J. David Rogers, Ph.D., P.E., P.G., C.E.G., C.HG. .... â«Cut slope along Pleasant Hill Rd in Martinez, CA in 1954 (left) and 30 years later, in 1984 (at right).
SOME ENGINEERING ASPECTS OF EXPANSIVE SOILS J. David Rogers, Ph.D., P.E., P.G., C.E.G., C.HG.
Expansive soils cause billions of dollars in real property damage to structures in the United States each year. The only peril causing greater loss is that illustrated here: dry rot of wood elements. Never allow moss to grow on a shingle roof! Dry rot and expansive soils damage are typically uninsured perils.
Part 1
RECOGNITION OF FEATURES COMMONLY ASSOCIATED WITH EXPANSIVE SOILS FOUNDATIONS
Expansive Soils
Expansive soils are those containing sufficient quantities of clay, which tend to swell when they absorb moisture and shrink when they lose moisture A pattern of polygonal desiccation, or “shrinkage cracks”, results, shown here
Sidewalk heave is a common manifestation of expansive soils Excessive watering, leaky irrigation systems, and/ or poor drainage often accentuates this problem
Poor drainage adjacent to slabs and flatwork is a common problem in expansive soils-related damage Difficult to solve in flat-lying flood plains
Telltale signs of expansive soils behavior include lifting of lighter structural elements, as opposed to heavy elements, such as chimneys. Edge lift and shear cracks near corners are also common.
Not all structural distress is ascribable to expansive soils action. The employment of dissimilar materials within wood frame structures also leads to differential performance The most common problem is cross grain shrinkage of timber elements (typically shrinks 6% in cross grain direction)
Many homes in California, Colorado, and Texas are constructed on isolated piers and/or piers and grade beams to better resist seasonal heave and shrinkage.
Perimeter heave is common tendency during the first decade after construction, before soil moisture equilibrates beneath the structure. This is sometimes termed “edge curl”.
Soil moisture can be drawn through soil by temperature and pressure-induced flux, sketched above. This can cause pseudo settlement of perimeter foundation elements, due to tilt.
Typical Foundation Damage – Interior isolated piers
Isolated interior piers tend to perform poorly if water gets into the home’s crawl space, even for a brief period of time, as shown here.
Variances in framing loads acting on foundation elements limits the amount of heave that can occur. Heavy loads resist heave and uplift, while lighter loads cannot resist, leading to differential heave
The perimeter of drilled pier-supported structures may settle when soil moisture levels decline significantly about the perimeter of the structure, as sketched here. This commonly occurs during extended drought periods.
Estimate of swell pressure = 100(P.I.) – 1000 (in psf) where P.I. is the Plasticity Index. This would be the maximum value, without any vertical confinement A common problem with lightly loaded structures on expansive soils is differential heave, caused by water ponding on the shady side of such structures, leaky water lines, or natural variances in soil moisture content.
Past Land Use
A seemingly unlimited number of factors can affect differential heave, aside form asymmetric structural loads These include factors such as rotting of old root balls, shown here These create pockets of high soil moisture content, causing differential heave
A common cause of differential heave is asymmetric landscaping practices, shown here Allowing more water to penetrate the soil on one side of a structure than the other inevitably leads to differential performance and damage. The worst problems are usually caused by leaky water lines, sprinkler control boxes, or hose bibs
Part 2
Expansive Soils Promote Slope Creep Butt-bowed trees in northern California
Cut
slope along Pleasant Hill Rd in Martinez, CA in 1954 (left) and 30 years later, in 1984 (at right).
Many natural slopes are blanketed by residual soils, which are expansive These materials swell and shrink with seasonal regularity, during alternating wet and dry seasons This cyclic movement allows for plastic creep, or strain under sustained loading.
Slope
creep sketched by Sharpe (1938)
Apparent
creep of a cut slope in Lafayette, CA
Seasonal slope creep occurs on inclined ground, whether engineered fill or natural hillsides.
The
longer the slope, the greater the amount of creep, because strain increments are accumulated upslope, as an integral function. Foundation
elements situated on slopes can be expected to move with the slope, if not deeply embedded
So called “butt-bowed” or “pistol-butted” tree trunks are common descriptors for tree trunks that exhibit a curvalinear form, due to down slope creep, as shown in these aspen trees near Dunckley Pass, CO, around el. 9500 ft.
Slope creep accumulates upslope
Slope creep accumulates upslope as an integral function This shows flexural slope creep in the Vishnu Schist along Clear Creek, a tributary in the Grand a Canyon’s Granite Gorge
Maximum creep
Mimimal creep
Creep increases upslope
Desiccation
cracks forming more or less normal to the slope surface create zero tension boundaries, which allow for seasonal translation and bending, as sketched here
The annual or seasonal creep increment is caused by expansion and contraction of near surface soils, especially on sloping ground When soil expands, it heaves upward, more or less normal to the slope surface (expansion arrow). When the soil subsequently shrinks (contracts), it moves down, vertically (contraction arrow). The downslope offset between the beginning and ending points of a single cycle of heave and shrinkage is called the creep increment
As surface soils creep downslope, it may encounter a stiff or fixed structural element, such as a foundation. As the soil movement continues, passive soil pressures can develop against the embedded element, as shown in the inverted loading diagram at lower right.
Part 3
Behavior of Overconsolidated Clay
Normal versus Overconsolidated clay behavior
Clay retains a “memory” of its load history The effective cohesion of overconsolidated clay can be significant The effective cohesion can be lost through subsequent wetting, especially, load cycling with moisture
Mohr Circle plot of a typical overconsolidated clay soil, illustrating the artificially high cohesion engendered by tortuous desiccation. Note the apparent pre-consolidation pressure, caused by the severe shrinkage.
The fundamental problem with slabs-on-grade on expansive soils is differential swell beneath the corners, sides, and under the center of a uniformly-loaded slab, as sketched here.
Part 44 Part
MITIGATION OF FOUNDATION PROBLEMS ON EXPANSIVE SOILS
Schemes
employed to combat expansive soils
Solutions for Expansive Soil Problems
An alternative method for handling highly expansive soils using mass grading is to cover them with 15 feet of select nonexpansive fill, as sketched here.
Theory of Drilled Piers
Drilled piers and grade beam foundation systems must extend below the zone of seasonal moisture variation, which can be difficult to predict with any meaningful certainty, because it often depends on landscape watering by the homeowner
Drilled piers are intended to extend below the zone of seasonal moisture fluctuation, holding the structure down, when lightly loaded elements are subjected to moisture absorption and heave.
Many factors can contribute to poor foundation performance in areas underlain by expansive soils Most of these are beyond the control of the geotechnical engineer who was associated with the foundation recommendations
Geotechnical engineers must design redundant support systems; sufficiently robust so as to resist long-range human and environmental factors beyond their control, such as water leaks and abnormally wet weather.
Part 5
Ascertaining Depth of Seasonal Fluctuation and Depth of Desiccation
Inserting meter stick into desiccation crack
The depth of desiccation is typically estimated from examination of soil moisture content with depth (shown at left) The depth of seasonal soil moisture content fluctuation is usually much less, also shown at left.
Drainage improvements, like these concretelined drainage interceptor ditches, can be difficult to maintain when constructed upon expansive soils This is because these soils are prone to seasonal slope creep, differential heave, and desiccation, as shown here
Part 6
SPECIAL PROBLEMS: SHALE BEDS and SWIMMING POOLS
Shale lenses
One of the most vexing problems are discrete beds of expansive shale that often cross a developed pad These seams will swell asymmetrically, as a function of effective normal force, as sketched here
Differential heave is a major problem for highways wherever they cross inclined strata containing clay (upper left) Similar phenomena can occur due to frost heave, shown at lower left
Seepage from swimming pools constructed of porous gunite shells with water-cement ratios
