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Subgrade Modulus of soil
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motahar
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PostPosted: Sun Jun 21, 2009 11:04 am    Post subject: Subgrade Modulus of soil Reply with quote

Dear all,
I know this question might be funny but i do not know the exact difference between subgrade modulus and bearing capacity of soil. And how can we convert one to another.
In safe program, the subgrade modulus should be inserted or bearing capacit of soil?
thanks
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Dr. N. Subramanian
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PostPosted: Sun Jun 21, 2009 1:51 pm    Post subject: Subgrade Modulus of soil Reply with quote

Dear Er Motahare,

Some times silly questions or questioning the status quo will produce great inventions. Think of the person who questioned why not we get the photos immediately from a camera-which resulted in Polaroid cameras-although it has become obsolete due to the invention of digital cameras.

Coming to your question: Please note that the unit for bearing capacity is KN/m2, whereas the unit for Subgrade modulus is kN/m3. It is also called as soil spring constant k = Pressure/settlement. It is normally used to solve problems of beams on elastic foundation using Winkler Model.

The allowable soil bearing capacity is the maximum pressure that can be permitted on foundation soil with adequate safety against soil rupture or excessive settlement.

There is no reliable correlation between modulus of subgrade reaction and soil bearing capacity-because they are measurements of entirely different characteristics of a soil. The k-value used for floor-slab design reflects the response of the subgrade under temporary (elastic) conditions and small deflections, usually 1.25 mm or less. Soil bearing capacity value (normally used to predict and limit differential settlements between footings or parts of a foundation) reflect total permanent (inelastic) subgrade deformations that may be 20 to 40 (or more) times greater than the small deflections on which k-values are based.

Best wishes
Subramanian
Quote:

From: motahare <forum@sefindia.org>
Subject: [SEFI] Subgrade Modulus of soil
To: general@sefindia.org
Date: Sunday, June 21, 2009, 4:35 PM

Dear all,
I know this question might be funny but i do not know the exact difference between subgrade modulus and bearing capacity of soil. And how can we convert one to another.
In safe program, the subgrade modulus should be inserted or bearing capacit of soil?
thanks
     



     


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sspawar
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PostPosted: Sun Jun 21, 2009 4:33 pm    Post subject: Re: Subgrade Modulus of soil Reply with quote

drnsmani wrote:
Dear Er Motahare,

Some times silly questions or questioning the status quo will produce great inventions. Think of the person who questioned why not we get the photos immediately from a camera-which resulted in Polaroid cameras-although it has become obsolete due to the invention of digital cameras.

Coming to your question: Please note that the unit for bearing capacity is KN/m2, whereas the unit for Subgrade modulus is kN/m3. It is also called as soil spring constant k = Pressure/settlement. It is normally used to solve problems of beams on elastic foundation using Winkler Model.

The allowable soil bearing capacity is the maximum pressure that can be permitted on foundation soil with adequate safety against soil rupture or excessive settlement.

There is no reliable correlation between modulus of subgrade reaction and soil bearing capacity-because they are measurements of entirely different characteristics of a soil. The k-value used for floor-slab design reflects the response of the subgrade under temporary (elastic) conditions and small deflections, usually 1.25 mm or less. Soil bearing capacity value (normally used to predict and limit differential settlements between footings or parts of a foundation) reflect total permanent (inelastic) subgrade deformations that may be 20 to 40 (or more) times greater than the small deflections on which k-values are based.

Best wishes
Subramanian
Quote:

From: motahare <forum>
Subject: [SEFI] Subgrade Modulus of soil
To: general@sefindia.org
Date: Sunday, June 21, 2009, 4:35 PM

Dear all,
I know this question might be funny but i do not know the exact difference between subgrade modulus and bearing capacity of soil. And how can we convert one to another.
In safe program, the subgrade modulus should be inserted or bearing capacit of soil?
thanks
     



     


Dear All,
I fully agree with the comments given by Er. drsmani over Modulas of Subgrade of Soil and Bearing Cpacity of Soil.
If Unit of Bearing capacity of soil is Kg/cm2 then Unit of K value will be kg/cm2/ cm.
Bearing Capacity of soil means after the said value no increase of pressure will be sustained and it is total faluire of strata. Graph totally break there. Hence safe bearing capacity is always considered in design with a factor of safety over ultimate Bearing Capacity. For load bearing structures like Column footings or any deep or shallow foundations which recieve  pressure of wider area and transfer to the soil through a rigd structure over a rather smaller area.
Modulus of reaction is a pressure sustained by a surface with respect to a unit settelment in its top crust. Here no total faluire causes but with respect to a standard settlment pressure is measured. and this value is used to measure the behaviour direct over the surface of structures like pavemnets, subgrades , subbases of roads. It is very much may corelate with the CBR values but not with the Bearing Capacity of Soil.
with regards
S S Pawar
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JVCSNL
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PostPosted: Mon Jun 22, 2009 4:04 am    Post subject: Subgrade Modulus of soil Reply with quote

Dear Member,

The modulus of subgrade reaction is a conceptual relationship between soil pressure and deflection.

Recall from the plate load test being done during geotechnical engineering subject and available in almost all soil reports.

The modulus of subgrade reaction k is slope of P vs. Delta graph.
Where,
P = load applied (kN/sqm) and
Delta = deformation (m).

Hence, unit of k is (kN/sqm)/(m). This is nothing but the stiffness of soil (Pressure required for unit deformation) Many people give unit as kN/cu.m. which many times confuses. Hence, using k in stiffness term always helps better understanding.

Though, modulus of subgrade reaction and bearing pressure is related to each other, there is no definite relationship between them. The reason, soil and its properties vary from place to place, there is significant effect on the type of soil in layers below etc.

However, J E Bowles has attempted to give a relationship which is
k = 40 x FOS x Qa,
where
FOS is factor of safety considered in Bearing pressure calculation and
Qa is allowable bearing pressure.

Hope this explanation will help you.

Regards,

Jignesh Chokshi

Quote:
Quote:
Quote:
forum@sefindia.org 21-06-2009 >>>

Dear all,
I know this question might be funny but i do not know the exact difference between subgrade modulus and bearing capacity of soil. And how can we convert one to another.
In safe program, the subgrade modulus should be inserted or bearing capacit of soil?
thanks

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JVCSNL
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PostPosted: Mon Jun 22, 2009 4:11 am    Post subject: Subgrade Modulus of soil Reply with quote

Dear Member,

In SAFE program, you have to supply modulus of subgrade reaction as you are suppliying the stiffness properties of soil.

Regards,

Jignesh Chokshi

Quote:
Quote:
Quote:
forum@sefindia.org 21-06-2009 >>>

Dear Er Motahare,

Some times silly questions or questioning the status quo will produce great inventions. Think of the person who questioned why not we get the photos immediately from a camera-which resulted in Polaroid cameras-although it has become obsolete due to the invention of digital cameras.

Coming to your question: Please note that the unit for bearing capacity is KN/m2, whereas the unit for Subgrade modulus is kN/m3. It is also called as soil spring constant k = Pressure/settlement. It is normally used to solve problems of beams on elastic foundation using Winkler Model.

The allowable soil bearing capacity is the maximum pressure that can be permitted on foundation soil with adequate safety against soil rupture or excessive settlement.

There is no reliable correlation between modulus of subgrade reaction and soil bearing capacity-because they are measurements of entirely different characteristics of a soil. The k-value used for floor-slab design reflects the response of the subgrade under temporary (elastic) conditions and small deflections, usually 1.25 mm or less. Soil bearing capacity value (normally used to predict and limit differential settlements between footings or parts of a foundation) reflect total permanent (inelastic) subgrade deformations that may be 20 to 40 (or more) times greater than the small deflections on which k-values are based.

Best wishes
Subramanian
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PostPosted: Mon Jun 22, 2009 4:26 am    Post subject: [SEFI] Re: Subgrade Modulus of soil Reply with quote

Dear Member,

The modulus of subgrade reaction is a conceptual relationship between soil pressure and deflection.

Recall from the plate load test being done during geotechnical engineering subject and available in almost all soil reports.  

The modulus of subgrade reaction k is slope of P vs. Delta graph.  
Where,
P = load applied (kN/sqm)  and
Delta = deformation (m).  

Hence, unit of k is (kN/sqm)/(m).  This is nothing but the stiffness of soil (Pressure required for unit deformation) Many people give unit as kN/cu.m. which many times confuses.  Hence, using k in stiffness term always helps better understanding.

Though, modulus of subgrade reaction and bearing pressure is related to each other, there is no definite relationship between them.  The reason, soil and its properties vary from place to place, there is significant effect on the type of soil in layers below etc.

However, J E Bowles has attempted to give a relationship which is
k = 40 x FOS x Qa,
where
FOS is factor of safety considered in Bearing pressure calculation and
Qa is allowable bearing pressure.

In SAFE program, you have to supply modulus of subgrade reaction as you are suppliying the stiffness properties of soil.

Hope this explanation will help you.

Regards,

Jignesh Chokshi
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motahar
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PostPosted: Mon Jun 22, 2009 4:52 am    Post subject: Another Question Reply with quote

Dear drnsmani and sspawar,
Let me first express my best gratitude to you and all the other sefinda friends for helping us by giving good and clear responses to our questions.
Related to the previous question I have got another question. In CSI SAFE program, the soil property defined must be subgrade modulus or bearing capacity. If subgrade modulus is needed to be defined there, so how does the program calculates the bearing capacity needed for design since as you said there is not a reliable correlation between these two.
With warm regards,
Motahare
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PostPosted: Mon Jun 22, 2009 5:11 am    Post subject: Reply with quote

Dear Er. Motahare,

In SAFE, you are supplying the stiffness of soil in terms of subgrade modulus.

At each node, the program then calculate the linear spring value (based on tributary area) to perform the analysis.

Since, the entire founation is now resting on set of spring in both the directions, based on the loading applied, the springs will deform.  This deformation value is calculated. Based on this value, the program will further calculate the bearing pressure below soil based on soil spring stiffness and deformation obtained.

Hence, in SAFE, you need not supply any input related to allowable bearing pressure.  In the graphical output, you can very well see the pressure induced below the slab elements.  This bearing pressure you shall check with allowable bearing pressure of soil.

Regards,

Jignesh Chokshi
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vikram.jeet
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PostPosted: Mon Jun 22, 2009 5:17 am    Post subject: Subgrade Modulus of soil Reply with quote

Dear All,

The bearing capacity is calculated based on shear failure criterion and also based
on settlement criterion and lesser of two is adopted as alowable sbc with a FOS.
In most soils ,I think it is the settlement criterion which decides the SBC vis-a vis
Local/General shear failure criterion

Kindly enlighten about:

i) IF Net SBC of soil is 10 T/m2 for isolated footings (permissible settlement=50mm)
FOS for SBC computation = 2.5

Value of subgrade modulus as per definition ,k= 10/0.050 = 2000 T/m3

As per Bowels k= 40*2.5*10 = 1000 T/m3

There seems difference in the two computations

ii) If shear criterion governs for deciding the SBC, then k canot be taken = Net SBC/settlement

kindly enlighten

regards

vikramjeet

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PostPosted: Mon Jun 22, 2009 6:18 am    Post subject: Reply with quote

Dear member

In SAFE No where you have an option of giving SBC of the soil,
In fact it is provided as Subgrade Modulus or Ks (stiffness of the soil)
This Ks is given by SBC by Settlement, in whick settlement is calculated by the the plate load Test

MODULUS OF SUBGRADE REACTION (Ks)

The Modulus of sub-grade reaction is a conceptual relationship between soil pressure and deflection that is widely used in the structural analysis of foundation members.

It is used for continuous footings, mats, and various types of pilings.  The basic equation for Ks when using the plate load test data is given by

          Ks = q / d

The plot of q versus d from load tests shows non-linear variation.  If this plot of curve is used to obtain Ks, the value of Ks depends on whether it is a tangent or secant modulus and location of the coordinates of q and d.

However, the other earliest method suggested by Terzaghi (1955) to evaluate the Ks for full sized footings from plate load tests for different types of soil are

1)     For footings on clay:
     Ks = k1 B
2)     For footing on sand (including size effects):
     Ks = k1 [(B+1)/2B]2        
3)     For a rectangular footing on sand of dimensions B x mB:
     Ks = k1 [(m+0.5m)/1.5m]
     Where,
     Ks = desired value for full-sized footings and
k1 = value from a 1 x 1 ft. square plate load test.

Vesic (1961) proposed that the Modulus of sub-grade reaction could be computed using the stress strain modulus Es as,

Ks = Es / [B (1-m2)]

     Comprehensive accounts of the evaluation of Modulus of sub-grade reaction from plate load tests are given by Terzaghi (1955), Bowles (1977) and summarized by Nair (1974).

     It is observed that ks is not a unique property of the soil medium and the following factors can affect the determination of  Ks from a plate loading test.

a)     Size of plate
b)     Shape of plate
c)     Embedded depth of plate

Vesic and Johnson (1963) have suggested an expression for the evaluation of  ks in terms of Modulus of Elasticity Es and Poissons ratio ms as

ks = [0.65Es / B(1- ms2)] [EsB4 / EIf]1/12

However, it has been found that the bending moments and the computed soil pressure are not very sensitive to what is used for Ks.This is because the structural member stiffness is usually 10 or more times as great as the soil stiffness as measured by Ks.

     Recognizing this, based on the suggestion given by Bowles (1977), the following method is presented on the assumption that the allowable soil pressure is based on some maximum amount of deformation (Si) including on a factor of safety (Fs).

     Thus, the Modulus of subgrade reaction is
     Ks = [(Fs)qa]/SI

For a settlement of 0.254m and factor of safety 3, Ks can be taken as
Ks = 120 qa kN/m3                              

Over the contract area, the value of Ks can be assumed to constant.The use of constant value of Ks beneath mats and other footings (spread and combined) is usually adequate owing to the effect of superstructure rigidity contributing to the foundation element and the fact that the element Flexural rigidity (EI) decidedly predominates.

For some foundations such as those of oil and water tanks resting on ground the foundation rigidity is often very low and it may be necessary to vary the ks values over the contract area to obtain results consistent with field measurements (Bowles 1977).
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