Shear strength testing

[masood053]

Hello

We are requried to caclulate the bearing capacity of the soil under a proprsed weigh in motion railyard scale. the scale has a slab foundation (rectangular) that is to be placed approx. 5 feet deep below ground surface. the soil is fine grained (Clay and silt). To calcualte the bearing capacity, we need to estinate the shear strength of the soil for use with the bearing cpacity equations.

i am trying to figue out which Triaxial test (UU) or (CU) to run on the samples to obtain shear stregnth for bearing capacity calcuation. I am not sure where the groundwater or in other words is the smaple saturated at 5 feet deep. I was comptempalting a UU test to get a shear stregnth for the short term loadng conditions that is going to govern in this application. However if the sample is not saturated then the UU could give pretty high shear strengths. Any suggestions would be very helpful

thanks

 

[oldestguy]

I'd be much more concerned with differential settlement than soil shear strength, which likely has no use there.

 

[BigH]

Agree with oldestguy - this is more in line with serviceability issues than shear issues since the weighing scales probably, if they are like ones I've dealt with in the past, very sensitive to settlement - in fact on one job they said "no settlement". Further, the fact that you are removing 5 ft of soil - and the pit is likely to behave as a bathtub - you therefore can counterbalance your loads to the removed soil and bearing shear would be of little concern.

You do need to characterize the soil - carry out a proper investigation which you seem not to have done as yet since you have indicated you do not know where the groundwater is - and not much as far as classification of the soil. You can always do in situ shear vane tests to determine the shearing strength - but you need to have more information as to compression behaviour of the soil. Consolidation testing - you also need, because you are digging a pit need to know how much rebound you will get. You can also do a CIU test with pwp measurements.

 

[oldestguy]

There also is the potential for overall "rocking" as the load comes on or goes off. With rail connections at each end of the "box", I would think the width and overall length of the supporting slab under the "box" may have to be sized for a limiting deflection at the junction with nearby ground supported rails. Perhaps handle this with an approach slab under the rails, as is done at highway bridges. I've noticed that under most rail traffic a noticeable deflecti0pn of the rails occurs as the loads go by.

I have not done tests to measure elastic action, etc. so perhaps some member here can elaborate.

I once had the job of evaluating a distorted (during placement) corrugated metal culvert pedestrian walkway under a railroad. I measured deflection of the "roof" as well as side to side "squashing" effect. With the locomotive and its distributed loadings not much effect. But cars loaded with iron ore really created deflections, but left no permanent distortion.