Question Bearing Capacity

[GeoGrouting]

Page 7.2 - 142 of NAVFAC has TABLE 1 for Presumptive Values of Allowable Bearing Pressures for spread footing.

It suggests allowable bearing pressures of 1 - 2 (average 1.5) tsf. for loose fine to medoium sand.

This value seems to be high.

Would appreciate your comment.

 

[Ron]

Nope..that's about right. 2 to 3 ksf is pretty common for sands.

 

[GeoGrouting]

I have the following results for the allowable bearing capacities which are considerably different. Which one would an experienced engineer rely on.

Assumptions: b = 2.2 m, loose sand with corrected SPT = 6, D = 1.2 m, gama = 180 kN/m3

Answers:

Terzaghi and Peck formula = 51 kPa

Teng = 77 kPa

Ditti Teng (1969)= 79 kPa

Terzaghi equation = 170 kPa !

NAVFAC Preassumptions = 100 kPa to 200 kPa

Terzaghi & Peck 1948 Chart = 70 kPa

Terzaghi & Peck 1967 Chart (from Tomlinson book) = 45 kPa

 

[VAD]

Geogrouting:

One of the issues with those who write textbooks etc wrt to soil mechanics or geotechnical engineering do not necessarily provide all the relevant information that the persom who developed the relationship took into consideration. Unfortunately we often use the relationships because thay are simple to use-go in a graph and pick a result.

I would suggest that you take a look at Peck, Thorburn and Hanson - Foundation Engineering, and Tezaghi, Peck and Mesri-Soil Mechanics in Engineering Practice. You will find that there is alot more to the charts that you are using to ensure that they are used correctly.

I would also look at the reduction in phi value that is required for theoretical bearing capacity calculation using formulae.

You will also learn that the Chart in your second reference is based on a FOS of 2.

As you get more involved with the subject you often refelect on whether you were taught anything correctly at University and end up with the situation of now trying to learn what you thought you knew.

Good luck as I am sure that you will pull a few hair out of your head as you proceed to demystify the subject.

 

[GeoGrouting]

The diffrerence between the calculated value of 170 kPa and the other references are great and I would appreciate further comments on these. The 70 kPa belongs to Peck, Thorburn and Hanson - Foundation Engineering.

 

[VAD]

I think that you may wish to do some reading since there are different scenarios. The use of the conventional Terzaghi equation does not invoke settlement. The other charts are based on allowable pressures that provide for 25 mm settlement.

The allowble pressures are not necessarily the bearing capacity of the soil which the conventional bearing capacity equations provide. You can calculate the settlement based on 170kPa using elastic theory. Your 180kN/m3 for gamma for loose sand is not correct and you may not have used this value in your calculation.

Further, you need to distinguish whether the value is gross allowable or net allowable since the presumptive values and the values from th charts are generally net allowable. Other aspects that you need to address when using the equations is whether you are calculating for a strip foundation or rectangular etc. I presume that you are addressing a strip footing.

 

[GeoGrouting]

All the calculated values are for net snd square footing. for the 170 kPa, I had assumed gama was 17 kN/m3. So it is mainly the settlement criteria. Would nappreciate further comments.

 

[ishvaaag]

For weak sands 1 kgf/cm2 = 1 TSF is a common minimum. At corrected 6 SPT blow count this normally won't be a very weak sand and so the 1.5 tsf range seems a reasonable allowable compression when the settlement is not to exceed 1 inch. Very weak sands should show lower blow counts and for these even 1 TSF would be too much, and the geotechncal information would show. On the other hand, some called weak sands are not as weak and so the upper 2 TSF range given would be reasonable. But if really "weak" 2 would be a bit too far by traditional practice. Almost any soil where is reasonable to take footings as foundation should allow 1.5 TSF.

Other thing is that to the light of the many things that may happen to weak sands related with water, this cover-all old procedures are simply not enough to describe what at hand. But we can't expect the mere extraction of a value from a table to cover such things as potential liquefaction, that have evolved separate procedures for the checks.