ENVIRONMENTAL GEOMECHANICS CE-641 Lecture No. 19 Prof. D N Singh - - PowerPoint PPT Presentation

ENVIRONMENTAL GEOMECHANICS

CE-641 Lecture No. 19

• Prof. D N Singh

Department of Civil Engineering

28.10.2018 Lecture No. 19 Lecture Name: Geomaterial Characterization

Sub-topics

• Cracking Characteristics
• Electrical Characterization
• Magnetic Characterization

• Fine-grained soils such as clays, expansive soils, active clay

minerals are prone to cracking (development of shrinkage cracks) due to loss of moisture (drying)

• Occurs in earthen dams, landfill liners & covers (Radioactive

wastes), embankments, earth slopes, cricket pitches, tennis courts/turfs etc.

• Understanding of cracking is necessary for assessing the

safety of structures built on or with soil mass The cracked soil mass would exhibit extremely high hydraulic conductivity, gas permittivity and reduced strength (and hence soil may not be useful for containment). Linked with tensile strength of soils

Cracking Characteristics of Fine-grained Soils

Cracking of Structural Fills Typical cracking patterns Measurement of crack propagation depth

Crack Development: Conceptual Model

Due to the buildup of tensile stresses (on drying of the soil mass)

Mechanisms of Mobilization of The Tensile Strength

Soil considered to be a ‘Three phase system (Soil-Water-Air interaction) Dry soil : apparent cohesion Partially saturated soils: Surface tension Tensile strength plays a vital role in cracking of the soils Soils that swell, exhibit higher tensile strength σt = f( w, t , soil type, CL, CEC, PI, SSA, ψ)

Tensile strength Sensitivity

Sensitivity  shear strength of soil in undisturbed and remolded forms Type of loading

• tensile
• compressive
• shear

most critical

Maximum exposed surface area

Effect of External Loading on the Soil Mass

Exposition of greater surface area of grains and hence more activity

• r reactivity

Tensile loading: Compression loading: Reduction in the exposed surface area of grains and hence less activity

Determination of Tensile Strength of soils

Direct measurement Laboratory or in situ Based on softwares and image analysis

Accurate ( & direct) measurement of crack pattern, geometry, area included in each segment, intersection of cracks and its initiation is a difficult and cumbersome task. Linking these parameters to basic soil properties (Physico-chemico- mineralogical) and its unsaturated sate (as soil dries up) has not been done yet. These investigations should yield a generalized model that would imbibe: Soil Properties Environmental Conditions Loading Conditions Sample size

Triaxial tests

2 4 6 8 10 12 14 16 200 400 600 800 1000

MT (3) kPa 25 50 100 200 300

 (%) d(kPa)

Empirical Relationships from the Literature

Incomplete Relationships, Soil and Methodology Dependent and are not generalized (only a few parameters are involved)

σt=f(LL, PI, CEC, CL, ψ, Ac)

σt = 632.10+38.23 CL σt =7.6 CL-59.2 σt = -5.77(w-womc)+29.4 σt=1.2.LL-4.8, σt=2.1.PI+9.3, σt=1.15.CL+9.0 σt = 31.44+1.24 PI -0.018 PI2 +0.00011 PI3 σt = 39.8 - 850.33/(1+exp(Ac+2.29)/0.67 ) log(σt)= 5.12–2.32 log(w) σt = -39+16.7 CEC σt = -125.21+21.10 CEC σt= 638.46+(-106.02-638.46)/(1+(ψ/1105.72)1.109) σt =10.3+331.2.exp{-0.5.(ln(ψ/15388.92)/2.187)2} σt=-95.89+400.9/{1+exp(-(ψ -566.3)/609.49)}

Generalized Relationships σt = 0.01 CL1.5 CEC0.5 ψ0.5 σt = CL 0.5 CEC

σt is tensile strength CL is clay content CEC is cation exchange capacity ψ is suction

100 200 300 400 500 600 700 100 200 300 400 500 600 700 Three parameters relation ship Two parameters relationship Triaxial test results



t(Com)

t(Mes)

Development of Crack patterns over prolonged durations (due to loss of moisture)

Direct Images

Image Analysis

CW is average crack width CW max maximum crack width : volumetric moisture content s : volumetric moisture content at saturation a: soil dependent parameter

Image Analysis of the Crack

Laser Microscopy

3-D Laser microscopy OLS LEXT-4000 (Olympus, Japan)

Measurement of Tensile Stress in Thin Films

Experimental setup (Shinde, 2009)

Laser Source Position Sensitive Detector Clamped Silicon wafer Soil sample  X L2 L1 Deflection Angle

 

         



L tan t t L t t E σ

Laser 1 f s f f 3 s s f

X(t) (t) (t) 12 (t) t E (t)

r fo f

t t

 

fo f f

t t 1 S  

Computation of Tensile Stress

200 400 600 800 1000 1200 1400 1600 1 2 3 4 5 6 7 8

MTT4,6

W (mg) t (s)

Eo = 4.5710

• 6 g/s

tcr

500 1000 1500 2000 2500 1.2 1.6 2.0 2.4

MTT12,5

X (V) t (sec)

400 800 1200 4 8 12 16 20

MTT12,5

Stress (kPa) t (sec)

(806,13)

SEM images of wafer specimen (MT) after completion of test

50X 250 X 500X 2000 X

SEM Micrographs

Important Relationships σt = a·(Sf)b

10 20 30 40 50 60 70 80 90 100 50 100 150 200

Sf (%)

t (kPa)

1 2 3 4 5 6 7 8 9 10 10 20 30 40 50 60 70 80 90 100

L/S 5 6 7 8 9

R

2 = 0.85

t (kPa)

L/S

σt = c·(L/S)d

Self-healing/Self Sealing Minerals

Minerals which posses built-in ability to stop swelling/shrinking and cracking properties Intelligent minerals Their synthesis, characterization and application in various projects (related to Civil, Geotechnical Engineering, Concrete etc.) is a real challenge