11) Allowable Bearing Pressure in Sand Based on Settlement ...

Geotechnical Engineering

SNU Geotechnical and Geoenvironmental Engineering Lab.

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11) Allowable Bearing Pressure in Sand Based on Settlement

Consideration

� Meyerhof �

For B ≤ 1.22m,

6012

)( )(98.11)/( NmkNq allnet =

For B > 1.22m

2601

2)( )

28.3

128.3()(99.7)/(

B

BNmkNq allnet

+= (B in meters)

where 601 )(N is the corrected penetration resistance.

� Too conservative.

� Later, Meyerhof suggested that )(allnetq should be increased by

50%.

� Bowles �

For B ≤ 1.22m

)4.25/()(16.19)/( 6012

)( SFNmkNq dallnet =

For B > 1.22m

)4.25

()28.3

128.3()(98.11)/( 2

6012

)(

SF

B

BNmkNq dallnet

+=

where, 33.1)/(33.01 ≤+== BDfactordepthF fd

S = tolerable settlement in mm.



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� Empirical correlations for )(allnetq with CPT( cq ) based on 1 inch

settlement (Meyerhof)

For B ≤ 1.22m

15/)( callnet qq =

For B > 1.22m,

22

)( )28.3

128.3(

25)/(

B

BqmkNq c

allnet

+=

in meters and 2/mkN .

� Notes

1)

2)

3)



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12) Field Load Test (Plate Load Test)

� Test Method

- Steel plate : 25mm thickness and 150mm~762mm diameter

(or square plate with 305mm x 305mm)

- A hole is excavated with a minimum diameter of 4B to a depth of Df.

(Df : depth to the proposed foundation)



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� For tests in clay,

)()( puFu qq =

where,

)(Fuq = ultimate bearing capacity of the proposed foundation

)( puq = ultimate bearing capacity of the test plate

� For tests in sand,

p

FpuFuB

Bqq )()( =

where,

FB = width of the foundation

pB = width of the test plate

� Major drawback of plate load test

Effect of footing size difference

⇒

⇒



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13) Tolerable Settlements

- Non-homogeneous subsoil conditions + wide variation of the load

carried by various shallow foundations.

�

�

- The parameters for definition of tolerable settlement

* TS = Total vertical displacement at point i

* TS∆

=

=

Differential settlement between i and j

)()( iTjT SS −

*∆ = Relative deflection

*ω = Tilt

* β = Angular distortion = ijT lS /∆

* L/∆ = Deflection ratio

- Criteria for tolerable settlement are determined based on types and

functions of structures.

ijl

l

)(iTS ω

∆ β

)( jTS



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� Skempton and McDonald

- Maximum settlements, (max)TS

In sand 32mm

In clay 45mm

- Maximum differential settlements, (max)TS∆

Isolated foundations in sand 51mm

Isolated foundations in clay 76mm

Raft in sand 51~76mm

Raft in clay 76~127mm

- Maximum angular distortion 1/300

� Soviet Code of Practice

- Allowable deflection ratio is a function of L/H, the ratio of the length to

the height of a building.

Type of Building HL / L/∆

Multistory building and civil

dwelling

3≤ 0.0003 (for sand)

0.0004 (for clay)

5≥ 0.0005 (for sand)

0.0007 (for clay)

One-story mills 0.001 (for sand and clay)



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� Bjerrum

Category of potential damage maxβ

Safe limit for flexible brick wall (L/H > 4)

Danger of structural damage to most buildings

Cracking of panel and brick walls

Visible tilting of high rigid buildings

First cracking of panel walls

Safe limit for no cracking of building

Danger to frames with diagonals

1/150

1/150

1/150

1/250

1/300

1/500

1/600

- Allowable deflection ratio is a function of L/H, the ratio of the length to

the height of a building.

� European Committee

Item Parameter Magnitude Comments

TS 25mm

50mm

Isolated shallow foundation

Raft foundation

TS∆ 5mm

10mm

20mm

Frames with rigid cladding

Frames with flexible cladding

Open frames

Limiting values

for serviceability

(European Committee

for Srandardization, 1994a)

β 1/500 -

TS 50 Isolated shallow foundation

TS∆ 20 Isolated shallow foundation

Maximum acceptable

foundation movement

(European Committee

for Srandardization, 1994b)

β ≈ 1/500 -



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14) Mat Foundations

� When individual footing plan areas cover 50~75% of the plan area of the

structure, use a mat foundation.

� Types of mat foundations (based on rigidity of footing)

a) Flat plate

b) Flat plate thickened under columns

c) Beams and slabs

d) Flat plates with pedestals

e) Slab with basement walls as a part of the mat



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� Points for design

� Bearing Capacity

- Bearing capacity can be obtained with the same formula for spread

footing.

idsqiqdqsqcicdcscu FFBFFFFqNFFFcNq γγγγ++= 2/1

- If embedded, the net applied load, nq

fdesignn Dqq γ−=

- So, for bearing capacity stability

nfu)net(all qFS/)Dq(q ≥−= γ

(or designuall qFSqq ≥= / )

Df

- Compensated foundation ;

No increase of the net

pressure on soil below a

mat foundation.

� 0=−= fdesignn Dqq γ

γ/designf qD = �

Effective on

reducing the

settlements.

qdesign



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For NC clays

'p (log scale)

e∆ excavation

e e∆ e loading

(a) No compensating (b) Fully compensating

- Allowable bearing pressure using SPT tests (Bowles);

From Eq.(5.79),

)4.25/))(/(33.01()28.3

128.3(98.11)/( 22

)( efcornetall SBDB

BNmKNq +

+=

where eS =settlement, in mm.

For mat foundation, B is very large. => BB 28.3128.3 ≈+

)4.25/))(/(33.01(98.11)/( 2

)( efcornetall SBDNmKNq += ≤ )4.25/(93.15 ecor SN

- Based on 2 inch total settlement and conservatively assuming that

1))/(33.01( =+= BDF fd

cornetall NmkNq 96.23)/( 2

)( =

↓ po’(=pc’)

po’+∆p

p’(log scale)

po’-γDf po’



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� Structural Design of Mat foundations

i) Conventional rigid method

- Bearing pressure : Uniform distribution for no eccentric loading

Varying linearly across the footing for eccentric

loading.

- Mat : Infinitely rigid and an inverted simply loaded two-way slab.

(Fig. 6.11b)

ii) Beam(slab) on elastic foundation (Winkler Foundation)

- Define the relationship between bearing pressure and settlement using

the modulus of subgrade reaction, )//,/( 2 mmkNqk δ=

- Then model the foundation soil as a bed of springs with a stiffness Sk .

(Fig. 6.12b)



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* Case study

1. Bearing capacity failure (Coduto, p181, Collapse of the Fargo Grain

Elevator)

2. Field settlement observations for foundations (p.304, 5.5)



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