Lecture 2: Soil Properties and Groundwater Flow

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1 ENGI 7718 Environmental Geotechniques ENGI 9621 Soil Remediation Engineering Lecture 2: Soil Properties and Groundwater Flow Spring 2011 Faculty of Engineering & Applied Science 1

2 2.1 Soil properties What is soil? Each discipline defines soil in a different way, depending on how soil affects it In an engineering sense, soil is the relatively loose agglomerate of mineral and organic materials and sediments found above the bedrock. R.D. Holtz and W.D. Kovacs (1981) Soils equation Soil = f (parent material, climate, biota, topography, time) 2

3 Soil composition About 50% of the soil solid particles 45% Minerals 5% Organic matter About 50% of soil should be pore space 25% Air/Pore space 25% Water 3

4 Pore spaces location of air and water Soil particles mineral and organic Soil composition 4

5 Air Water Soil Soil composition by phase: ssoil (dry) wwater aair vvoid (pores filled with water or air) V is volume M is mass 5

6 2.1.2 Physical characteristics of soil (1) Soil profile Soil profile layers of soil are called horizons Typical profile A Horizon topsoil B Horizon subsoil C Horizon bedrock A Horizon B Horizon C Horizon 6

7 (2) Soil texture Soil texture the way the soil feels depends on the amount of each size of mineral particles in the soil Sand, silt, and clay are names that describe the size of individual mineral particles in the soil Sand the largest particles and they fell gritty Silt medium sized, and they feel soft, silky or floury Clay the smallest sized particles, and they feel sticky 7

8 Soil texture: Relative size comparison of soil particles 8

of coarse and fine contents produce different soil textures A loam a

9 Soils are more cohesive when they have more fine particles Clay Soils are more loose when the have more coarse particles Sand Different combinations of coarse and fine contents produce different soil textures A loam a mixture of sand, silt and clay Soil texture triangle represents 12 textural classes 9

10 (3) Soil structure Soil structure the shape that the soil takes based on its physical and chemical properties How particles cluster together single grain granular (best for most plants) blocky platy massive Single grain associated with sandy soils 10

11 Granular the best for most plants particles cling together to form rounded aggregates it is commonly found in A horizon peds are small usually between 1 to 10 millimeters Platy large, thin peds platelike and arranged in overlapping horizontal layers 11

12 Blocky particles cling together in angular aggregates typical of soils with high clay content typical of B horizon peds are large about 5 to 50 millimeters Massive soil has no visible structure hard to break apart & appears in very large clods 12

13 (4) Soil consistency Soil consistency describes the general organization of the soil hold a moist sample between the thumb and forefinger, and gently squeeze it until it falls apart is classified by the following categories Loose you have trouble picking out a single sample and the structure falls apart before you handle it Friable the sample breaks with a small amount of pressure Firm the sample breaks when you apply a good amount of pressure and dents your fingers before it breaks Extremely firm the sample can't be crushed with your fingers (you need a hammer!) 13

14 Loose Friable Firm Extremely Firm Soil consistence 14

15 (5) Soil bulk density (ρ d ) Bulk density a measure of soil compaction soil weight per unit soil volume Sample is made of solids and pore spaces 1.33 gms 1 cm (so, there is 1 cubic centimeter of soil) To calculate bulk density: Volume = 1 cm 3 Weight = 1.33 g Bulk density = W T V T Bulk density = Bulk density = 1.33 g/cm 3 15

16 Low BD = high porosity High BD = low porosity Depth Bulk Density (grams/cm 3 ) 0 inches 7 inches 8 inches 9 inches 10 inches Plow layer Compacted zone Uncompacted subsoil Bulk density and compaction zones 16

17 (6) Soil moisture content Soil moisture content the quantity of water contained in a soil Volumetric moisture content, θ defined mathematically as θ = V V w T where V W = the volume of water V T = V S + V V = V S + V W + V a = the total volume (that is soil volume + water volume + air space) 17

18 Gravimetric moisture content, u expressed by mass (weight) as follows: u = M M w s where M W = the mass of water M S = the mass of soil 18

19 Soil at different moisture levels Groundwater Pore Spaces are filled with water Soil Pore Space Water on soil particle surface 19

20 2.1.3 Chemical characteristics of soil (1) Soil ph or Hydrogenion activity Soil ph a measure of the relative amount of H + ions indicates the acidity or the alkalinity of a solution (a soil solution) ph meter ph = log [H + ] In a soil it is driven by the ionization of soil water ph scale ranges from 0 to 14 7 is considered neutral Everything greater than 7 is considered alkaline (basic) Everything less than 7 is considered acidic 20

21 (2) Soil surface charge Coarsegrained soil such as gravel, sand and silt are chemically inert The surface of clay minerals and organic matters (OM) in soil generally carry electronegative charges Clay Particle 21

22 Sources of charge on clays Ionizeable H + on edges it is phdependent, similar to charge on OM just as in the case of a weak acid Isomorphous substitution in clays it is not affected by ph often referred to as permanent charges Substitution of Al 3+ for Si 4+ in the tetrahedral layer of clays Substitution of Mg 2+ for Al 3+ in the octahedral layer of clay 22

23 (3) Cation exchange capacity (CEC) CEC = quantity of exchangeable cations per unit weight of soil The capacity of a soil to adsorb and exchange cations (positively charge ions, Ca 2+, Mg 2+, K +, Na +, NH 4+, Al[OH] 2 +, Al 3+, and H + ) CEC due to the net negative charge of soil colloids (clays and organic matter) Both ionizable H + and isomorphous substitution impart CEC to clays Total CEC of the soil is dependent upon the amount of these sources and also upon the surface area of clays exposed 23

24 With Magnets In soil Unlikes Attract CLAY CLAY + + NH 4 + Ammonium K + Potassium Likes Repel CLAY + + NO 3 Nitrate 24

25 Cation exchange the replacement of one adsorbed cation for another from solution..na + + [Ca 2+ ]..Na + Dissolved in soil solution..ca 2+ + [Na + ] [Na + ] Negativelycharged clay 2XNa + + Ca 2+ XCa Na + 25

Circle the correct (best) terms inside the brackets:

1 Circle the correct (best) terms inside the brackets: 1) Soils are [consolidated / unconsolidated] [natural / artificial] bodies at the earth s surface. Soils contain mineral and organic material, which