b'Cohesive Soil (Clays & Silts)Cohesive soil is soil that is generally classified asafinegrainedclaysoiland/orsilt.By Table 5. Properties of Cohesive Soilcomparison, granular soils like sands and gravelsaresometimesreferredtoasnon-cohesiveorcohesionless soil. SoilDensity - N 2 CompressiveDescription SPT Blow Count Undrained lb/ftc Unconfined 2Shear Strength Strength lb/ft Claysorcohesivesoilsaredefinedassoilswhere the internal friction between particles isapproximately zero.This internal friction angleis usually referred to asor phi. Cohesivesoilshavearigidbehaviorwhen Very 02 250 500exposedtostress.Stiffclaysactalmostlike Softrock.They remain solid and inelastic until they Soft 24 250500 5001,000fail.Soft clays act more like putty.The soft Firm 48 5001,000 1,0002,000clay bends and molds around the anchor whenunder stress. Stiff 815 1,0002,000 2,0004,000Undrained Shear Strengthc: The undrained Veryshear strength of a soil is the maximum amount of Stiff 1532 2,0004,000 4,0008,000shear stress that may be placed on the soil beforethesoilyieldsorfails.Thisvalueofconly Hard 3248 4,0006,000 8,00012,000occurs in cohesive soils where the internal friction Veryangle,,ofthefinegrainparticlesiszeroor Hard 48 6,000 12,000Table 4.Cohesive Soil Classification nearly zero.The value of c generally increaseswithsoil density;therefore,onecanexpectthatSoil Description USCS Density Density 3Symbol Description lb/ft stiffclayshavegreaterundrainedshearstrengththan soft clay soil.It is easy to understand thatInorganic silt, rock flour, Soft 90 when dealing with cohesive soils; that the greatersilty or clayey fine sand or ML Stiff 110 the shear strength c of the soil, the greater thesilt with low plasticity Hard 130 bearing capacity.It also follows that the capacityInorganic clay of low to Soft 90 of the soil tends to increase with depth.medium plasticity, sandy CL Stiff 110 Cohesive Bearing Capacity Factor - N c : Theclay, gravelly clay, lean bearingcapacityfactorforcohesionisanclay Hard 130 empiricalvalueproposedbyMeyerhofintheOrganic silts and organic Soft 75 Journal of the Geotechnical Engineering Division,silty clays, low plasticity OL Stiff 90 ProceedingsofASCE,1976.ForsmallshaftHard 105 helicalpilesortiebackanchorswithplateSoft 80 diametersunder18inches,thevalueoftheInorganic silt, fine sandy or CohesiveBearingCapacityFactor, N c wassilty soils, elastic silts - MH Stiff 93 foundtoavalueofapproximately9,thereforehigh plasticity Hard 105 N c= 9 is a generally accepted value to use whenSoft 90 determining capacities of helical piles and anchorsInorganic clays of high CH Stiff 103 embedded in cohesive soils.plasticity, fat clay, silty clayHard 115 WhendeterminingtheultimatecapacityforaSoft 75 TorqueAnchorsituatedincohesivesoil,Organic silts and organic Equation 1 may be simplified because the internalclays of medium tohigh OH Stiff 95 friction, , of the soil particles can be assumedplasticity Hard 110 to be zero and the N c= 9 is assumed.EquationPeat and other highly PT 1 can be modified when dealing with cohesive soilorganic soils as shown below:ECP Helical Torque Anchors Technical Service Manual2021 Earth Contact Products, L.L.C.2021-09 Chapter 1 - Page 18 All rights reserve'