Geomorphology Chapter 3 Soils 1 Chapter 3 Soils Perhaps the most important part of weathering is the development of soil D Easterbrook Soil makes plant life possible Plants make animal life possible 2 Chapter 3 Soils Definitions Processes Characteristics Classification 3 Chapter 3 Soils Definition of soil varies according to discipline Geology Soil science Engineering 4 Q What is Soil A Depends on who you ask To a geologist residual material formed in situ from weathering of a parent material either bedrock or sediment not a sediment Profile from alteration of parent material residual vs transported soils 5 Q What is Soil A Depends on who you ask To a soil scientist organic rich material that supports plant growth The life zone intersection of hydrosphere atmosphere lithosphere and biosphere soil scientists largely trained work in agricultural related areas very specialized consider economic significance of agricultural grains fruits vegetables feed stock ethanol etc grains biggest U S export important considerations fertility texture drainage 6 Q What is Soil A Depends on who you ask To an engineer any unconsolidated material above bedrock important considerations thickness strength water conditions unimportant considerations origin true soil or sediment fertility 7 General problems associated with various soil types engineering and environmental perspectives Clays shrink and swell w moisture changes esp montmorillonite fat clays compressibility sensitivity e g liquefaction poor drainage slow or negligible transport Silt easily erodable loses strength when saturated highly susceptible to frost heave and piping Sand Gravel fine sands may behave similar to silts good drainage rapid transport Organics highly compressible non uniform 8 9 Chapter 3 Soils Characteristics unconsolidated porous differentiated by depth into horizons upper horizon s organic rich soft weak compressible compared to rock 10 11 Chapter 3 Soils Processes 12 Soil Forming Factors Climate T H2O Parent material composition structure texture Vegetation influences runoff erosion depends on climate Topography slope influences runoff erosion Time weathering may reach equilibrium given enough time 13 14 Processes and Resulting Profile differentiated by depth into horizons i e soil profile CaCO3 Bk Zone of leaching Clays oxides 15 16 17 18 19 20 Chapter 3 Soils Classification 21 Classification Geology Grouped by texture Coarse grained gravels and sands Well sorted Poorly sorted Grouped by texture Fine grained silts and clays Engineering Grouped by texture Granular or non cohesive Poorly graded Well graded Grouped by texture and plasticity Cohesive 22 23 24 25 Example 1 42 sand 20 clay 38 silt Loam Example 2 10 sand 46 silt 44 clay 26 10 sand 46 silt 44 clay Silty Clay 27 Group Symbol Category Coarse Gravels Clean Gravel Grained Soils Gravel with Fines Sands Unified Soil Classification System USCS Clean Sands Sand with Fines Liquid Limit 50 Gravel well graded GW Gravel poorly graded GP Gravels mixed non plastic fines GM Gravels clayey plastic fines GC Sands well graded SW Sands poorly graded SP Sands mixed plastic fines SM Sands clayey plastic fines SC Mineral silts low plasticity ML Fine Silts Grained and Clays mineral low plasticity CL Soils Clays Organic silts low plasticity OL Mineral silts high plasticity MH Clays mineral low plasticity CH Liquid Limit 50 Organic clays high plasticity Highly Organic Soils Organic soils as Peat 28 OH Pt 68 gravel 28 sand 4 silt 0 clay Sample 1 29 68 gravel 28 sand 4 silt clay Sample 1 D60 3 5mm 30 68 gravel 28 sand 4 silt clay Sample 1 D10 0 23mm 31 Grain Size Analysis cont Sorting Grading Parameters Uniformity coefficient Cu Cu D60 D10 3 5mm 23mm 1 5 32 Grain Size Analysis cont Sorting Grading Parameters coefficient of concavity Cc Cc D30 2 D10 x D60 1 8 2 0 23 x 3 5 4 33 Group Symbol Category Coarse Gravels Clean Gravel Grained Soils Gravel with Fines Sands Unified Soil Classification System USCS Clean Sands Sand with Fines Liquid Limit 50 Gravel well graded GW Gravel poorly graded GP Gravels mixed non plastic fines GM Gravels clayey plastic fines GC Sands well graded SW Sands poorly graded SP Sands mixed plastic fines SM Sands clayey plastic fines SC Mineral silts low plasticity ML Fine Silts Grained and Clays mineral low plasticity CL Soils Clays Organic silts low plasticity OL Mineral silts high plasticity MH Clays mineral low plasticity CH Liquid Limit 50 Organic clays high plasticity Highly Organic Soils Organic soils as Peat 34 OH Pt Our sample is 68 gravel 28 sand 4 silt clay 35 Our sample is 68 gravel 28 sand 4 silt clay Cu 1 5 Cc 4 36 17 gravel 38 sand 45 silt clay Sample 2 37 Our sample is 17 gravel 38 sand 45 silt clay YES NO 38 Our sample is 17 gravel 38 sand 45 silt clay YES NO 39 Our sample is 17 gravel 38 sand 45 silt clay YES SC YES OR YES LAB FIELD 40 Why didn t we just use grain size curve to determine silt vs clay 17 gravel 38 sand 45 silt clay Sample 2 1 Often don t have curve for fraction smaller than silt just clay silt combined 2 Clay silt distinction is based as much on behavior plastic or non plastic as actual grain size distribution 41 42 43 Clay Mineralology Activity PI clay Non expansive clays low PI kaolinite illite Expansive clays high PI montmorillonite 44 45 46 47 SAND 48 SILT 49 CLAY 50 SAND CLAY SILT 51 SOIL DENSITY 52