CHAPTER 4SOIL WATER CHARACTERISTIC CURVE

Presented by:Yusep Muslih Purwana

Lab. Mekanika Tanah FT UNSJl Ir Sutami 36 a Surakarta

The value of soil suction is strongly affected by its moisture content. The variation of moisture content versus suction in particular soil is presented graphically by a curve, namely Soil-Water Characteristic

Curve (SWCC)

Definition of SWCC

• SWCC is a curve representing the relation between suction and moisture content.

• SWCC quantify the capacity of soil to hold water with particular degree of suction

Some other terms used for Suction-water content relationship

• Soil-water characteristic curve, SWCC• Soil-water content relationship• Moisture retention curve• Soil moisture retention curve• Water retention curve

Note:In Civil engineering, the SWCC is commonly accepted rather than

“retention”. The term characteristic is related to behavior, whereas the term retention is more closely related to retaining water for agricultural purpose.

Typical SWCC (Drying)

• SWCC consists of 3 zone; boundary effect, transition, and residual.

• At maximum water content, suction is near zero

• At minimum water content, suction is very high

• At boundary effect zone, all pores are occupied by water (saturated condition) surface tension is not developed yet.

Typical SWCC (Cont.)• When higher suction is

applied, pore water is expelled from the largest pores first and pores started to be occupied by air.

• Suction required for removing water from largest pore is known as bubbling pressure (ua – ub)b or air entry value yb

• The area between zero suction and AEV is boundary effect zone.

Typical SWCC (Cont.)• Beyond HAV, the increase of suction

causes rapid loss of water content until residual water content

• Suction corresponding to residual water content is refer to as residual suction yr, at which desaturation ends.

• Starting from yr, water begins to be held by adsorption force.

• At residual zone the curve forms asymptotic line at low degree of saturation.

• At residual zone, water still exists but need considerable suction to expel even small amount of water.

Some important things about SWCC

• No single or unique SWCC for particular soil• Variables affect the shape of SWCC: grain size

distribution, initial water content, stress history, natural or remolded, temperature.

• SWCC also exhibits hysteresis phenomenon (drying and wetting SWCC is different)

• The term moisture content can be: gravimetric (w), volumetric q, or degree of saturation (S)

Typical shape of SWCC from different soils

Effect of stress state on SWCC

Hysteresis on SWCC

General procedure for SWCC

1. Sample saturation, determine w0, e0, Gs, W0

2. Apply certain value of suction (ua – ub)1, maintain until equilibrium. Determine W1, e1, w1 and S1

3. Apply higher value of suction (ua – ub)2 until equilibrium. Determine W2, e2, w2 and S2

4. Repeat step 3 for higher suction until low water content attained wn

5. Plot all suction values versus water content w0 until wn

Laboratory apparatus for SWCC

• Pressure plate • Tensiometer• Filter paper

Testing procedure for those apparatus is presented in another chapter.

Mathematical Model of SWCC

• Unsaturated soil testing is time consuming and costly. To overcome this situation, researches have been conducted to utilize SWCC fro predicting soil property functions.

• Some mathematical models have been proposed to relate SWCC and other soil parameters.

• Models mostly use saturated water content, high air entry and residual suction as base parameters

• Common models: Gardner (1958), Brooks and Corey (1964), Farel and Larson (1972), van Genuchten (1980), William (1983), McKee and Bumb (1984), Fredlund and Xing (1994). The last one is the most commonly used in Geotechnical Engineering.

Mathematical Models of SWCC

• According to Leong and Rahardjo (1997), the SWCC equation can be derived from generic form:

Where an and bn are constants, Q is normalised water content;

𝑎1Θb1 +a2 exp(a3Θb1) = a4ψb2 + a5 exp൫a6ψb2൯+ a7

rs

r

Brooke and Corey model (1964)

sb for

b

rsfor rb

Van Genuchten model (1980)

Where a, b, c are constant

Fredlund and Xing model (1994)

Where a, n, m are constants determined by non linear regression procedure. a is corresponding to (but generally higher than) AEV, n is relating to slope at inflection point, C(y) is correction factor

Home work

• Model the SWCC using van Genuchten, Brooke and Corey, and Fredlund and Xing.

TO BE CONTINUED NEXT WEEK