GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE...Inclinometer Piezometer Settlement gauge PVD up to mid depth of blanket Prof. J. N. Mandal, Department of Civil Engineering, IIT

GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE

Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]

Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay


Module - 9LECTURE - 46

Geosynthetics for ground improvement

Recap of previous lecture…..


Bearing capacity and settlement

Principles and functions of vertical drains

Basic principles of prefabricated vertical drains

Design charts without and with smear effect

DESIGN CHART WITH SMEAR EFFECT AND WELL RESISTANCE

Design Procedure:

Step 1: Determine the characteristics of soilCh = Coefficient of consolidation for horizontal drainagem2/sec or m2/month,Kh = Coefficient of horizontal permeability in undisturbed zone,(m/sec),Ks = Coefficient of permeability in the disturbed or smear zone(m/sec)

Step 2: Determine following drain propertiesdw = Diameter of the drain, m,ds = Diameter of the disturbed (smear) zone, m, andqw = Discharge capacity of the drain, m3/sec


Step 3: Determine the following consolidation requirementsU = Average degree of consolidation, and

t = Time required to achieve the degree of consolidation

Step 4: Determine factor for soil disturbance (Fs)

Assume, ds/dw = 2 to 8

Now, Fs can be determined from following Figure.

w

s

s

hs d

dln1KKF


Soil disturbance factor (Fs) versus Kh / Ks


Step 5: Determine factor for drain resistance (Fr)

w

hr q

KzLzF

z = Distance from the drainage end of the drain (m),

L = Length of the drain when drainage occurs at oneend only (Put ‘L/2’ replacing ‘L’ when drainage occursfrom both the ends)


Well resistance (Fr) versus Length (L) Prof. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Product of Ch and t versus time (t)

Step 6: Determine the required consolidation time, t inmonths and obtain product (Ch x t) from the following Figure.


Step 7: Determine the parameter, λ from the followingFigure based on the required degree of consolidation (U)and (Ch x t).

Product of Ch and time (t) versus Lambda (λ)Ch = Coefficient of consolidation in horizontal direction, t = Required time for consolidation


Step 8: Add Fs and Fr obtained from steps 4 and 5respectively.

Fsr = Fs + Fr

Determine the design diameter ‘D’ (in meter) from thefollowing Figure based on λ and appropriate Fsr for n = 20.

2h8 = D F1ln

1

C t

U

43

dDln

n1

43)nln(

n1n)n(F

w2

2

F = Fsr + F(n)

Fr = Well resistance factor Fs = Smear effect factorF(n) = Spacing factorn = spacing ratio


Design diameter, D (m) versus Lambda, λ for n = 20


Step 9: Find the spacing (S)

D = 1.05 S (For triangular pattern), and

D = 1.13 S (For square pattern)

The above design is developed by Mandal and Fulzele(2008).

The similar design charts are also developed byMandal and Shiv (1992) for prefabricated geocompositedrain.


Example:L = Length of the soil strata = 20 m, Kh/ Ks = 5,

Ch = 3.9 x 10-1 m2/month, ds/dw = 2, U = 80 %, t = 11 months

Determine the design diameter of the drain influence zone.Solution: Kh/Ks = 5, and ds/dw = 2

From Figure, Fs = 2.9.


L = 20 m, and Kh/qw = 0.001

Well resistance (Fr) versus Length (L)

From Figure, Fr = 0.9


Therefore, Fsr = Fs + Fr = 2.9 + 0.9 = 3.8

U = 80 % and Ch x t = (3.9 x 10-1) x 11 ≈ 4

From Figure, λ = 20


Fsr = 3.8 ≈ 4 and λ = 20

From Figure, design diameter of influence (D) = 1.8 m

D = 1.05 S (For triangular pattern), andD = 1.13 S (For square pattern)


1. Marking of Points 2. Installation of PVD 3. Sand Blanket4. Geotextile placement 5. Instruments Installation 6. Preloading stages

Instrumentation of embankment on soft sol with PVDsInstrumentation of embankment on soft sol with PVDs

PVD spacing

Inclinometer Piezometer Settlement gauge

PVD up to mid depth of blanket


Pneumatic Piezometer: To monitor the pore water pressureat the desired depth of the compressible soil

Inclinometer: To measure the lateral movement of the soilat the site slopes.

Settlement gauges: To measure the long term settlement atthe original ground surface of drainage blanket.

Settlement Plate: To monitor the settlement readings

Water stand pipe: To monitor the level of water in thecompressible soil layer either the Piezometers orsettlement gauges are installed immediately or during theinstallation of the vertical drains.


1

2

3

4

5

0 10 20 30

Time in days

Settl

emen

t (m

)Without PVD

32

10

10 100 1,000 10,000 100,000

With PVD

1

2

3

4

5

0 10 20 30

Time in days

Settl

emen

t (m

)Without PVD

32

10

10 100 1,000 10,000 100,000

With PVD

1

2

3

4

5

0 10 20 30

Time in days

Settl

emen

t (m

)

1

2

3

4

5

0 10 20 30

Time in days

Settl

emen

t (m

)Without PVD

32

10

10 100 1,000 10,000 100,000

With PVD

Settlement versus time curve without and with PVD


Installation of PVD:

The vertical drains are availabel in the form of roll.

The band drain is wrapped around a base plate or asteel bar at the bottom of the hollow steel lance.

The lance, band drain and steel rod will be pushedinto the soft cohesive soils to the desired depth.

When the band drain is reached to the desireddept, the lance is to be withdrawn keeping behind thesteel rod and band drains.


Installation of PVD drain (Photo Compliments of DBM Geotechnics)


Installation of PVD at JNPT for national HighwayAuthority of India

Installed 7,00,000 Rmt (Sohams Foundation)


Installation of PVD atJNPT for Maersk India Ltd

Installed PVD 8,50,000 Rmtin 40 Days Period(Sohams Foundation)


Cross section of mandrel anddrain (After Rixner et al., 1986)

Different base plates to maintainwick drains at the base of theinstallation lance


Band drains can be installed very fast. It isprefabricated, lighter as well as easy to transport andstore.

It is also economical with respect to the conventionalsand drains. The equipment is lighter than the rigs.

15 m long drains at spacing of 1 m to 2.5 m at aplacement rate of 375 m/hr is possible (Hausmann,1990).


Summary on band drain Geotechnical engineers generally faced problem toconstruct structures on very soft soil.

It is necessary to prevent excessive settlement as well asimprove the bearing capacity of soft soil. Therefore, groundimprovement techniques are needed.

Combination of preloading and installation of prefabricatedvertical band drains in the subsoil layers can serve thepurpose.

Prefabricated vertical drains accelerate the consolidationprocess as well as dissipate excess pore water pressurewithin very short period.


The PVD drains have many advantages comparedto the sand drains.

Rapid settlement of the soft soil is required forconstruction of embankments, oil tank foundations,landfills and underwater constructions.

PVD drains are efficient and most economical.


Example:

Compute time required for 60%, 70%, 80% and 95%consolidation at different spacing of PrefabricatedVertical Band Drain.

Size of PVD drain: 100 mm x 5 mm (TriangularPattern) Assume Ch = 6.27 x 10-6 m2/min


Solution:

Consolidation time,

a = 100 mm (width of band drain)b = 5 mm (thickness of band drain)

dw = equivalent diameter of PVD = 2(a + b)/

D = equivalent diameter of influence = 1.05 × S (forTriangular pattern)S = Drain Spacing (c/c between drains) (m)

Ch = 6.27 x 10-6 m2/min

U11ln

43

dwDln

C8Dt

h

2


l = Length of prefabricated vertical drain (m)


Spacing (m) Consolidation time, t (months) S (m) 60% (U) 70% (U) 80% (U) 95% (U)

2 5.029 6.608 8.833 16.441.71 3.484 4.577 6.119 11.391.43 2.246 2.951 3.944 7.342

1.333 1.899 2.495 3.336 6.2091.24 1.584 2.082 2.783 5.181.14 1.301 1.71 2.286 4.2551.05 1.049 1.378 1.842 3.43


Consolidation time versus spacing for Prefabricated vertical drain under different degree of consolidation


Example: Design steps for Prefabricated Vertical Drain(PVD) in a runwaySubsoil condition: Available borehole information showsthat very soft silty clays exist up to approximately 10 m to14 m below the existing ground level underlain by firm tostiff clays.

Standard penetration test value (N) within the very softlayer does not exceed 5, whereas it exceeds 10 in thefirm/stiff layer. The stratum is found to contain considerableorganic materials.

Area for the proposed runway extension is a low linearea where the ground water table has been observe closeto the ground level approximately 1.0 m to 1.5 m below theexisting ground level.


For the construction of runway and taxiway extension,the existing ground level is needed to be raised by fillingabout 2 meter to 3 meter.

After construction, considerable live loads are expecteddue to heavy aircraft movements. It is required to improvethe very soft sub-soil by accelerating its consolidation;thereby increasing its bearing capacity as well.

Most cost effective way to improve the underlying verysoft clay is installation of prefabricated vertical drains(PVD) or band drains and preloading.

The required drain spacing, time required forconsolidation etc. are designed based on the soilcharacteristics and period available for consolidation.


The pre-loading requirements consist of the following:

(a) Load from pavement

- Concrete- 0.45 m thick x 2.4 t/m3 = 1.1 t/m2

- GSB layer- 0.90m thick x 1.8 t/m2 = 1.7 t/m2

(b) Live load from aircraft on pavement (estimated) = 4.6 t/m2

Total load from aircraft at sub-grade level = 7.4 t/m2


Pavement cross-section


Solution:

Step 1: Calculate safe bearing capacity of soft soil stratum

. .c

uCNQF S

Qu = Safe bearing capacity (t/m2),C = Undrained cohesion (t/m2),Nc = Bearing capacity factor (5.7), F.S. = Factor of safety

Calculation of Safe Bearing Capacity

Input Data Values

1 Cohesion (T/sqm) 2.52 Bearing Capacity Factor 5.73 F.S. 3

CALCULATE


Value of Safe Bearing Capacity

Safe Bearing Capacity (T/sqm) 4.75

Back Go to Main

Safe bearing capacity of soil = 4.75 t/m2 < 7.4 t/m2.

Therefore, we have to go for ground improvement.


Step 2: Calculate the settlement of soft soil stratum usinggeneral consolidation theory

0

0 0

log1

cC H p pe p

δ = Settlement (m),Cc = Compression index of soil,H = Thickness of compressible clay layer (m),p0 = Effective overburden pressure at the middle ofcompressible clay layer (kPa),Δp = Increase in surcharge (kPa), ande0 = Initial void ratio of soil


Calculation of settlement

In put data Values

1 Thichkness of compressible clay layer (m) 102 Height of embankment (m) 4.353 Compression Index 0.2434 Initial void ratio 1.25 Bulk Unit weight of soil (T/cum) 1.76 Unit weight of embankment (T/cum) 1.8

CALCULATE Go to Main

Output value of settlement

563.497Total Settlement of compressible clay layer in mm


Step 3: Calculate the time required for total settlement(calculated in earlier step)

2v

v

T HtC

2

4vT U (For U ≲ 60%)

1.781 0.933log(100 %)vT U (For U > 60%)

t = Time to achieve required degree of consolidation (month),Tv = Time factor,H = Thickness of compressible clay layer in meter (H forsingle way drainage and H/2 for both way drainage),Cv = Coefficient of consolidation of soil in m2/month, andU = Degree of consolidation in %


General Consolidation Theory

Design Input

Vertical coefficient of consolidation (Cv) in m2/month 0.334

Average degree of consolidation (U) % 90

Thickness of the compressible layer (H) in meter 10

CALCULATE

General Consolidation Theory

Design Output

Time required for the given degree of consolidation (years) 21.167

Time Factor (Tv) 0.848

Time required to attain 90% consolidation without anyground improvement is 21.167 years.


Step 4: Calculate the time required for particular degreeof consolidation using PVDs

2 2

2

ln / 3 ( / ) 1ln8 4 11 /h

D dD d DtC Ud D

t = Time required for the given degree of consolidation,D = Effective diameter of influence for the PVD

=1.13 S (for square pattern),= 1.05 S (for triangular pattern)

S = Spacing of PVDsd = Equivalent diameter of the PVD

2( )b td

b =Width of the PVD, t = Thickness of the PVD,

Ch = Horizontal coefficient of consolidationProf. J. N. Mandal, Department of Civil Engineering, IIT Bombay

Designing with Prefabricated Vertical Drains

Design Input

Horizontal coefficient of consolidation (Ch) in m2/month 6.7E-01

Average degree of consolidation (U) % 90

Width of the PVD (b) in mm 100

Thickness of the PVD (t) in mm 4

PVD Spacing (s) in m 1.2

CALCULATE


1.52

1.81(With Square Pattern)

1. For Triangle Pattern =1.05 × Spacing 1.262. For square Pattern =1.13 × Spacing 1.356

Effective Diameter of influence of PVD (D)

Time required for the given degree of consolidation with PVD (month)(With Triangular Pattern)

Time required for the given degree of consolidation with PVD (month)

Back MAIN SCREEN

Go for Stage loading

In triangular pattern, time required for 90% consolidationwith PVD is 1.52 months i.e. 45 days

In square pattern, the time required for 90%consolidation with PVD and 1.81 month i.e. 55 days


Step 5: Calculate the required height of embankment thatcan be safely placed on soft soil stratum

usafe

Soil

Qh

hsafe = Height of embankment that can be safely placed on soft soil stratum in meter,Qu = Safe bearing capacity of soil = 4.75 t/m2, and γsoil = Unit weight of compressible soil = 1.7 t/m3

m37.175.4hsafe


Step 6: Decide the number of stages and days required forapplication of embankment loading in such a manner thatthe total number of days (D) should be greater than thedays required for desired settlement using the PVDs.Step 7: Determine the improvement in initial cohesion and thus in shear strength in each stage

100%U.(%)I.P0045.015.0

pc )increased(u

)increased(u)initial(u)final(u ccc

Cu(increased) = Increment in initial cohesion,

Cu(final) = Cohesion after the improvement

p100

%U.(%)I.P0045.015.0cc )initial(u)final(u


Cu(initial) = 2.5 t/ m2

U% = Desired degree of consolidation = 91%, 55%, 33%P.I. = Plasticity Index of soil = 27%, andΔp = Increase in overburden pressure

= height of fill x fill density, fill density = 1.8 t/m3

Improvement in cohesion due to consolidation in stage I,

Improvement in cohesion due to consolidation in stage II,

Improvement in cohesion due to consolidation in stage III,

Δp = (3 x 1.8) t/ m2

Δp = (4 x 1.8) t/ m2

Δp = (5 x 1.8) t/ m2

2)final(u m/t83.3)8.13(

10091270045.015.05.2c

2)final(u m/t9.4)8.14(

10055270045.015.083.3c

2)final(u m/t7.5)8.15(

10033270045.015.09.4c


Please let us hear from you

Any question?


Prof. J. N. Mandal

Department of civil engineering, IIT Bombay, Powai , Mumbai 400076, India. Tel.022-25767328email: [email protected]


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GEOSYNTHETICS ENGINEERING: IN THEORY AND PRACTICE...Inclinometer Piezometer Settlement gauge PVD up to mid depth of blanket Prof. J. N. Mandal, Department of Civil Engineering, IIT