Soil Mechanics

Ds

Dw

Us

Uw

Ms+Mw

Vt

Ms

Vt

Ws+Ww

Vt

Wt

Vt

Ws

Vt

Ww

Ws

Mw

Ms

Vv

Vs

The main reason for this section is for students who want some clarity on basic soil calculations but also for people who are attempting Do It Yourself projects and want to do a quick evaluation of how well suited their soils are for the project they are considering.

Vt = Volume total

Va = Volume air (gas)

Vw = Volume water (liquid)

Vs = Volume solids (soil particles)

Vv = Volume of voids = Va+Vw

Mt = Mass total

Ma = mass air (gas)

Mw = Mass water (liquid)

Ms = Mass solids (soil particles)

Wt = Weight total

Wa = Weight air (gas) = 0

Ww = Weight water (liquid)

Ws = Weight solids (soil particles)

Pt = Unit mass (density) total

Pw = Unit mass (density) water = 1 gram/cubic centimeter (g/cm^3) = 1,000 kilogram/cubic meter (kg/m^3)

Ps = Unit mass (density) solids

Ut = Unit weight total

Uw = Unit weight water = 62.4 pounds/cubic foot (lb/ft^3)

Us = Unit weight solids (soil particles)

Basic Soil Calculations:

Specific Gravity of Solids (Gs) - ratio of the density or unit weight of the material being considered to

that of water:

Gs = =

**This value is often given or assumed to be between 2.65-2.7

Moist Density (Pm): Pm =

Dry Density (Pd): Pd =

Moist Unit Weight (Um): Um = =

Dry Unit Weight (Ud): Ud =

Water Content (w): w = *100 = *100

Void Ratio (e): e = =

Porosity (n): n =

Saturation (s): s =

Mass soilds (Ms): Ms =

Volume water (Vw): Vw = s*e*Vs

Equation that can be used if 3 of 4 variables are know or can be calculated: Gs*w = s*e

Relative Density (Dr) = *100

Liquid Limit (LL) = water content (w) that divides a soil between liquid and plastic states

Plastic Limit (PL) = water content (w) that divides a soil between plastic and semi-solid states

Plasticity Index (PI): PI = LL - PL

Va+Vw

Vs

Vv

Vt

Vw

Vv

Mt

w+1

e(max) - e

e(max) - e(min)

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