# Geotech Formula

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Index and Classification Properties of Soils

Unit weight : = V

W

Dry unit weight : d = e+1

Gws

Moist unit weight : = dry ( 1 + )

Saturated unit weight: sat = e+1

)e+Gs(w

Zero air void unit weight:

zav = e+1

Gsw

Moisture content (water content)

= s

w

W

W

Degree of saturation : s = e

Gs

Porosity : n = e+1

e

Air void ratio = n (1 s)

where : W = total weight

V = volume

Gs = specific gravity

e = void ratio

w = unit weight of water

= 9.81 kN/m3 ( 1gram/cm3 )

Phases of Soil G.W.T. = ground water table

DRY

MOIST

SATURATED

H1

H2

H3

G.W.T.

GEOTECHNICAL 1

Relative density ( Density index )

Dr = minmax

max

e-e

e-e

or

Dr = min

min max

1/ -1/

1/ -1/

d d

d d

e = in-situ void ratio

emax = void ratio in loosest condition

emin = void ratio in densest condition

d = dry unit weight in the field

d(max) = dry unit weight in densest condition

d(min) = dry unit weight in loosest condition

Shrinkage limit : S.L.

S.L. = w

2

21

2

21 m

)V-V(-

m

)m-m( in %

m1 = initial mass in saturated state

m2 = final mass in dry state

V1 = initial volume in saturated state

V2 = final volume in dry state

Plasticity Index : P.I.

P.I. = L.L. - P.L.

L.L. = liquid limit

P.L. = plastic limit

Liquidity Index : L.I.

L.I. = .I.P

.L.P-

= in-situ moisture content

P.L. = plastic limit

P.I. = plasticity index

Consistency Index : C.I.

C.I. = .I.P-.L.L

-.L.L

Soil Characteristic Liquidity Index

brittle solid < 0

plastic < 1

liquid >1

Shrinkage Ratio : S.R.

S.R. = w2

2

V

m

Specific Gravity: Gs

Gs =

100

SL-

SR

1

1

SOIL CLASSIFICATION

U.S. Dept of Agriculture (USDA Method) Gravel Sand Silt Clay

> 2 mm 2 to 0.05mm 0.05 to 0.002mm < 0.002 mm

AASHTO Method Gravel Sand Silt Clay

76.2 to 2mm 2 to 0.075mm 0.075 to 0.002mm < 0.002 mm

Unified Soil Classification System Gravel Sand Fines (Silt & Clay)

76.2 to 4.75 mm 4.75 to 0.075 mm < 0.075 mm

GEOTECHNICAL 2

Particle Size Distribution

Effective Size, D10

- is the diameter in the particle size

distirbution curve corresponding to 10%

finer.

Uniformity coefficient, Cu

Cu = 10

60

D

D

Coefficient of gradation or curvature, Cc

Cc = 1060

2

30

DD

)D(

D30 = particle diameter corresponding to 30% finer

D60 = particle diameter corresponding to 60% finer

Sorting Coefficient, So

So = 25

75

D

D

D75 = particle diameter corresponding to 75% finer

D25 = particle diameter corresponding to 25% finer

AASHTO Classification System

Group Index, G.I. = (F 35)[ 0.2 + 0.005(LL 40)] + 0.01(F 15)(PI 10)

Partial Group Index = 0.01(F 15)(PI 10)

F = % passing sieve # 200

LL = liquid limit

PI = plasticity index

Group index must be whole and positive, if the

computed value is negative use zero. If the group

classification is A-2-6 & A-2-7 attached Partial

Group Index only.

SOIL COMPACTION

Compaction is the densification of soils by

the application of mechanical energy. It may

also involve a modification of the water

content as well as gradation of the soil.

Some methods for determining density of

soil in the field:

1. Sand Cone Method

2. Balloon Method

3. Oil Method

To determine the dry unit weight of

compaction in the field:

d = holetheofvolume

holethefromexcavatedweightunitdry

Volume of hole, V = c

( )

-Ws

d sand

W

Ws = weight of sand to fill the hole and cone

Wc = weight of sand to fill the cone only

Relative Compaction, R

R = ( )

(max -lab)

d field

d

d(field) = compacted field dry unit weight

d(max-lab) = maximum dry unit weight

determined in the laboratory by Proctor

Test

GEOTECHNICAL 3

PERMEABILITY, SEEPAGE of WATER

in SOIL

Coefficient of Permeability, k ( Laboratory Test)

Constant Head Test : k = thA

QL

Q = volume of water collected

A = area of cross section of soil specimen

t = duration of water collection

i = hydraulic gradient = L

h

Falling Head Test: k = 2

1

h

hln

tA

La

a = cross-sectional area of the stand pipe

A = cross-sectional area of soil specimen

t = duration of water collection

h1 = initial head when t1 = 0

h2 = final head when t2 = t

L = length of soil specimen

i = hydraulic gradient = L

hh21

Rate of water flow in soil, Q

Q = k i A

k = coefficient of permeability

i = hydraulic gradient

A = cross-sectinal area of soil sample

Discharge velocity, V

V = k i

Seepage velocity, Vs

Vs = n

V

n = porosity

Absolute Permeability, K

K = w

k

k = coefficient of permeability

= viscosity of water

w = unit weight of water

Transmissibilty of Soil Stratum, T

T = K b

K = average coefficient of permeability

b = thickness of aquifer

Equivalent Coefficient of Permeability in

Layered Soil:

Equivalent Horizontal Coeffient, KH(eq)

KH (eq) = H

1(k1H1 + k2H2 + + knHn)

Equivalent Vertical Coefficient, KV(eq)

KV(eq) =

n

n

2

21

k

H+...+

k

H+

k

H

H

k1

k2

H1

H2

GEOTECHNICAL 4

Vertical Stresses in Soil

1. Total stress at C : c = d h1 + sat h2

2. Pore water pressure at C : uc = w h2

3. Effective stress at C = Total stress Pore pressure

c = c uc

1. Total stress at C : c = d h1 + sat h2

2. Pore water pressure at C : uc = w h

3. Effective stress at C = Total stress Pore pressure

c = c uc

1. Total stress at C : c = d h1 + h2 B = d h1 2. Pore water pressure at C : uB = -S w h2 uc = 0

3. Effective stress at C = Total stress Pore pressure

c = c uc

B = B (uB)

Compressibility of Soil

Settlement the total vertical deformation at

the surface resulting from the load. When a soil

deposit is loaded (by a structure or a man-made

fill) deformation will occur. The movement may be

downward with an increase in load or upward

(called swelling) with a decrease in load.

Components of Settlement:

S = S1 + S2 + S3

S = total settlement

S1 = immediate or distortion settlement

S2 = primary consolidation settlement

S3 = secondary consolidation settlement

Primary Consolidation Settlement, S2 Normally Consolidated Clays

S2 = o

o

0

c

P

P+Plog

e+1

HC

Cc = compression index

eo = in-situ void ratio

Po = effective overburden pressure at the

middle of the clay layer

P = average increase of stress on clay layer

H = thickness of clay layer

G.W.T.

Dry, d

Saturated, sat

A

B

C

h1

h2

G.W.T.

Dry, d

Saturated, sat

A

B

C

h1

h2

h

G.W.T.

Dry, d

Moist,

Degree of saturation, S

A

B

C

h1

h2 zone of capillary rise

S

GEOTECHNICAL 5

Over Consolidated Clays

1. when Po + P < Pc

S2 = o

o

0

s

P

P+Plog

e+1

HC

Cs = swell index

= ranges from 1/5 to 1/10 of Cc

Pc = preconsolidation pressure

2. when Po + P > Pc

S2 = o

c

0

s

P

Plog

e+1

HC+

c

o

0

c

P

P+Plog

e+1

HC

Secondary Consolidation Settlement, S3

S3 = Ca H log 1

2

t