Pile Capacity Analysis using SHAFT (by Ensoft) 2

[pelelo]

Hello,

I have been working on the axial capacity of drilled shafts using SHAFT.

To my knowledge, shaft is one of the most popular softwares available to compute axial capacity.

Most of the soils I deal with are mixed soils such as Clayey Sand (SC), Silty sand (SM), Silty gravel (GM), etc.

I would like to know how do you consider these mixed strata for analysis purposes. I usually treat them as sand (if the case is SM or SC) and as gravel (if the case is GM or GC).

I understand this assumption is not 100% accurate as for these mixed soils, at some point I will need to add some cohesion value, but of course this involves additional lab tests, therefore more time.

I would like to know what is your position about this?

Thanks

 

[FixedEarth]

For the soils you describe, I use only friction and ignore end bearing capacity to obtain the ultimate axial capacity. As for cohesion, for P.I. > 10, I use cohesion value of 0.7*(0.5*penetrometer), but rarely go over 1.5 ksf. See attached example and plug your own numbers. Look for a program called LATERAL FOUNDATION next month.

 

[pelelo]

Fixedearth,

Thanks a lot for your reply. It was very helpful.

1- you said if PI>10, you use 0.7*(0.5*penetrometer), what happens if PI is less than 10?, Do you use the penetrometer value as it is?.

2- Is this just a rule of thumb based on your experience or can you please give any reference regarding c values from pocket penetrometer?.

3- What would you do in case you have a slope stability analysis under the same soil conditions (SM, GM, SC, and SG), how would you "guesstimate" c?, I know slope stability is another topic different than evaluating Axial capacity, this question just popped on my mind.

Thanks

 

[FixedEarth]

pelelo;

1- As you know, the higher the P.I. the more Clayey the soil is. The GM and the SC are both in the coarse grained soils of the USCS Table (less than 50% is passing the #200 sieve). So we are really not in fine grained soils-However you have some cohesion in the soil. So in theses soils, it is better to perform a direct shear test and use the cohesion intercept value. The penetrometer values for GC soils in particular is not reliable as it may hit on fine aggregates as you advance the hand penetrometer or the pocket Torvane. So you will have a range of descriptions on most sites. You have Sand like soils (SW, SP, SM, GW, GP, GM), then Clay like soils (CL, CH, MH) and then you get transitional soil like SC, ML, GC. So our concern is the transitional. You can do -#200 seive, Atterberg limits, direct shear and in some cases hydrometer tests. These 4 test results will indicate how "Clayey" the materials are. The penetrometer value is very close 2 times the Cohesion strength. However, instead of just using 50% of the penetrometer as the cohesion strength, I factor it further by 0.7.

2- I remember seeing a good discussion of points 1) and 2) in "Physical and Geotechnical Properties of Soils", 2nd ed, Joseph E. Bowles. Don't remember what page though, sorry.

3- For slope stabilty, if a structure is within 15 feet of the slope crest, I use 70% of the cohesion intercept of RESIDUAL direct shear test. If a structure is greater than 15 feet, I use 100% of the Cohesion intercept of RESIDUAL direct shear test result. If no structures I use approx. half way value between the RESIDUAL strength and the PEAK strength values. See if you can get a copy of "Soil Strength and Slope Stability" by Duncan and Wright. it has good section on shear strength.

For earth retaining structures, I ignore cohesion on the active or at rest side of the wall and only use 70% of the cohesion on the passive side. For bearing capacity, excluding (CL, CH or MH, Claystone), I use the RESIDUAL srength values of direct shear test but then look at GWT. If historical GWT is within 5B (upper 10 feet) or you think that the soils can become saturated at any point, then you can reduce the cohesion value even further than 70% may be to as small as 33% of RESIDUAL strength. A lot of this information is from experience heavily tilted towards residential and light commercial jobs.