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vikram.jeet
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PostPosted: Fri Aug 27, 2021 4:00 am    Post subject: Reply with quote

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
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PostPosted: Fri Aug 27, 2021 5:24 am    Post subject: Reply with quote

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
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PostPosted: Fri Aug 27, 2021 5:50 am    Post subject: Reply with quote

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
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PostPosted: Sat Aug 28, 2021 2:40 am    Post subject: Reply with quote

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
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PostPosted: Sat Aug 28, 2021 5:20 am    Post subject: Reply with quote

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
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PostPosted: Sat Aug 28, 2021 6:45 am    Post subject: Reply with quote

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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PostPosted: Sat Aug 28, 2021 9:42 am    Post subject: Reply with quote

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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PostPosted: Sat Aug 28, 2021 1:05 pm    Post subject: Reply with quote

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 SP-7, 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 SP-7 )

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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PostPosted: Sat Aug 28, 2021 2:42 pm    Post subject: Reply with quote

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
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PostPosted: Sat Aug 28, 2021 3:12 pm    Post subject: Reply with quote

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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