Advanced Engineering Geology & Geotechnics
VARIOUS ASPECTS OF EXPANSIVE SOILS RELEVENT TO GEOENGINEERING PRACTICE Simple Correlations B
etween Soil Plasticity and Expansion Potential
Soil Expansion Potential (ASTM D-4829) This test was developed in Orange County, California in the mid-1960s and introduced in the 1973 Uniform Building Code as UBC Test Standard 29-2. It was re-designated as UBC Test Standard 18-1 in the 1994 code. This standard was adopted by ASTM in 1988. Soil material is disaggregated and passed through the #4 sieve and then brought to approximately the optimum moisture content (as determined by ASTM D-1557). The optimum moisture content equates to approximately 80 to 85% of saturation. After setting for 6 to 30 hours, the moisture-conditioned soil is compacted into a 4-in diameter mold. The moisture content is then adjusted, if necessary, to bring the sample to 50% saturation. A 144 psf surcharge is applied and the sample is wetted and monitored for 24 hours, measuring the volumetric swell. The Expansion Index is calculated as follows: EI = 100 x ?h x F Where ?h = percent swell and F = fraction passing No. 4 sieve Section 1803.2 of the 1994 Uniform Building Code directs expansive soil tendency be graded by this method. The UBC mandates that “special [foundation] design consideration” be employed if the Expansion Index is 20, or greater (UBC Table 18-1-B). UBC Table 18-1-C may be applied to gain a “weighted index”, allowing for a lessening of expansion with increasing depth (confinement). EI 0 to 20 21 to 50 51 to 90 91 to 130 >130 Expansion Potential Very Low Low Medium High Very High
According to ASTM, “The expansion index has been determined to have a greater range and better sensitivity of expansion potential than other indices” (such as Atterberg limits).
Example Design Input from Geotechnical Consultant Two soil samples from a structure exhibiting distress from apparent uplift were subjected to the UBC Expansion Index test. The two samples were observed to swell 12.8 to 13.2%. These values are then multiplied by 100 to obtain the Expansion Index, in this case, 128 and 132. The UBC index test does not attempt to replicate any particular moisture or loading conditions that actually exist in the field, it is simply a relative index of swell potential. The soil specimen is arbitrarily subjected to moisture absorption at 50% saturation, under a normal load of 144 psf. An Expansion Index (EI) of 100 would correspond to a volume increase of 10%. In this example, the volume increase from 50% saturation was significant. The Uniform Building Code states that EI’s between 91-130 are considered to have a “High Expansion Potential” and any values in excess of 130 are to be termed “Very High Expansion Potential”. The distinctions are contained in Table 18-I-B of the UBC.
Empirical estimate of uplift pressures
Clayey materials can undergo relatively large volume changes in response to fluctuations in water content. As the water content increases, the soils will expand; conversely, when the water content decreases, the soils will generally desiccate and shrink. In the mid 1960s, the San Francisco firm of Lowney-Kaldveer Associates developed the following empirical relationship: Soil Uplift Pressure = 100 (Plasticity Index) – 1000 in psf A material with a PI of 45 could be expected to swell as much as 3500 psf. A PI of 18 could be expected to exert as little swell pressure as 800 psf. These values can also be useful in making preliminary estimates of active and passive soil pressures acting against continuous strip footings of foundations.
Atterberg Limits and clay content can be combined into a single parameter called Activity. Skempton (1953) defined the term as follows: Plasticity Index Activity (Ac) = ----------------------------------% finer than 2?m Skemton suggested three classes of clays according to activity: ? inactive for activities less than 0.75 ? normal for activities between 0.75 and 1.25 ? active for activities greater than 1.25 Active clays provide the most potential for expansion. Typical values of activities for the three principal clay mineral groups are as follows: Mineral Montmorillonite Exchangeable Ion Na+1 K+ Ca+2 Na+1 K+1 Ca+2 Na+1 K+1 Ca+2
LL (%) 710 660 510 120 120 100 53 49 38
PL (%) 54 98 81 53 60 45 32 29 27
PI (%) 656 562 429 67 42 55 21 20 11
SL (%) 9.9 9.3 10.5 15.4 17.5 16.8 26.8 24.5
Activity 7.2 1.5 0.9 0.33-0.46 -
WATER AND EXPANSIVE SOILS
Water in the soil occurs as three types: 1. Gravitation water: Water free to move downward from the force of gravity, or water that drains from a soil. 2. Capillary water: Water held in the capillaries or pores of the soil 3. Hygroscopic water: Moisture that remains after capillary and gravitational waters are removed. This moisture is in the form of a thin film held by each grain of soil. It also has a chemical affinity for the soil particle and its neighbor particle to tightly bond them together. It is also in balance with the humidity of the air. Today, we will focus on the capillary water that heavily influences expansive spoils.
Expansive soils typically arise as a result of an increase in water content in the upper few meters from ground surface. There have been instances of deep-seated heave, but these rare. The water contents in the upper few meters are influence by climatic conditions and environmental factor. This zone is generally termed either the zone of seasonal fluctuation or the active zone.
FIGURE 1 - Typical plots of moisture content versus depth for exploratory borings in expansive soils. Note how the zone of seasonal moisture fluctuation and maximum depth of dessication can be estimated from such data.
Laboratory tests used in identification of expansive soils
Test Reference Properties Investigated Parameters Determined
Atterberg Limits Liquid Limit (LL) Plastic Limit (PL) Shrinkage Limit
ASTM Standards 1991 ASTM D-4038 ASTM D-4318 ASTM D-427
Plasticity, consistency Upper limit water content of plasticity Lower limit water content of plasticity Lower limit water content of soil shrinkage Distribution of fine-grained particles sizes Mineralogy of clay particles Characteristic crystal dimensions Characteristic reactions to heat treatments Size and shape of clay particles Charge deficiency and surface activity of clay particles Page 1 of 2
PI = LL - PL = plasticity index w-LL L1= -----------= liquidity index LL-PL R = shrinkage ratio LS = linear shrinkage Percent finer than 2 ?m
Clay content Mineralogical tests X-ray diffraction Differential thermal analysis Electron microscopy Cation-exchange capacity
ASTM D-422 Whiting (1964) ASTM STP (1970) Barshad (1965) McCrone & Delly (1973) Chapman (1965)
Basal spacings Area and amplitude of reaction peaks and thermograms Visual record of particles CEC (meq/100 g)
Laboratory tests used in identification of expansion soils (continued)
Test Reference Properties Investigated ParametersDetermined
Free swell test
I-Ioltz and Gibbs (1956)
Swell upon wetting of unconsolidated unconfined sample of air dried soil One dimensional swell and pressure of compacted, remolded sample under semi strain controlled conditions One dimensional swell under I psi surcharge of sample compacted to 50% saturation initially. One dimensional swell under surcharge pressure of compacted, remolded samples on partial wetting. Linear strain of a natural soil clod when dried from 5 psi (33kPa) to oven dried suction Page 2 of 2
Free swell = (Valet - Vdry)fVdry *100% Swell Index (lb/ft2) (SI) Potential volume change (PVC)
Potential volume change meter
Expansion index test
Uniform Building Code
Expansion Index (EI)
California bearing ratio test
Yoder and Witczak (1975); Kassiff et al (1969)
Percent swell CBR (%)
Coefficient of linear extensibility (COLE) test
Brasher et al. (1966)
COLE and LE (%)
Relationship between various suction units used
Height of Water Column (cm) 1 10 102 103 104 105 106 107
pF* 0 1 2 3 4 5 6 7
psi 0.0142 0.1422 1.422 14.22 142.2 1,422 14,220 105
Kgf/cm^2 0.001 0.01 0.1 1 10 102 103 104
kPa 0.0981 0.981 9.81 98.1 981 9,810 98,100 981,000
Bars 0.00098 0.0098 0.098 0.981 9.81 98.1 981 9,810
Atmospheres 0.00097 0.00968 0.0968 0.968 9.68 96.8 968 9,680
* The pF value is simply the logarithm to the base 10 of the capillary head (i.e., suction head) measured in centimeters of water. Note: Typical units of insitu suction values may range from zero to over. 15,000 psi (100,000 kPa). Suction values as high as 150,000 psi have been reported. These high values include a predominant component of osmotic suction.
Coefficient of Linear Extensibility (COLE)
This test is a shrinkage test used routinely by the U.S. Soil Conservation Service, National Soil Survey Laboratory, for characterizing expansive clays. The COLE test determines the linear strain of an undisturbed, unconfined sample on drying from 5 psi suction to oven dry suction. The procedure involves coating undisturbed soil samples with a flexible plastic resin. The resin is impermeable to liquid water, but permeable to water vapor. Natural clods of soil are brought to a soil suction of 5 psi in a pressure vessel. They are weighed in air and water to obtain their volumes. The samples are then oven dried and another volume measurement is performed in the same manner. COLE is a measure of the change in sample dimension from the moist to dry state and is estimated from the bulk densities of the clod at a suction of 5 psi and oven dry moisture conditions. The value of COLE is given by:
COLE = ?L /?LD = (γds/γdM)0.33 -1 where ?L /?LD = linear strain relative to dry dimensions γdB = dry density of oven dry sample γdM = dry density of sample at 5 psi suction The National Soil Survey uses Linear Extensibility (LE) as an estimator of clay mineralogy. The ratio of LE to clay content is related to mineralogy as follows: LE/Percent Clay __________ Mineralogy >0.15 0.05-0.15 <0.05 Smectites Illites Kaolinites
Estimate of Volume Change
A constitutive relationship for volume change of an unsaturated soil may be related to the stress state variables using appropriate constitutive relationships. Because the stress state variables are independent, the stress-strain relationships must be depicted on three-axis plots, such as the one shown for void ratio below. The constitutive surfaces can be linearized by plotting the volumeweight parameters (void ratio, water content or saturation) versus the logarithm of the stress state variables. The constitutive surface shown in the figure below can be represented by an equation as follows (Fredlund, 1979):
?e = Ct*?log (σ - ua) + Cm* ?log (ua- uw) Where: e Ct (σ - ua) Cm (ua - u,,,) = void ratio = compression index = saturated effective stress state variable = suction index in terms of void ratio and matric suction = matric suction
The constitutive relationship for the water phase may be similarly presented: ?w = Dt*?log (σ - ua) + Dm* ?log (ua - uw)
Dt = water content index with respect to saturated effective stress state variable Dm = water content index with respect to matrix suction
FIGURE 2 - Idealized three-dimensional constitutive surface for unsaturated soils in terms of void ratio independent stress state variables.