Modulus of Subgrade Reaction of Soil (Bowles Approach and Basic Approach)

The modulus of subgrade reaction (k) is used as a primary input for rigid pavement design. It estimates the support of the layers below a rigid pavement surface course (the concrete slab). The k-value can be determined by field tests or by correlation with other tests. There is no direct laboratory procedure for determining k-value.
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1- BASIC APPROACH Method
The modulus of subgrade reaction came about because work done by Westergaard during the 1920s developed the k-value as a spring constant to model the support beneath the slab.
The reactive pressure to resist a load is thus proportional to the spring deflection (which is a representation of slab deflection) and k.
P=k* delta
Typically, the modulus of subgrade reaction is estimated from other strength/stiffness tests, however, in situ values can be measured using the plate bearing test.
2- Joseph E. Bowles Approach Method
Bowels recommended following approximation to determine Ks based on allowable bearing capacity as suggested by geotechnical engineer.
(SI Units) Ks = 40(SF)qa kN/m3
(FPS Units) Ks = 12(SF)qa k/ft3
Where
SF = Safety Factor
qa = Allowable Bearing Pressure in ksf or kPa
Keywords
modulus of subgrade reaction for raft foundation
modulus of subgrade reaction for Isolated Footing
modulus of subgrade reaction for Combined Footing
modulus of subgrade reaction for Slab on Grade
modulus of subgrade reaction for Mat foundation
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#SoilModulus #safe #SubgradeReaction

[@civilmdc]

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[@haidermumtaz6065]

one of the BEST VIDEOS on youtube , BRILLIANT bro keep it up !!!

[@civilmdc]

Thanks 😊

[@tcsengenharia9569]

thank you for your time. Your explanation was very good. Eng Thiago from Brazil

[@civilmdc]

Welcome Dear ❤️

[@BashirAhmed-jo4zq]

Nice work bro keep it up, I tried to comment on all videos, its an appreciation of your hard work.

[@mgkhantlinn5442]

Sir How may i calculate for Line spring? Thank you

[@joserhidalgozapata5159]

From my point of view. Bowles method is used to run strength combinations. On the other hand, common approach is used to service load. I'm glad to read your opinions.

[@civilmdc]

Thanks..... Both methods are good.

[@mariabridges4767]

Thank you so much. This was very helpful!

[@md.ashrafulalam4732]

Please make a tutorial about how to design foundation in SAFE using soil test report. Also how to input in SAFE from soil test report.

[@gouthamjayabalan4775]

Hello sir, for designing mat/raft foundation can use Bowels method ?? Else Bowels method having any restriction to use in the types of foundation ??

[@civilmdc]

Yes, absolutely, You can USE.

[@bdsoilfoundation3592]

please provide an excel sheet for calculation of modulus subgrade reaction both horizetally and vertically

[@subhashbiswas6961]

Thanks for sharing your knowledge. One question. Normally foundations are below approx 1.5m for more, below existing ground. The Spring value is determined in this video, at that level. Now when we are designing the Slab on grade which is at ground level or for a compacted ground in fill area (say the final ground level is developed by Cut and fill, for a project). How the spring value can be calculated for analysis (other than field test, e.g. plate load test etc). Thanks

[@civilmdc]

You have to add an overburden on that level.... Amount of weight... Which is removed of soil

[@archiepamatmat3142]

Just want to ask, based on your SAFE model examples the bearing pressure you got was greater than 100kPa, does it mean its not adequate? Or do you need to divide the bearing pressure obtained from SAFE to Safety Factor then check it to allowable bearing pressure? Thank you.

[@civilmdc]

Your question first part... It will not adequate

[@archiepamatmat3142]

​@@civilmdc thank you very much for your reply. Just a follow up question. How about if I use Ultimate Load Combination instead of service combination for the loads, do i have to multiply the allowable bearing pressure to SF (safety factor) and compare it to the pressure results provided by the software?

[@suhaibkhan7356]

Why you not multiply to 2.5 * 4000 for subgrade modulus??

[@mohammadrahal8080]

Thankyou for the explanation sir but I have a small question. For the raft foundation, you said in a previous video that the allowable settlement is 50mm (5 cm). If we want to apply Bowles approach in obtaining Ks, the factor 40 should be changed to 20 in the formula as follows: ks=qult/deltH = qult * 1/0.05 = qult * 20 = qallowable * F.S * 20. Is this correct? I'm asking this question because I realized that some people use for the raft foundation also the factor 40 instead of 20 (assuming 2.5 cm for the raft as well). So I was wondering what is the correct approach.

[@rezafauzinirwan9471]

how can i find the q ult value if i only have CBR test data ? please help me

[@civilmdc]

Dear.... Write this line on Google and make the decision... "cbr correlation to bearing capacity"

[@aashishp007]

Why are springs provided only at the periphery of footing and not uniformly?

[@civilmdc]

It is just a modelling view .... Actually present under the whole footing

[@aashishp007]

@@civilmdc Thanks for prompt reply. Have a good day 👍

[@sagurkhan5795]

Thank you so much.. you did good job please make a video on how to use safe 2016 except update crack file?

[@user-yt9kj6gx4x]

which load combination did you check punching shear? UDCONU2 or SERVICE LOAD

[@civilmdc]

Both approaches are discussed in this lecture 1 service and 2 ultimate

[@jayshah6293]

Can I use Bowles approach for Pile for subgrade reaction vertical and horizontal both????

[@civilmdc]

Yes

[@jayshah6293]

Bcz I'm doing thesis in Mtech on offshore structure that's why for soil structure interaction .... I must be assigned soil spring both vertical and horizontal direction..... Again asking its use for both vertical soil spring and horizontal soil spring???

[@civilmdc]

@@jayshah6293 yes

[@jayshah6293]

@@civilmdc okay thank you sir.... So if in viva asked why i use Bowles for both directions soil spring on pile.... What I answer in that sir ?? Bcz bowles method on plate so if you help

[@civilmdc]

@@jayshah6293 read the book... You will get the confidence

[@ajazsayyed8487]

Asslamualikum sir. Best

[@civilmdc]

W. Salam Thanks

[@birukabera5895]

Thank you sir for the tutor I have 1 question how to use TMD on reinforced concrete structure please do some video about TMD thank you 🙏

[@birukabera5895]

And how to model it by using sap or etabs

[@mkdmanohar9744]

in bowels approch im getting very less value of shear and moment in footing when compare to basic approch ...i checked with model..tell ur opinion

[@civilmdc]

Basic approach is too conservative and Safe, Approach ... that is why .... Basic approach ... Always give more moment and shear.

[@engrgep]

@@civilmdc Sir i get so confused. Pls help. Regarding subgrade modulus. 1. Can u xplain in laymans term it's definition. Does it mean that IT IS THE PRESSURE THE FDN MUST RESIST TO AVOID SETTLEMENT? ?? 2. U said Bowles approach is conservative and basic approach is MUCH CONSERVATIVE. but base on the bowles formula. q*F.S./allowsettlement As u increase FS k value increase As u decrease allow settlement k value also increase. Meaning as the value of K increases it is much conservative. So base on your example 10,000 is more conservative than 4,000 3. Ex using bowles method. Ur examlple is 100*2.5/0.025=10 000 If the soil report allow 60mm settlement 100*2.5/0.06= 4166.67 So fr that the k=10 000 is much conservative (because allow settlement is lower) than 4166.67 as the k value increases is much conservative? So in your sample (just compare the value and not the formula) 10 000 is much conservative. So if the high value is much conservative why shear and moment is lesser? The main question is what is much conservative bet high and low value of K. Thanks and more power!!!

[@civilmdc]

@@engrgep good observation... I said in the video.. that... and Again conclusion is Bowels approach is Okay to use.... I mean.. without Safety factors... At the actual soil conditions ( bowls approach ) is also okay... Because now we have to design member as per actual loading...

[@engrgep]

@@civilmdc thanks for your reply. Its another scenario. In a soil report soil bearing and allowable settlement is given. 250 kpa and 60 mm Using bowles safety factor 3. 250*3/.06 = 12 500 But the soil report also recommended to use K=20 000 Which is more conservative 12500 or 20 000??? For me 20 000 because when i "back sub" and recalculate using bowles formula it will have LARGER SAFETY FACTOR. 20 000*0.06/250 = 4.8 (SF) Does increasing safety factor safe? And also does decreasing allowable settlement? Both of them will generate larger K values. Any comment? Thanks again!

[@civilmdc]

@@engrgep more is 12 500.. But you can use 20,000 which is recommended by geotechnical engineer

[@khalildawar812]

AoA Sir. Sir From which book you have taken the above document from which you explained Modolus of subgrade, If You have this book Please share with me

[@civilmdc]

W.Salam Dear, The book Name is "Foundation Analysis and Design by Joseph E. Bowles (5th Edition)" civilmdc.com/2020/03/09/foundation-analysis-and-design-by-joseph-e-bowles-5th-edition/ Book Download link is added in the description of the video also...

[@TheWhosayn]

which way is correct? defining column or stiff slab instead of it?

[@civilmdc]

Both are okay

[@musaabduljabbar4390]

Hello sir, How we apply spring assignment in pile model?

[@civilmdc]

Hi, Dear here is the full course lectures... kzbin.info/aero/PL0h14oMuZXQ6l-O7aqF0hYsZbyJ7eFt1H Your question answer is ''See lecture 23 CSI SAFE"

[@jayshah6293]

Hi Abdul you got your answer??

[@er.kalyanikaremore528]

How to calculate spring stiffness for pile group

[@civilmdc]

See the lecture of Pile cap design Here is the full course link on CSI SAFE kzbin.info/aero/PL0h14oMuZXQ6l-O7aqF0hYsZbyJ7eFt1H

[@chastie_6821]

can soil base pressure exceed soil bearing capacity?

[@civilmdc]

Then Increase footing size

[@surajgurung5856]

What about Line Spring properties and Point Spring Properties??? Plz Help

[@kshitizgautam5412]

Not possibe in Etabs for area and line springs.. U can use plaxis for that.

[@muhammadsajidaslam1328]

Asslamualikum sir, is there any option in ETABS & SAFE from where we can select Rebar Grade for slab reinforcement design???

[@civilmdc]

W.Salam, Right click on slab and select material overwrites... select steel grade for slab design.

[@muhammadsajidaslam1328]

@@civilmdc thanks alot sir .... 👍

[@civilideas1925]

👍🏻

[@SBPatil-eh6vr]

Your English accent is good, but try to talk normal, it's hearing like you are pretending to talk in a high class accent. Thank you..