University:West Bengal University of Technology
Course: B.Tech Civil Engineering
Subject : Narula soil solve
Year of Question Paper : 2012
Group b
Difference between shallow foundation and deep foundation:
A foundation is shallow if its depth is equal to or less than its width and in case of deep foundation the depth is equal to or greater than the width.
Spread footing, strap footing, combined footing and mat/raft footing are the different types of shallow foundation and pier foundation, pile foundation and well foundation are the different types of deep foundation.
Difference between gross pressure and net pressure:
How is the effective overburden pressure for partly submerged soil determined?
Ans: effective pressure = σ ’ = σ-ua + χ (ua+uw)
Where,
Ua = pore air pressure
Uw= pore water pressure
χ = factor of unit cross section area occupied by water Aw/A
Aw= area of water
A= area of cross section of soil
Terzaghi’s genral equation:
Qu= CNC+ϒDNq+.5ϒBNϒ
Where,
C= cohesion
Nc= bearing capacity factor depending on angle of internal friction
Nq= bearing capacity factor depending on angle of internal friction
Nϒ= bearing capacity factor depending on angle of internal friction
ϒ= bulk density/ unit weight
D= depth of the footing
B= width of the footing
Qu= ultimate bearing capacity
Assumption:
Soil is homogenous and isotropic and it’s shear strength is represented by coloumb’s equation.
Strip footing has a rough base and the problem is essentially two dimensional
Elastic zone has straight boundaries inclined at ψ=φ to the horizontal and the plastic zones fully developed
Pp consists of 3 components which can be calculated separately and added although the critical surface of these components are not identical
The shear resistance of soil above the base is considered equivalent to a surcharge σ=ϒD
Problem
D=1
B=1.5
qnu= 1.3C’NC’+ ϒD(Nq’ -1)+.4 ϒBN ϒ’
=1.3*10.13*11.8+17.8*1*(3.8-1)+.4*17.8*1.5*1.3=219.12KN/m2
qns=(qnu/Fs)+ ϒ D=(219.12/4)+17.8*1=72.58KN/m2
Gross load=qs*area=72.58*1.52=163.31KN
ϒ= 17.8KN/m3
C= 15.2 kn/m2
Φ=20 degree
NC=17.7
Nϒ= 5.0
Nq=7.4
FS= 4
NC’=11.8
Nϒ’= 1.3
Nq’=3.8
C’=2C/3=2*15.2/3=10.13
Discuss spacing of boreholes with special reference to the provisions made in the relevant I.S code:
Caisson: The term caisson is derived from French word caisse meaning a chest or box. Caisson has come to mean boxlike structure, round or rectangular which is sunk from the surface of either land or water to some desired depth.
Types:
Box cassions
Open caissons
Peneumatic caissons
Well shape types:
Single circular
Twin circular
Dumb well
Double D
Twin hexagonal
Twin octagonal
Rectangular
Problem
Standard penetration test: The test is performed in a clean hole, 55-150 mm in diameter. A casing or drilling mud is used to support the sides of the hole. A thick wall split tube sampler 50.8mm OD and 35mm ID is driven in to the undisturbed soil at the bottom of the hole under the blows of a 63.5kg drive weight 75cm free fall. The minimum open length of the sample should be 60cm.
Correction:
Correction for overburden: The N value for cohesion less soil shall be corrected for overburden to get the corrected value No.
No= Cn*N , where Cn= normalizing correction factor
Correction due to dilatency/ submergence: The values NO obtained after applying overburden correction is corrected further for dilatancy if the stratum consist of fine sand and silt below water table, for values of N>15 using Ne=15+(NO-15)
Sampling of soil:
Disturbed sampling: disturbed sampling can be obtained by direct excavations, augers and thick wall samplers. For sampling saturated cohesion less soils a trap valve or a spring retainer is inserted in the drive shoe.
Un-disturbed sampling: undisturbed samples may be required for tests, consolidation and permeability. They can also be used for other test like the disturbed samples. Undisturbed samples are obtained by forcing a thin wall sampler in to the soil at the bottom of the bore hole or in a test pit. The penetration of the sampler into the soil should be continuous and rapid.
Pic punmina 865
Rock core sampling:
Group c
11.
a. sub soil exploration program:
Open excavations: test pits and trenches can be used for all types of soils. Soils can be inspected in their natural condition and samples, disturbed or undisturbed can be conveniently taken.
Boring: the methods of boring or drilling are auger and shell, wash boring, percussion boring and rotary boring.
Sub-surface soundings: sub surface includes the term sampling. Sampling are of two types, disturbed and undisturbed. Sounding tests and penetrations test are also part of this phase. Standard penetration test and Dutch cone test are under this phase.
Geographical method: the geographical methods of exploration were developed in connection with prospecting for useful minerals and oils. The major methods are gravitational, magnetic, seismic and electrical resistivity.
difference between disturbed and undisturbed sample:
Disturbed sample Undisturbed sample
Obtained by direct excavations, augars and thick wall samplers. Obtained by forcing a thin wall sampler.
Sampler’s area ratio> 10%-25% Sampler’s area ratio < 10%-25%
A disturbed sample is that in which natural structure of soils get partly or fully modified and destroyed. A undisturbed sample is that in which the natural structure and properties remain preserved.
Mode of operation :Open drive Mode of operation: stationary piston and rotary but open drive can also be used.
Correction of N value:
Correction for overburden: The N value for cohesion less soil shall be corrected for overburden to get the corrected value No.
No= Cn*N , where Cn= normalizing correction factor
Correction due to dilatancy/ submergence: The values NO obtained after applying overburden correction is corrected further for dilatancy if the stratum consist of fine sand and silt below water table, for values of N>15 using Ne=15+(NO-15)
Draw a typical cutting edge of a sampler and explain inside clearance, outside clearance and area ratio:
The area ratio should be as low as possible. It should not be greater than 25% for soft sensitive soil; it should preferably not exceed 10%. The inside clearance should lie between 1%-3%. The outside clearance should be not much greater than the inside clearance.
Area ratio = (D22-D12)/D12 *100 %
Inside clearance= D3-D1/D1 *100 %
Outside clearance= D2-D4/D4 *100 %
D4
D3
D2
D1
10.
Problem:
ϒsat =(G+e)/(1+e) * ϒw= (2.65+.6)/(1+.6) *9.81= 19.33kn/m3
ϒ’ = 19.33-9.81= 10.12kn/m2
qnf=C NC+ σ(Nq-1)+.5B ϒ’N ϒ
= C NC+ ϒD(Nq-1)+.5B ϒ’N ϒ [water table at the base of the footing, σ=ϒD]
Let us use IS code method,
Nq = tan2(45o+φ/2)eπtanφ= tan2(45o+36o/2)eπtan36 =37.75
Nc= (Nq-1)cot φ = (37.75-1) cot36=50.58
Nϒ= 2(Nq+1)tanφ=56.31
qnf=0+16*1(37.75-1)+.5B*10.12*56.31=588+284.9B
qs= qnf/F+ ϒD=(588+284.9B)/3+16*1=212+95B
Actual load intensity= q=1000/(B*1)=1000/b m2
So, 1000/B=212+95B or, B2+2.23B-10.53=0
So, B≈2.32m
Correction of penetration test:
Correction for overburden: The N value for cohesion less soil shall be corrected for overburden to get the corrected value No.
No= Cn*N , where Cn= normalizing correction factor
Correction due to dilatency/ submergence: The values NO obtained after applying overburden correction is corrected further for dilatancy if the stratum consist of fine sand and silt below water table, for values of N>15 using Ne=15+(NO-15)
12.
Failure of soil under a foundation with neat sketches:
Mode of failure Condition
General shear failure Footings on the ground or at shallow depth, in very dense sand.
Footing on saturated, normally consolidated clay, under undrained loading.
Punching shear failure Very deep footing in dense sand.
Footing on surface or shallow depth in loose sand.
Footing on saturated, normally consolidated clay under drained loading.
Local shear failure Footing on ground surface or at shallow depth in soils of high compressibility.
Footing at ground surface or at shallow depth in sands of relative density between 0.3 to 0.7.
Footing at great depth in sand of relative density between 0.7 to 0.9.
Ultimate bearing capacity (qf): the ultimate bearing capacity is defined as the minimum gross pressure at the base of the foundation at which the soil fails in shear.
Net ultimate bearing capacity (qnf): it is the minimum net pressure intensity causing shear failure of soil. qf = qnf±σ , σ= effective surcharge at the base level of the foundation.
Safe bearing capacity (qs): the maximum pressure which the soil can carry safely without risk of shear failure is called the safe bearing capacity. qs = qnf/F +ϒD.
How bearing capacity of footing on layered soils is considered:
13.
Problem
14.
a. how pile capacities are estimated by static and dynamic formulae as per as IS code recommendation in clayey soils?
Static formulae: the static formula are based on assumption that the ultimate bearing capacity Qup of a pile is the sum of the total ultimate skin friction Rf and total ultimate point or end bearing resistance Rp.
Qup = Rf + Rp or, Qup = As*rf+Ap*rp
Where,
As= surface area of pile upon which the skin friction acts
Ap= area of cross section of pile on which bearing resistance acts.
rf= average skin friction
rp= unit point or toe resistence
For cohesive soil rf= αc or mc
Rp=CpNc=9Cp
Qup=mcAs+9CpAp
Where,
M or α =adhesion coefficient
c= average undrained cohesion along the length of pile
Cp= average undrained cohesion of soil at the pile tip
For clayey soil N value <4 and value of α is 0.7 and 1.0 for bored piles and drives cast in situ piles respectively.
Allowable load =Qa=Qup/F where F= factor of safety=2.5 or 3
The IS code recommends that for working out safe load a minimum factor of safety of 2.5 should be used.
Dynamic formulae: according to IS code recommendation Hiley’s formula is used as dynamic formula. IS:2911(PART 1)-1964 gives the formula based on Hiley’s expression.
Qf=(ƞhWHƞb)/(S+.5C)
Where,
Qf = ultimate load on pile
W= weight of hammer in kg
H= height of drop of hammer in cm
S= penetration or set in cm per blow
C= total elastic compression = C1+C2+C3
C1+C2+C3= temporary elastic compression of dolly and packing, pile and soil respectively
ƞb =efficiency of hammer blow= (W+e2P)/(W+P)
ƞh= efficiency of hammer = (W+e2P)/(W+P) – {(W-eP)/(W+P)}2
P= weight of pile helmet
e= coefficient of restitution.
C1=.006m, C3=.025m
C1=1.7(Qu/Ap)=.006 or, Qu=.0000528
Ap= (π/4)*(.3/2)2
C2=QuL/Ap = .0000528*10/.0176 =.03
C= .006+.025+.03=.061
S= 12.5/6=2.083mm=.002m
W<Ep so ƞb =(W+e2P)/(W+P)=3.395
Where, W=30 , P=18
b.
problem
Qu=(ƞhWHƞb)/(S+.5C)
=( 70*30*3.395*.91)/(.002+.5*.061)= 199626kn
Safe pile load =Qs=Qu/Fs= 199626/2.5
= 79850.4kn [ assume Fs =2.5]
c.
Sinking of well foundation and the situations where such types of foundations are used:
Laying the well curb: if the river bed is dry laying of well curb presents no difficulty. In such a case excavation up to half a metre above subsoil water level is carried out and the well curb is laid. The inside shuttering of the curb is generally made of brick masonary built to proper profile is preferable. The outer shuttering is made of wood/steel. All connecting in the well curb should be done in one continuous operation.
Masonary in well steining: the well steining should be buit in initial short height of about 2m only. It is absolutely essential that the well steining is built in one straight line from bottom to top. Steining should not be allowed to be built more than 1m at a time. After sinking of one stage is complete all the damged portions of the steining at the top of the first stage should be repaired properly before masonary in the next stage is started.
Sinking operations: a well is ready to be set in after having cast the curb and having built first short stage of masonary over it. The well is sunk by excavating material from inside under the curb. In the initial stage of sinking the well is unstable and progress can be very rapid with only little material being excavated out.
Tilts and shifts: the primary aim in well sinking is to sink them straight and at the correct position. Suitable precautions should be taken to avoid tilts and shifts. Also proper record of tilts and shifts should be maintained and measure should be taken to counter act tilts and shifts.
Forces for which well foundation is designed:
Bracking and tractive effort of the moving vehicles
Force on account of resistance of the footing of the bearings
Water current force
Wind forces
Seismic forces
Earth pressure
Centrifugal forces
15.
a.
How SCPT & DCPT correlated with SPT: the cone test is considered very useful in determining the bearing capacity of pits in cohesionless soils, particularly in fine sands of varying density. The cone resistance qc is approximately equal to 5 to 10 times the penetration resistance N.
Problem
Total over burden pressure = 127kn/m2
So Cn= .9 [ approx according to Robertson and companella chart]
So qcn= qc*Cn =.9*8.8=9.77
For saturated silty soil qc/N=2 so N=9.77/2=4.89
As N=4.89 from the value N and Empirical correlation for cohesive soil unconfined compressive strength=5t/m2
b.
Vane shear test: vane shear test is a quick test used either in the laboratory or in the field to determine the undrained strength of cohesive soil.
Problem
T=35knm=35000knmm
H=100mm and d=50mm
T= πd2τf[H/2+d/6] or, 35000=π 502 τf[100/2+50/6] or, τf= 35000/(2500 π*58.33) =.076kn/mm2=C
Again, T= 5knm so τf= 5000/(2500 π*58.33)= .0109kn/mm2
So sensitivity = .076/.0109=6.97
Course: B.Tech Civil Engineering
Subject : Narula soil solve
Year of Question Paper : 2012
Group b
Difference between shallow foundation and deep foundation:
A foundation is shallow if its depth is equal to or less than its width and in case of deep foundation the depth is equal to or greater than the width.
Spread footing, strap footing, combined footing and mat/raft footing are the different types of shallow foundation and pier foundation, pile foundation and well foundation are the different types of deep foundation.
Difference between gross pressure and net pressure:
How is the effective overburden pressure for partly submerged soil determined?
Ans: effective pressure = σ ’ = σ-ua + χ (ua+uw)
Where,
Ua = pore air pressure
Uw= pore water pressure
χ = factor of unit cross section area occupied by water Aw/A
Aw= area of water
A= area of cross section of soil
Terzaghi’s genral equation:
Qu= CNC+ϒDNq+.5ϒBNϒ
Where,
C= cohesion
Nc= bearing capacity factor depending on angle of internal friction
Nq= bearing capacity factor depending on angle of internal friction
Nϒ= bearing capacity factor depending on angle of internal friction
ϒ= bulk density/ unit weight
D= depth of the footing
B= width of the footing
Qu= ultimate bearing capacity
Assumption:
Soil is homogenous and isotropic and it’s shear strength is represented by coloumb’s equation.
Strip footing has a rough base and the problem is essentially two dimensional
Elastic zone has straight boundaries inclined at ψ=φ to the horizontal and the plastic zones fully developed
Pp consists of 3 components which can be calculated separately and added although the critical surface of these components are not identical
The shear resistance of soil above the base is considered equivalent to a surcharge σ=ϒD
Problem
D=1
B=1.5
qnu= 1.3C’NC’+ ϒD(Nq’ -1)+.4 ϒBN ϒ’
=1.3*10.13*11.8+17.8*1*(3.8-1)+.4*17.8*1.5*1.3=219.12KN/m2
qns=(qnu/Fs)+ ϒ D=(219.12/4)+17.8*1=72.58KN/m2
Gross load=qs*area=72.58*1.52=163.31KN
ϒ= 17.8KN/m3
C= 15.2 kn/m2
Φ=20 degree
NC=17.7
Nϒ= 5.0
Nq=7.4
FS= 4
NC’=11.8
Nϒ’= 1.3
Nq’=3.8
C’=2C/3=2*15.2/3=10.13
Discuss spacing of boreholes with special reference to the provisions made in the relevant I.S code:
Caisson: The term caisson is derived from French word caisse meaning a chest or box. Caisson has come to mean boxlike structure, round or rectangular which is sunk from the surface of either land or water to some desired depth.
Types:
Box cassions
Open caissons
Peneumatic caissons
Well shape types:
Single circular
Twin circular
Dumb well
Double D
Twin hexagonal
Twin octagonal
Rectangular
Problem
Standard penetration test: The test is performed in a clean hole, 55-150 mm in diameter. A casing or drilling mud is used to support the sides of the hole. A thick wall split tube sampler 50.8mm OD and 35mm ID is driven in to the undisturbed soil at the bottom of the hole under the blows of a 63.5kg drive weight 75cm free fall. The minimum open length of the sample should be 60cm.
Correction:
Correction for overburden: The N value for cohesion less soil shall be corrected for overburden to get the corrected value No.
No= Cn*N , where Cn= normalizing correction factor
Correction due to dilatency/ submergence: The values NO obtained after applying overburden correction is corrected further for dilatancy if the stratum consist of fine sand and silt below water table, for values of N>15 using Ne=15+(NO-15)
Sampling of soil:
Disturbed sampling: disturbed sampling can be obtained by direct excavations, augers and thick wall samplers. For sampling saturated cohesion less soils a trap valve or a spring retainer is inserted in the drive shoe.
Un-disturbed sampling: undisturbed samples may be required for tests, consolidation and permeability. They can also be used for other test like the disturbed samples. Undisturbed samples are obtained by forcing a thin wall sampler in to the soil at the bottom of the bore hole or in a test pit. The penetration of the sampler into the soil should be continuous and rapid.
Pic punmina 865
Rock core sampling:
Group c
11.
a. sub soil exploration program:
Open excavations: test pits and trenches can be used for all types of soils. Soils can be inspected in their natural condition and samples, disturbed or undisturbed can be conveniently taken.
Boring: the methods of boring or drilling are auger and shell, wash boring, percussion boring and rotary boring.
Sub-surface soundings: sub surface includes the term sampling. Sampling are of two types, disturbed and undisturbed. Sounding tests and penetrations test are also part of this phase. Standard penetration test and Dutch cone test are under this phase.
Geographical method: the geographical methods of exploration were developed in connection with prospecting for useful minerals and oils. The major methods are gravitational, magnetic, seismic and electrical resistivity.
difference between disturbed and undisturbed sample:
Disturbed sample Undisturbed sample
Obtained by direct excavations, augars and thick wall samplers. Obtained by forcing a thin wall sampler.
Sampler’s area ratio> 10%-25% Sampler’s area ratio < 10%-25%
A disturbed sample is that in which natural structure of soils get partly or fully modified and destroyed. A undisturbed sample is that in which the natural structure and properties remain preserved.
Mode of operation :Open drive Mode of operation: stationary piston and rotary but open drive can also be used.
Correction of N value:
Correction for overburden: The N value for cohesion less soil shall be corrected for overburden to get the corrected value No.
No= Cn*N , where Cn= normalizing correction factor
Correction due to dilatancy/ submergence: The values NO obtained after applying overburden correction is corrected further for dilatancy if the stratum consist of fine sand and silt below water table, for values of N>15 using Ne=15+(NO-15)
Draw a typical cutting edge of a sampler and explain inside clearance, outside clearance and area ratio:
The area ratio should be as low as possible. It should not be greater than 25% for soft sensitive soil; it should preferably not exceed 10%. The inside clearance should lie between 1%-3%. The outside clearance should be not much greater than the inside clearance.
Area ratio = (D22-D12)/D12 *100 %
Inside clearance= D3-D1/D1 *100 %
Outside clearance= D2-D4/D4 *100 %
D4
D3
D2
D1
10.
Problem:
ϒsat =(G+e)/(1+e) * ϒw= (2.65+.6)/(1+.6) *9.81= 19.33kn/m3
ϒ’ = 19.33-9.81= 10.12kn/m2
qnf=C NC+ σ(Nq-1)+.5B ϒ’N ϒ
= C NC+ ϒD(Nq-1)+.5B ϒ’N ϒ [water table at the base of the footing, σ=ϒD]
Let us use IS code method,
Nq = tan2(45o+φ/2)eπtanφ= tan2(45o+36o/2)eπtan36 =37.75
Nc= (Nq-1)cot φ = (37.75-1) cot36=50.58
Nϒ= 2(Nq+1)tanφ=56.31
qnf=0+16*1(37.75-1)+.5B*10.12*56.31=588+284.9B
qs= qnf/F+ ϒD=(588+284.9B)/3+16*1=212+95B
Actual load intensity= q=1000/(B*1)=1000/b m2
So, 1000/B=212+95B or, B2+2.23B-10.53=0
So, B≈2.32m
Correction of penetration test:
Correction for overburden: The N value for cohesion less soil shall be corrected for overburden to get the corrected value No.
No= Cn*N , where Cn= normalizing correction factor
Correction due to dilatency/ submergence: The values NO obtained after applying overburden correction is corrected further for dilatancy if the stratum consist of fine sand and silt below water table, for values of N>15 using Ne=15+(NO-15)
12.
Failure of soil under a foundation with neat sketches:
Mode of failure Condition
General shear failure Footings on the ground or at shallow depth, in very dense sand.
Footing on saturated, normally consolidated clay, under undrained loading.
Punching shear failure Very deep footing in dense sand.
Footing on surface or shallow depth in loose sand.
Footing on saturated, normally consolidated clay under drained loading.
Local shear failure Footing on ground surface or at shallow depth in soils of high compressibility.
Footing at ground surface or at shallow depth in sands of relative density between 0.3 to 0.7.
Footing at great depth in sand of relative density between 0.7 to 0.9.
Ultimate bearing capacity (qf): the ultimate bearing capacity is defined as the minimum gross pressure at the base of the foundation at which the soil fails in shear.
Net ultimate bearing capacity (qnf): it is the minimum net pressure intensity causing shear failure of soil. qf = qnf±σ , σ= effective surcharge at the base level of the foundation.
Safe bearing capacity (qs): the maximum pressure which the soil can carry safely without risk of shear failure is called the safe bearing capacity. qs = qnf/F +ϒD.
How bearing capacity of footing on layered soils is considered:
13.
Problem
14.
a. how pile capacities are estimated by static and dynamic formulae as per as IS code recommendation in clayey soils?
Static formulae: the static formula are based on assumption that the ultimate bearing capacity Qup of a pile is the sum of the total ultimate skin friction Rf and total ultimate point or end bearing resistance Rp.
Qup = Rf + Rp or, Qup = As*rf+Ap*rp
Where,
As= surface area of pile upon which the skin friction acts
Ap= area of cross section of pile on which bearing resistance acts.
rf= average skin friction
rp= unit point or toe resistence
For cohesive soil rf= αc or mc
Rp=CpNc=9Cp
Qup=mcAs+9CpAp
Where,
M or α =adhesion coefficient
c= average undrained cohesion along the length of pile
Cp= average undrained cohesion of soil at the pile tip
For clayey soil N value <4 and value of α is 0.7 and 1.0 for bored piles and drives cast in situ piles respectively.
Allowable load =Qa=Qup/F where F= factor of safety=2.5 or 3
The IS code recommends that for working out safe load a minimum factor of safety of 2.5 should be used.
Dynamic formulae: according to IS code recommendation Hiley’s formula is used as dynamic formula. IS:2911(PART 1)-1964 gives the formula based on Hiley’s expression.
Qf=(ƞhWHƞb)/(S+.5C)
Where,
Qf = ultimate load on pile
W= weight of hammer in kg
H= height of drop of hammer in cm
S= penetration or set in cm per blow
C= total elastic compression = C1+C2+C3
C1+C2+C3= temporary elastic compression of dolly and packing, pile and soil respectively
ƞb =efficiency of hammer blow= (W+e2P)/(W+P)
ƞh= efficiency of hammer = (W+e2P)/(W+P) – {(W-eP)/(W+P)}2
P= weight of pile helmet
e= coefficient of restitution.
C1=.006m, C3=.025m
C1=1.7(Qu/Ap)=.006 or, Qu=.0000528
Ap= (π/4)*(.3/2)2
C2=QuL/Ap = .0000528*10/.0176 =.03
C= .006+.025+.03=.061
S= 12.5/6=2.083mm=.002m
W<Ep so ƞb =(W+e2P)/(W+P)=3.395
Where, W=30 , P=18
b.
problem
Qu=(ƞhWHƞb)/(S+.5C)
=( 70*30*3.395*.91)/(.002+.5*.061)= 199626kn
Safe pile load =Qs=Qu/Fs= 199626/2.5
= 79850.4kn [ assume Fs =2.5]
c.
Sinking of well foundation and the situations where such types of foundations are used:
Laying the well curb: if the river bed is dry laying of well curb presents no difficulty. In such a case excavation up to half a metre above subsoil water level is carried out and the well curb is laid. The inside shuttering of the curb is generally made of brick masonary built to proper profile is preferable. The outer shuttering is made of wood/steel. All connecting in the well curb should be done in one continuous operation.
Masonary in well steining: the well steining should be buit in initial short height of about 2m only. It is absolutely essential that the well steining is built in one straight line from bottom to top. Steining should not be allowed to be built more than 1m at a time. After sinking of one stage is complete all the damged portions of the steining at the top of the first stage should be repaired properly before masonary in the next stage is started.
Sinking operations: a well is ready to be set in after having cast the curb and having built first short stage of masonary over it. The well is sunk by excavating material from inside under the curb. In the initial stage of sinking the well is unstable and progress can be very rapid with only little material being excavated out.
Tilts and shifts: the primary aim in well sinking is to sink them straight and at the correct position. Suitable precautions should be taken to avoid tilts and shifts. Also proper record of tilts and shifts should be maintained and measure should be taken to counter act tilts and shifts.
Forces for which well foundation is designed:
Bracking and tractive effort of the moving vehicles
Force on account of resistance of the footing of the bearings
Water current force
Wind forces
Seismic forces
Earth pressure
Centrifugal forces
15.
a.
How SCPT & DCPT correlated with SPT: the cone test is considered very useful in determining the bearing capacity of pits in cohesionless soils, particularly in fine sands of varying density. The cone resistance qc is approximately equal to 5 to 10 times the penetration resistance N.
Problem
Total over burden pressure = 127kn/m2
So Cn= .9 [ approx according to Robertson and companella chart]
So qcn= qc*Cn =.9*8.8=9.77
For saturated silty soil qc/N=2 so N=9.77/2=4.89
As N=4.89 from the value N and Empirical correlation for cohesive soil unconfined compressive strength=5t/m2
b.
Vane shear test: vane shear test is a quick test used either in the laboratory or in the field to determine the undrained strength of cohesive soil.
Problem
T=35knm=35000knmm
H=100mm and d=50mm
T= πd2τf[H/2+d/6] or, 35000=π 502 τf[100/2+50/6] or, τf= 35000/(2500 π*58.33) =.076kn/mm2=C
Again, T= 5knm so τf= 5000/(2500 π*58.33)= .0109kn/mm2
So sensitivity = .076/.0109=6.97
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