content(D w). These values can then be used to find the suction compression index as noted by the equation C h = -10*(D h/D w) -2 (Gordon, 2001). Using assumptions for the lateral restraint factor (f) and the coefficient for heave reduction due to load (s) in addition to the measured soil thickness ( D t), predicition of the soil heave(D H) can be calculated as D H=C h* D h*D t*f*s (Gordon, 2001). Table 1 provides an example for soil heave found from samples taken from a tested site as defined in "Investigating Field Behavior of Expansive Clay Soils". The supplied data accounts for vertical movement so it shall be noted that 3D expansion need also to be considered when designing or analyzing foundations.

Types of Failures
A failure due to expansive soils should be considered as a cause when a structure exhibits slab cracking that may have propagated to adjacent walls. Cracking in asphalt as demonstrated by Figure 1 can be sorted into six types of failures, longitudinal, transverse, block, yield, spot ridge, and green zone cracks (Dafalla 2001). The same sort of cracking appears in building applications, but will usually fall into the categories of longitudinal, transverse, and block cracking. Soils that display cyclic heaving and shrinkage will ultimately cause slab and wall cracking.

Expansive Soils cause failures in two phases: swelling (expansion) and shrinkage (contraction). Longitudinal and Transverse cracking occurs when a subsurface flow of moisture travels in one of the two directions and causes vertical road/slab movement. Block cracking occurs in roads and slabs when a specific patch is introduced to moisture changes. This can also happen in below-grade foundation walls if a flow of water is constricted between soil layers. Yield, Spot Ridge, and Green Zone cracks are more specifically for asphalt applications. Yield and Spot Ridge are caused by cyclic exposure to water while Green Zone cracking is a result of moisture runoff from green spaces.

Investigation into the cause of damages or collapse due to soil heaving should be considered when the structural slab exhibits lifting or cracking, walls are seperated or cracked, and walls adjacent to one another are separated at the adjoining corner. To see examples of soil swelling related cracking to residential structures, refer to [|Basement and Foundation Damages due to Expansive Soils].

Light Structures Damages in Ankara, Turkey
The city of Ankara has a flat topography surrounded by elevated land and exhibits hot a dry summers as well as cold and rainy winters (Ozer 2011). In June 2009 a retaining wall spanning the Gazi University campus garden toppled over. The collapse sparked an investigation of the failed retaining wall and four other light single-story structures that have shown evidence of expansive soil damages prior to the wall collapse. The 40m (131 ft.) long and 2.2m (7.2 ft.) high wall collapsed after a long period of heavy rains in the spring of 2009. Heaving of the supported soil and the period of heavy rain gave obvious signs of an expansive soil failure. The four light structures all demonstrated wall cracking near their corners and windows. Some of the found cracks were repaired with a cement mortar, but most of these temporary repairs have and continue to fail.

The toppled retaining wall was constructed using andesite block and cement mortar. Measurements from sample points found that the horizontal heave was about 40 cm (15.7 in.) and the vertical heave of the sidewalk that spanned the wall was found to be 8.5 cm (3.3 in.). Most of the light structures of the campus are 50-year-old buildings with ground-bearing concrete floor slabs. In the cases where cracking due to soil swelling was evident, no drainage of the site was installed. The damaged structures were used as laboratories or workshops and ranged in vertical heave from 48mm (1.9 in.) to 100mm (3.9 in.). Numerous cracks were sustained by each structure to the slab and/or walls.

Testing of the soil near the wall consisted of taking samples using sample points, using X-ray diffraction, ASTM testing methods, measurements of swelling properties, and shear strength of the fill material. Samples of the soil near the light structures were taken using boring points and the testing procedures were similar to those noted in the testing of the retaining wall. The soil was found to be mainly comprised of clay like soil with a fill material thickness around 4-5m or 13-16.4 ft.

Residential Home Damages in Sudan, Africa
Sudan is an African country where one million sq. km of its land is approximated to have expansive soils. After a yearlong investigation of different regions in this country, an estimated $6,000,000 in damages was calculated to be caused by heaving and shrinking soils.

One of many examples, the Rahad Project is an irrigated farm project where the staff housing buildings have succumbed to expansive soils and were found to have large cracks in interior walls. Many factories and production plants around the region were found to have similar cracking to their structures. The Asalaya and Sennar Sugar Factory, Friendship Cotton Mill, and Gezira Tannery are all examples of properties where structures were found to have cracking or failed completely. As shown in Figure 2, damages found in structures due to expansive soils are located in close proximity to the Nile River. In Khartoum, the capital and largest city of Sudan, damages to roads, buildings, and utility piping are common and amount to millions of U.S. dollars per year for repair costs. Many one and two story buildings using shallow foundations have collapsed or have been heavily damaged due to these soils.

Despite efforts in foundation design, many larger buildings such as the Rahad Project and the Asalaya Suger Factory have succumbed to expansive soil damages. A specific type of soil known as Black Cotton Soil exists throughout the entire Rahad property and the heaving from water absorption caused many concrete piles to fail where the reinforcement was terminated. In the case of the Asalaya Factory, the potential for water seepage into the slab was accounted for by encasing the top 3m (9.8 ft) of the supporting piles in plastic pipe and providing polyethylene sheeting beneath the slab. Despite these efforts, damages from the soil have been estimated to exceed the original construction costs by a factor of three (Wayne 1984).

Courtyard House Damages in Adalaide, Australia
In March of 1993, a site investigation was conducted to determine the causes of cracking to a courtyard styled residence in Adalaide, Australia. After documenting the status of the slab, walls, ceiling, and inoperable doors in regards to cracks, a level survey was performed to pin point deflections. The failures within the house were results of soil heaving due to inadequate site drainage. No significant damage was done to the structure to cause a complete failure.

Prior to construction, site testing through 3 bore holes was performed per AS 2870. The Australian Standard was used to classify the soil as Class E or extremely reactive. To combat the reactive soil conditions, a stiffened raft slab and stiffened grade beams with a 550mm (21.6 in.) depth were used to support the house. To figure out the causes of cracking to the slab in spite of the design measures taken to prevent cracking, a back-analysis was performed which determines the stress levels in the slab and free soil mound shape of the expansive soil underlying the building ( Li 2002). The back analysis yielded depth measurements of the free swelling mound shape for the floor plan of the building as can be seen by Figure 3.

Elementary School Failure in Western Pennsylvania
Soon after completion in September of 1961, lifting of the slab at the south exit door of the school was observed towards the start of 1962. This first observances lead to claims and other related failures that would soon become a part of the life of the school. Pyrite, a mineral that is common to sedimentary rocks, was found in the soils underneath and around the structure. Under ideal conditions pyrite can expand to 30 times its size. Weathering of this material in the short amount of time following the school's completion lead to the slab lifts and other similar issues found throughout the years.

Pyrite which is a specific type of expansive soil will expand when introduced to warm oxygen-rich ground water. Cracking and movement, as seen in Figure 4, of the structure were reported in early 1962 when issues were found with the finishes, slab, and walls. After years of on-going construction, claims, and investigations, pyrite was found to be the cause of the damages after boring tests were conducted in March of 1968 (Parfitt 2011). Testing to see if the source of these problems were due to granulated slag fill was also conducted. These tests determined that the slag fill was not a contributing cause. The years following this and other related investigations were riddled with trials which lead to confusion and hampered the remediation of the issues. 40 years post initial construction, the school continued to undergo investigations considering the ongoing structural issues involving pyrite.

All of the original classroom, office, and gymnasium building sections, with the exception of the 1965 classroom addition, were abandoned and demolished in the early 2000's based on safety concerns relayed to the school district by their consulting engineer (Parfitt 2011). Even as early as 1968, repair costs to the school were in excess of $545,000; a little more than half the original construction cost of the school. A full and proper investigation into the site prior to construction could have prevented problems considering that cases with as little as 0.1% sulfide sulfur (chemical composition of pyrite) can cause issues and tend to be hard to find when doing a straight forward site investigation. Buildings including the elementary school that were built in the early 1960's are considered to be among the first to encounter and document failures due to pyrite. These examples should be heavily considered for current construction and forensic procedures since proper site evaulautions at this time were not typically conducted.

Prevention and Repairs
When dealing with expansive soils, it is best to take a proactive approach rather than a reactive one. To prevent damage from expansive soils, procedures in slab design or soil manipulation should be considered. A posttensioned slab, suspended floor, pre-swelled soil, and removal of suspect soil material are ways to mitigate the issues that can stem from soil shrinkage and heaving. In the case of repairing a damaged structure, a perimeter drainage system and removal of the damaged slab or wall followed with the proper structural replacement are possible fixes. There are no cheap remediation methods in the industry which includes the ones previously mentioned.

Post-tensioning the slab using guidelines such as the ones described by the Post-Tensioning Institute is a proactive approach for expansive soils design that can be implemented in any size structure. As demonstrated by Figure 5, the slab should be designed for "Center Lift" and "Edge Lift". The process for designing this type of slab system includes the differential movement to be expected, maximum anticipated vertical differential soil movement (y m ), and horizontal distance of moisture variation from the slab perimeter (e m ) to be given by the soils or geotechnical engineer (Day 1994). Using this information, the structural designer is usually tasked with the actual design of the foundations.



Other remediation methods that can be applied to any size structure includes pre-swelling or removing the soil. To pre-swell the soil, excavation and re-compaction or pressure injection can be used. The first method removes the soil, mixes it with water to achieve an optimum moisture level, and then replaces and compacts it. The second pre-swell method uses driven rods that inject water under high pressures to mechanically fracture the soil. This method should only be used when dealing with clay like soil conditions. Completely excavating and replacing the fill material and expansive soil on site is an expensive, but more redundant method compared to pre-swelling.

Using a suspended floor which isolates the slab from the active ground through the use of piers provides a buffer zone for expansion. This design is often not considered for large structures and industrial buildings due to the costs and load considerations. All methods discussed above excluding the post-tensioned slab should not be considered for large industrial buildings with an area greater than 40,000 sq. m (430,556 sq. ft) because of cost and construction schedule impacts (Reed 2006).

The only remedy for a damaged structure due to expanded soils post construction is removal and replacement of the cracked members. Providing site drainage using traditional means or the "picture frame" alternative as described in "Alternative Earthwork Procedure For Expansive Soils" should also be considered. The "picture frame" method uses an earthen dam along the perimeter of the building to reduce moisture transfer to the interior clay.

Conclusion
Expansive soils damage is a common issue that is not often found untill after completion of a structure. These failures in alot of cases will often result in a large amount of money to fix and these costs can exceed the value of the building. The proper investigation of the site in consideration for a proposed structure should not be taken lightly. Following the right procedures and taking a proactive approach is the best way to prevent expansive soil damages. Slab and wall lifting, seperation, and cracking as experienced in the case studies above stem from a combination of factors, but the main culprit in all was the expansive soil content in their site.

Annotated Bibliography
This report elaborates on road damage due to expansive soils in Saudi Arabia. The study covers the region of Al Ghatt and classifies the different types of cracking found with images and figures.
 * 1.** **Dafalla, M. A., and Shamrani, M.A. (2011). “Road Damage Due to Expansive Soils: Survey of the Phenomenon and Measures for Improvement,”**
 * //ASCE Geotechnical Special Publication No. 219//, 73-80.**

This paper evaluates the effects that expansive soils have on slab-on-grade (SOG) construction. It also gives recommendations for a SOG built on expansive soils since a SOG is typical in construction.
 * 2.** **Day, R. (1994). “Performance of Slab-on-Grade Foundations on Expansive Soil ,” **
 * //Journal of Performance of Constructed Facilities//****, Vol. 8, No. 2, pp. 129-138.**

This piece describes the processes used to classify the varieties of expansive soils. Three separate sites were used for the methods described.
 * 3.** **Gordon, R. (October 2001) “Investigating Field Behavior of Expansive Clay Soils.”**
 * //Expansive Clay Soils and Vegetable Influence on Shallow Foundations//, pp. 82-94.**

This piece presents a case study on a particular courtyard house in Australia and how expansive soils have caused light to mid-range damage.
 * 4**. **Li,J. and Cameron D. (October 15, 2002). “Case Study of Courtyard House Damaged by Expansive Soils”**
 * //Journal of Performance of Constructed Facilities,// Vol. 16, No. 4, pp. 169-175.**

This paper evaluates failures in light structures at Gazi University in Ankara.
 * 5.** **Ozer, M., Ulusay, R., and Isik, N. (August 30, 2011). “Evaluation of Damage to Light Structures Erected on a Fill Material Rich in Expansive Soil,”**
 * //Bulletin of Engineering Geology and The Environment Vol. 71, No. 1,// pp. 21-36.**

This paper uses damages found in a private elementary school in conjuction with soil properties below grade to explain the effect of pyritic and expansive soils on a building.
 * 6. Parfitt, M., Jones, D. , and Garvin, R. (2011). ”Structural, Construction, and Procedural Failures Associated with Long-Term Pyritic Soil Expansion at a Private Elementary School in Pennsylvania.” //J. Perform. Constr. Facil.//, 25(1), pp. 56–66.**

This document gives descriptions on the processes used to deal with expansive soils in terms of earthwork procedures.
 * 7. Reed, R. (2006). “Alternative Earthwork Procedure for Expansive Soils,” //Unsaturated Soils 2006,// pp. 315-322**

This paper surveys the area of Sudan in Africa and the $6,000,000 in damages caused annually by expansive soils. The summarized information includes typical damages found in many structures of the area.
 * 8.** **Wayne, M. ASCE, Mohamed, and Elfatih (1984). “Construction on Expansive Soils in Sudan ,” **
 * //Journal of Performance of Constructed Facilities//****, //Vol. 110, No. 3//, pp. 359-374.**