Dealing with Expansive Soils
The problem of expansive soils was not recognized until the late 1930s. Then, cracks on brick wall structures of light buildings were attributed to bad workmanship or foundation settlements. The first recorded observation about soil heaving was made by USBR in 1938 associated with foundation for a steel siphon on Owyhee Project in Oregon.
For purposes of definition, expansive soils are (clayey) soils capable of undergoing considerable volumetric deformation usually along with variation of moisture content. While a rational analysis is sought about such soils, it is necessary to address two issues. One, why soil heaving takes place or what geological/chemical processes are responsible for swelling of some clayey soils. Other, how can the process of swelling be physically measured and quantified so that it may be useful for engineering decision making.
Nature of Expansive Soils
- Basalts of Indian Deccan Plateau, dolerite sills and dukes in Central South Africa have undergone considerable chemical residual weathering so that feldspar and pyroxene minerals have decomposed to montmorillonite.
- Expansive soil deposits in North America and Israel are mainly due to physical weathering of secondary sedimentary rocks containing montmorillonite minerals.
In the above context, the term montmorillonite mineral is used to refer to all clay minerals with an expanding lattice (except vermiculite), because montmorillonite is the clay mineral that presents most of the expansive soil problems. Normally clay minerals: montmorillonite, illite and kaolinite are formed through a complicated process of weathering or halmyrolysis of parent materials, especially, feldspar, mica and limestone. The process usually includes disintegration, oxidation, hydration and leaching. Formation of montmorillonite occurs under extreme disintegration, strong hydration and restricted leaching so that magnesium, calcium, sodium and iron cations may accumulate in the system. Thus an alkaline environment, presence of magnesium ions and lack of leaching aids formation of montmorillonite minerals. So semi-arid regions with relatively low rainfall or highly seasonal moderate rainfall, particularly where evaporation exceeds precipitation contains such deposits. Under these conditions, enough water is available for hydration but accumulated cations will not be removed by flush rain.
In the light of research works conducted during 1960s, swelling mechanism is attributed to the osmotic pressures developed between fresh rainwater and montmorillonite minerals with cation base on surface. Since montmorillonite minerals are smallest and permit maximum base cation exchange reactivity, it is capable of attracting water into interstitial spaces as adsorbed water. As osmotic pressure is the only internal pressure acting between particles, if the soil is subject to external pressure, the distance between particles decrease, water is squeezed out. As a result, cation concentration between particles increases and osmotic pressure in turn increases. Equilibrium is finally reached when osmotic pressure equals the external pressure. Subsequently, if there is a reduction in external pressure, suction of water by osmotic pressure occurs between the particles to dilute the concentration of ions, causes increase in volume and reduction in the osmotic pressure. This process continues until a new equilibrium is established.
Mechanics of Swelling
If the environment of the expansive soil has not been changed vis-à-vis – Release of pressure due to excavation, Desiccation caused by increase of temperature, And/or introduction of moisture – Swelling does not take place.
Moisture introduction into an expansive soil deposit can be expected if there is a potential gradient for migration of water and a continuous passage for transfer of water.
The gradient is usually gravity and/or capillary rise of groundwater. The possibility of transport by vapour, formed on hotter surface to cooler surface, say under pavements and can initiate swelling. The review panel of engineering concepts of Moisture Equilibria and Moisture Changes in Soil beneath Covered Areas, states that the major change induced in soil by a surface cover is the detraction in the rates and quantities of water able to enter and leave the soil at surface.
Kraynski (1973) explained the moisture content variation with depth in a homogeneous soil under covered and uncovered conditions. In a covered area, moisture content of soil reduced with the depth and there is no moisture exchange with the atmosphere. On an uncovered area too, moisture content reduces with depth below an intermediate zone, so that above this, surface evaporation is critical as well. At some depth called the depth of desiccation below the ground level the water content under uncovered area could be equal to that if area were covered, hence this depends on climatic condition and type of soil. The depth of desiccation represents the total thickness of material which has a potential to expand because of water deficiency. It is impossible to determine this depth, while it is at the maximum equal to depth of groundwater table. During the time of precipitation, moisture content near surface increases and again reduces during evaporation. However, the seasonal variations are limited generally, over what is called a depth of seasonal moisture content fluctuation, which may extend from about 1-3.5 m depending on the alternation of drying and wetting climate. Influence of other factors such as watering of lawns, trees and shrubs and domestic drainage on the variation of this depth cannot also be neglected. In case of covered areas moisture transport by gravity, capillarity and vapour migration occurs very slowly and causes swelling over several years after construction. For such span of time, the depth of seasonal moisture content fluctuation is equivalent to depth of desiccation.
Among various factors that influence the expansion are:
- Initial moisture content and dry density: Expansive soils do not swell or shrink unless there is an increase in moisture content. Dry soils expand the most and the slope of e-log p curve of swelling decreases as initial moisture content increases. It is also established that prolonged wetting well result in more swelling than short duration wetting. Also complete saturation is not required to result in large heaves; thus removing free water by drainage does not arrest foundation movement.
- Superimposed loads: Without superimposed loads, swelling cannot be controlled even with a minimum amount of moisture change. For lightly loaded structures, including slabs on grade, a short duration wetting can cause equally heavy damage as long duration wetting.
Since the characteristics of swelling were dependent on numerous parameters, it has been difficult to standardize a unique measure of swelling capacity. Nevertheless, the most common measure is the swelling pressure defined as pressure required to maintain zero volume change in undisturbed or maximum Procter density in remoulded samples. The swelling potential is measured using a consolidation test apparatus either by adjusting superimposed pressure such that volume is maintained constant or by allowing the soil to expand freely under a surcharge and measuring the pressure to regain original volume. The advantage is that the swelling pressure is not affected by initial moisture content, degree of saturation, stratum thickness or extent of shrinkage. It is affected only by the initial dry density as swelling potential increases exponentially with the dry density, in-situ. Thus for undisturbed samples, the swelling pressure at in-situ dry density can be used directly to describe the swelling characteristics. For remoulded soils, swelling pressure varies with the degree of compaction and maximum Proctor density (at optimum moisture content) is taken as a guide.
Foundation & Soil Engineering on Expansive Soils
A number of projects in this country are constructed on expansive soils. There are two approaches in engineering developments on such soils. One, the structural components such as foundations and slabs on grade that are directly in contact with such soils should be designed for possible upheaval and swelling pressure. The other way is to control moisture and stabilise soil with an intention to bring down the swelling potential of the soil. It cannot be overemphasized that, both these approaches or a mixture of these would not be possible, without a complete understanding of the nature and mechanics of swelling soils.
It is known that buildings can be constructed on swelling soils using straight shaft/under-reamed piles; however, the utility of under-reamed piles is questionable in many circumstances and there is a general tendency to misuse these. Besides, it is rarely appreciated that footings and rafts can also be conditionally used for buildings on expansive soils.
Installation of moisture diaphragms and barriers are sometimes effective in the control of swelling. soil stabilization can also be undertaken by ponding or chemical treatment.
Foundations in Expansive Soils
Straight shaft or under-reamed piles have been used on expansive soils for quite some time. The piles may be friction or end bearing or both and require the bulb to be placed below the depth of seasonal groundwater fluctuation. There should also be no direct contact between the soil and structure excepting the piers. For piles with straight shafts, the uplift pressure proportional to the swelling pressure of soil acts on the wetted surface of the piles and is withheld by the dead load pressure on the pile and skin friction along unwetted portion of the pile. Under-reamed piles, on the other hand, offers two additional components withholding the uplift viz.; the weight of soil in shaft described by the bulb and shearing resistance along a circular plane of failure caused by the bulb. Thus, whereas a straight pile shaft may be affected by loss of friction if the soil gets wet, undreamed piles provide a marked advantage.
Footings can be successfully used on expansive soil if sufficient dead load pressure is exerted on the foundation, the structure is rigid enough to withstand differential heaving without cracking and/or swelling potential of soil can be eliminated. Narrow continuous spread footings under masonry walls have been found to be effective in soils with low degree of expansion (with less than 1% swelling potential). Such spread footings are preferable to be constructed with RC walls or the entire structure can be converted into an RC box. Individual RC column footings may be used in cases where under-reamed piles are not economical, soil cannot mobilize skin friction, swelling potential is moderate and bearing capacity of soil is high. It is, however, important to strike balance between allowable bearing capacity and the swelling pressure, It must also be noted that swelling can be prevented only in localized zones beneath the footings where the stress induced by the foundation are concentrated.
Raft foundations on expansive soils are likewise to be designed for both the bearing pressure and the swelling pressure. The stiffness of the raft makes it advisable for simple lightly loaded buildings on moderately swelling soils. The design of the raft requires the value of a support index that depends on soil sensitivity and climatic rating. Hence, usually adoption of raft requires tests to be undertaken on undisturbed samples.
Slabs on Expansive Soils
By far, the most important problem in design of structures on expansive soils is that of slabs of grade. Slabs placed directly on grade with or without high reinforcement, crack heavily due to rise in water table. It is disputed about the use of gravelly layer beneath such concrete floors, but a major advantage of sub-floor drainage and even distribution of loads could be achieved. Use of structural slabs on grade and raised structural slabs with crawlspace or honeycombs separating from the soil are also clear but expensive solutions. Sometimes it is possible to insert slip joints between floor slabs and grade beams such that the slab is capable of independent movement. Such slabs are called floating slabs and prevent transfer of uplift pressure to column foundations. The slip joints are provided using asphalt felt expansion sometimes oiled with tempered masonite and silicone lubricant. It must, however, be noted that heaving of floor slab also results in cracking of partition walls directly supported on the ground and requires separate slip joints. Further, the doorframes are required to be hung from the top rather than supported on the floor.
It is possible to control swelling by maintaining moisture content of soil constant although the procedure is very difficult. Horizontal moisture barriers, such as polythene membranes buried near-surface, concrete aprons around building and asphalt membranes can limit direct percolation of water and rise in water table. Vertical moisture barriers such as sheet piles, concrete diaphragm walls or polythene membranes are required to arrest moisture and vapour transport from the surrounding area into the foundations. It is noted that compacted backfill can usually serve the purpose of a vertical diaphragm at a cheaper cost.
Subsurface drainage is installed for three purposes:
1. To intercept gravity flow of free water into the site using intercepting drains
2. To protect against formation of perched water tables
3. To arrest capillary and vapour movement of water by installation of peripheral drains around the building.
It is advisable to avoid lawn sprinkling, vegetative landscaping and faulty internal plumbing. More so, roof downspouts should be directed away from the ground.
Soil stabilization methods for expansive clay ponding and chemical stabilization.
The oldest is to flood the soil by ponding, so that swelling is achieved prior to construction. Pre-wetting takes a considerably long period of ponding (1-2months) for complete swelling due to impermeability of clay. Footings cannot be placed on pre-wetted soil that has low bearing capacity and pre-wetting can cause detrimental long-term effects as moisture travels to dry lower strata in course of time. These shortcomings may be removed by compacting the soil to low densities and high moisture contents, so that swelling pressure (of loose soil) is reduced. Replacement of soil unto that level where surcharge would compensate swelling is also a plausible but usually expensive solution.
For highway and airway pavements, lime stabilization by direct mixing or pressure injection has been successful. Lime can lower base-exchange capacity of soil, increase particle sizes by flocculation and hence reduce the plasticity of soil. Chemical stabilization may be done using cement, fly ash and organic or inorganic chemicals.