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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Fri Aug 27, 2021 4:00 am Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
SBC OF SOILS  SETTLEMENT OF COHESION LESS SOILS
1.0 BASED ON STATIC CONE PENETRATION RESISTANCE Ckd value
A curve is drawn indicating resistance value Vs depth of soil layers.
For each layer the readings of static cone resistance show approx vertical curve points.
The Settlements is worked out for each such layer and added to arrive at final Settlement of cohesionless soil
Sf = 2.303 Ht /C * LOG10 {( Po + ∆P)/Po}
Compressibility Constant C = 3/2 * Ckd / Po
Ht is the thickness of each soil layer as per nearly vertical curve readings
Po  initial overburden pressure at center of soil layer
∆P  Pressure increaments 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Fri Aug 27, 2021 5:24 am Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
SBC OF COHESIONLESS SOILS
ALLOWABLE BEARING PRESSURE  SETTLEMENT CRITERIA
2.0 PLATE LOSD TEST
Plate load test can be conducted as per IS 1888 . At desired founding level in a pit , plate load test is carried on 30x30cm plate with equipment and loading platform.
Load vs settlement curve is created based on various readings of settlement Vs load on test plate ,square 30 cm
Settlement of footing Sf=
settlement of plate Sp*[ B /Bp *(Bp+0.3)/(B+0.3) ]^2
The value of plate settlement is calculated from above formula by putting the value of footing settlement permissible as per IS1904.
From the test curve drawn for Load intensity Vs test plate settlement , the allowable SBC is read against calculated plate settlement.
INITIAL STRAIGHT /NEARLY STRAIGHT LINE OF CURVE REPRESENT SEATING OF PLATE , TO BE DEDUCTED.
REFER IS 1888 , IS 8009 PART1 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Fri Aug 27, 2021 5:50 am Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
SBC OF SOILS ON SHALLOW FOUNDATIONS
DETERMINATION OF SETTLEMENTS  COHESIONLESS SOILS
As stated in earlier posting that for cohesionless soils , the Settlement is immediate on application of load / pressure , and due to difficulties in sampling these soils , there is no laboratory procedures for getting compressibility characteristics. The settlement is worked out based on semi empirical methods . Static cone penetration resistance and plate load test have been discussed.
The third method of detemination of SETTLEMENT OF COHESIONLESS SOILS is based on SPT. N VALUE.
Referring to FIGURE 9 of IS 8009 PART 1 , for given width of footing , settlement in Meters per unit (1 kg/cm2) pressure with various curves for N Values are given in figure.
The illustration has already been given thru a hypothetical data in earlier post. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Sat Aug 28, 2021 2:40 am Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
SBC COHESIVE SOILS
DETERMINATION OF SETTLEMENTS
The Settlement of cohesive soils comprise of two parts , one being immediate Settlement plus other being Time dependent Consolidation Settlement.
Sf = Si + Sc
Working out of settlement depends on nature of cohesive soil:
The cohesive soils are NORMALLY CONSOLIDATED which are not subject to pressure more than existing .
OVERCONSOLIDATED Cohesive soils are precomressed soils . These were subjected to pressures higher than existing ones.
Also the determination of settlement needs to be seen in various scenarios in which clay / cohesive soil exist.
(I) Cohesive soil layer sandwiched between layers of cohesionless soil Or cohesive soil sandwiched between cohesion soil at top and rock at bottom
(ii) Precompressed soils
(iii) cohesive soil at top over cohesionless soil. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Sat Aug 28, 2021 5:20 am Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
SBC  COHESIVE SOILS
DETERMINATION OF SETTLEMENT
1.0 COHESIVE LAYER SANDWICHED BETWEEN COHESIONLESS LAYER (TOP) & COHESIONLESS LAYER /ROCK AT BOTTOM
1.1 In case of non PRECOMRESSED CLAYS
Sf= Si + Sc
Si = 0 ( assumed in this scenario)
Sc = Soed= Ht/(1+eo) *Cc *LOG10 {(Po +∆P)/Po)}
Ht is thickness of clay layer
eo= void ratio
Cc Compression Index
Po = Initial existing pressure at clay layer
∆P = pressure increaments
Compression Index can be calculated from rational formulas :
Cc= 0.009(wL  10)
Cc = 0.3(eo  0.27)
1.2 In case of PRECOMPRESSED CLAYS
The value of Compressibility will be much lower
Sc = ∆p *mv * H
mv coefficient of volume compressibility
mv can be determined from consolidation test results for range of loadings, though approximate. But for exact determination of field value , Appendix C of IS 8009 P1 to be referred. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Sat Aug 28, 2021 6:45 am Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
SBC  COHESIVE SOILS
DETERMINATION OF SETTLEMENTS
2.0 CLAY LAYER RESTING ON COHESIONLESS SOIL/ ROCK
Sf= Si+Sc
This is not a case of sandwiched clay layer .
The consolidation settlement in such situation is determined as per following relation:
Sc= Soed * £
Soed = as per formula given in previous post
£ = It is a settlement factor , can be read from curves given at Fig 10 of IS 8009P1 against pore pressure parameter A .
In absence of Pore pr parameter , the values of £ are taken from Table of Code and are presented hereinbelow:
For sensitive Clays (soft alluvial , marine clays)    1.0 to 1.2
For Normally consolidated clays    0.7 to 1.0
For OVERCONSOLIDATED Clays    0.5 to 0.7
For HEAVILY CONSOLIDATED CLAYS    0.2 to 0.5
Immediate Settlement Si
Si = p B (1  mu^2)/E * I
Si  Immediate Settlement at center/corner of footing
mu  Poisson ratio , 0.5 for clay
I = INFLUENCE FACTOR depend on L/B of footing
Values of I are given in code as :
Circular footing :
At center 1.0 ; At edge 0.65 : Average I value = 0.85
Square footing
At center 1.12 ; At corner 0.56 Average I value =0.95
Rectangular footing : As per IS based on L/B ratio ranged from 1.5 to 100. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Sat Aug 28, 2021 9:42 am Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
SBC
SETTLEMENTS
For soils comprising of different layers of soils , the Settlement of each layer is computed separately and then summed up to arrive at total Settlement. For cohesionless layers methods discussed viz Static Cone penetration resistance , Plate load test, SPT N value are adopted . For Cohesive soil layers the discussed methods to be adopted to work out settlements.
CORRECTIONS  TO CALCULATED SETTLEMENTS
FOUNDATION DEPTH CORRECTION  AS PER DEPTH FACTORS GIVEN IN FIGURE OF IS8009P1.
RIGIDITY OF FOUNDATION FACTOR  The rigid foundations like beam type Raft , massive pier foundation, the calculated Settlement to be multiplied by RIGITY FACTOR 0.8 .
COURTESY IS8009 PART 1. 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Sat Aug 28, 2021 1:05 pm Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
MODULUS OF SUBGRADE REACTION
The modulus of subgrade reaction is very important soil/rock parameter.
MODULUS K = (LOAD/AREA) /( CORRESPONDING SETTLEMENT)
Generally for field tests on 75cm circular plate , pressure for 1.25mm Settlement is taken .
Then K = p/0.125 MPA/cm = 10p / 0.125 kg/cm2/cm
P is the pressure for a settlement of 1.25mm on plate
REFER IS 9214 FOR DETERMINATION OF K VALUE
USEFUL SOIL DATA (AS PER SP7, NBC OF INDIA)
COHESION LESS SOIL
LOOSE SOIL
RELATIVE DENSITY < 20%
SPT N VALUE (BLOWS/30CM) < 10
MODULUS OF SUBGRADE REACTION K 15000KN/m2 (DRY,MOIST)
MOD OF SUBGRADE REACTION K 9000 kN/m2 (Submerged state)
MEDIUM SOILS
RD 20% TO 70%
SPT N VALUE 10 TO 30
K (DRY/MOIST)  15000  47000 KN/m2
K (Submerged state)  9000  29000 kN/m2
DENSE SOILS
RD> 70%
SPT N VALUE >30
K (DRY/MOIST)  47000  180000 kN/m2
K (Submerged) 29000  108000 kN/m2
ALL ABOVE DATA IS FOR COHESION LESS SOILS
USEFUL SOIL DATA (AS PER SP7 )
COHESIVE SOILS
SOFT TO VERY SOFT SOILS
SPT N VALUE < 4
UNCONFINED COMPRESSIVE STRENGTH  NOT GIVEN , MUST BE SMALL
MODULUS OF SUBGRADE REACTION K  NOT GIVEN
MEDIUM SOILS
SPT N VALUE 4 TO 8
UNCONFINED COMPRESSIVE STRENGTH Not given , but seems < 100 kN/m2 , since stiff values are given in range 100  200 kN/m2.
K value  not given , but seem < 27000 kN/m2 looking at range of stiff values
STIFF SOILS
SPT N VALUE 8 TO 15
UCS 100  200 KN/m2
Modulus of subgrade reaction K value 27000 kN/m2
VERY STIFF SOILS
SPT N VALUE > 15
UCS 200  400 KN/m2
Mod of subgrade reaction K value 27000 to 54000 kN/m2
HARD SOILS
SPT N VALUE > 15
UCS > 400 KN/m2
Mod of subgrade reaction K 54000 to 108000 kN/m2
UCS VALUES AND K VALUES ARE ON SQ PLATE 300X300MM
AV LOAD INTENSITY TO BE LESS THAN (ULTIMATE BC/2)
FOR MORE INFO REFER SP7 CHAPTER 6 ANNEXURE B 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Sat Aug 28, 2021 2:42 pm Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
MODULUS OF SUBGRADE REACTION
EMPIRICAL FORMULA FOR CALCULATING MODULUS K
K= 0.65 {(Es*B^4)/(E I)}^(1/12) * Es/(1  u^2) * 1/B
Approximately K can be taken as
K = 0.65 * Es/(1  u^2) * 1/B
(1/12 the power term taken as nearly unity)
Es  Modulus of Elasticity of soil
E  Young's modulus of foundation material
u  Poisson ratio of soil
B  Foundations width 

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vikram.jeet General Sponsor
Joined: 26 Jan 2003 Posts: 3153

Posted: Sat Aug 28, 2021 3:12 pm Post subject: 


STRUCTURE AND GEOTECHNICAL COMBINE
MODULUS OF SUBGRADE REACTION
FOR RIGID FOUNDATIONS :
MODULUS K = ( Average contact pressure)/(average settlement )
The ultimate BC = SBC* FOS
MODULUS K =( SBC * FOS) /(PERMISSIBLE SETTLEMENT)
The formula given by Foundation Design book by BOWELS is based on this. 

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