Basic mechanics of soils
Figure – Relationships between undrained shear strength and Standard . Table – Summary of effective stress (drained) shear strength .. shear strength (s = c = su, φ = 0) are performed using total stresses; effective. This term describes a type of shear strength in soil mechanics as distinct from drained strength. Conceptually, there is no. What is the relationship between he undrained shear strengh(Su) and the unconfined compressive strength(qu) of a soft clay? c or c' that is generally used to represent the cohesion intersept for drained shear conditions.
The vane is a simple test that gives much more reliable information. I assume that the qu you refer to is from a pocket penetrometer or did you conduct a test on an undisturbed sample?Drained and Undrained Soil Shear Strength
Still, it has been traditionally used for so many years. It is a valid estimation in my book given correct judgment on the final use of the numbers. I have developed correlations of Su and N for a number of clayey soils - it's deviation or scatter is no worse than for many other correlations. Many times, too, you have no recourse.
The drillers here do not have vanes with them - if you want them it is a real hassle. They are only set up for SPTs and thin-walled tubes that they "tap" in, of course!! But, most of all, with all correlations and most "test" results you need proper understanding of your problem and good judgment.
Relationship between Su, qu anc c Focht3 Geotechnical 8 Apr 04 While this represents a more desirable circumstance by cleaning up our nomenclatureit isn't true - yet. Total stress, not effective stress, concepts still dominate this area of geotechnical design.
Relationship between Su, qu anc c dirtguy4 Geotechnical 4 Jun 04 It is cheap, quick and easy to perform. The problem is that it measures the wrong parameters.
Shear strength, by definition, is the effective cohesion c' plus the effective stress times the tan of the effective angle of internal friction. I agree with jdmm on this one. An accurate laboratory vane, this is not a field vane or hand vane but a stationary laboratory, bench mounted lab vane apparatus that can measure the shear strength on shelby tubes samples will generally give better results, especially when testing clays and clay tills.
The beauty of the laboratory vane is one can get several tests throughout the length of one shelby tube sample. For example take one lab vane test, push out a little sample, take another lab vane test and so on. Averaging the results will give a far more reliable result that an unconfined, especially if the till is quite rocky.
Relationship between Su, qu anc c Focht3 Geotechnical 4 Jun 04 I'd love to run CU's with pore pressure measurements all the time ah, the fees!
G increases rapidly with depth around here. Once this peak strength caused by dilation has been overcome through continued shearing, the resistance provided by the soil to the applied shear stress reduces termed "strain softening".
Shear strength (soil)
Strain softening will continue until no further changes in volume of the soil occur on continued shearing. Peak strengths are also observed in overconsolidated clays where the natural fabric of the soil must be destroyed prior to reaching constant volume shearing.
Other effects that result in peak strengths include cementation and bonding of particles.
The constant volume or critical state shear strength is said to be extrinsic to the soil, and independent of the initial density or packing arrangement of the soil grains. In this state the grains being sheared are said to be 'tumbling' over one another, with no significant granular interlock or sliding plane development affecting the resistance to shearing. At this point, no inherited fabric or bonding of the soil grains affects the soil strength. The residual strength occurs for some soils where the shape of the particles that make up the soil become aligned during shearing forming a slickensideresulting in reduced resistance to continued shearing further strain softening.
This is particularly true for most clays that comprise plate-like minerals, but is also observed in some granular soils with more elongate shaped grains. Clays that do not have plate-like minerals like allophanic clays do not tend to exhibit residual strengths.
Relationship between Su, qu anc c - Soil testing engineering - Eng-Tips
The Critical State occurs at the quasi-static strain rate. It does not allow for differences in shear strength based on different strain rates.
Also at the critical state, there is no particle alignment or specific soil structure. Almost as soon as it was first introduced, the critical state concept has been subject to much criticism --chiefly its inability to match readily available test data from testing a wide variety of soils.
This is primarily due to the theories inability to account for particle structure. Further, an assumption commonly made to make the model mathematically tractable is that shear stress cannot cause volumetric strain nor volumetric stress cause shear strain. Since this is not the case in reality, it is an additional cause of the poor matches to readily available empirical test data.
Additionally, critical state elasto-plastic models assume that elastic strains drives volumetric changes.
Since this too is not the case in real soils, this assumption results in poor fits to volume and pore pressure change data. Steady state dynamical systems based soil shear [ edit ] A refinement of the critical state concept is the steady state concept. The steady state strength is defined as the shear strength of the soil when it is at the steady state condition. The steady state condition is defined Poulos as "that state in which the mass is continuously deforming at constant volume, constant normal effective stress, constant shear stress, and constant velocity.