Angle of internal friction of sand is found to decrease with increased overburden pressure (see attached figure). The curved nature of actual Mohr-Coulomb failure envelope (the straight line that we plot for Mohr-Coulomb failure envelope is merely a linear approximation) also indicates the same, i.e. a decrease in friction angle at higher normal stress.
In physical terms friction angle represents particle interlocking. Greater the interlocking between the particles, larger should be the friction angle and vice versa. At higher normal stress better interlocking is to be expected, which should translate into a higher friction angle but it isn't the case (considering the density remains constant). Particle crushing at high normal stress, might be one reason leading to low friction angle. My question is what are the other physical explanations of the decrease in friction angle at higher normal stress/overburden pressure?
Your question is the holy grail of shear strength , as you correctly understood at higher rate of normal stress aggregate interlocking forces will be broken and coarse particles will crush BUT your assumption is wrong,
higher load leads to more compaction but without required moisture for impersonating as lubrication, soil's particles ride on top of each other and this will leads to breaking of aggregate internal bonds as same as aggregates interlock forces , which consequently will reduce internal friction
I hope I have been of some help
So what I get from your answer is that it is only the particle breakage/crushing which is responsible for low friction angle at higher normal stress. But my original question stays unanswered, any other physical explanations of this phenomenon.
That is the case , but in terms of physical explanation, I try my best to elucidate ,
imagine a moving object like a brick on a rough surface when you push the brick slowly there is no evident move , again pushing a little harder nothing happens , again and again until your moving push reaches the ultimate friction between the bottom of the brick and the rough surface, at this moment your brick begins to move with constant rate ( with the pushing force remaining unchanged) , if you push harder the speed will increase and there is no sign of preliminary friction, I think this is the science behind all of that , when you apply load , friction increases and tries to resist but after reaching the ultimate resistance, with the higher normal stress the friction begins to decrease
so when you reach on the verge of movement and annihilation of friction with every increase in pushing force your speed increases, in the case of aggregate interlock force specially for aforementioned sand, higher stress at constant rate, first move aggregates close into each other and will compact them but after breaking of aggregate interlock forces and specially wearing of sharp edges of coarse particles , friction will diminish and final rupture ( same as incremental pushing force and consequent movement ) occurs.
hope it shed a light on your question
best regards
I believe in this figure, it is the peak friction angle that decreases with increasing confining stress. According to critical state soil mechanics, peak friction angle decreases due to lower dilation caused by higher confining stress. Critical state friction angle, however, should be independent to confining stress (of course, within a certain range).
Best,
Fei
First of all, the critical state friction angle does not change with the overburden pressure and soil density and thus, the discussion must be about the peak friction angle.
Many ordinary stress-dilation rules based on the balance between the input and dissipated energies (e.g., Taylor’s stress-dilatancy rule) highlight a clear direct linkage between the mobilized friction and dilation angles. Considering that the increase in the overburden pressure mitigates dilation, one can expect decrease in the peak friction angle with the increase in overburden pressure.
I think that the increasing friction angle is due to increased pressure in the sandy soil, and the question of Muhammad Irfan applies to clay soil.
friction angle in coarse , medium sands are more when compared to fine, very fine silt and clay materials . soil liquefaction instantly occurred in clay -silt soil nor in sandy soil . grain size is the physical explanation as the grain size increases the friction angle increases , as the grain sizes decrease ( clay) friction angle decreases.
If the angle friction of soil is increased what happen to shear strength of soil
Dear Muhammd,
Friction angle is an important parameter in the static and dynamic stability. The friction angle of sand decreases with increasing confining pressure or overpressure thus implying a curved soil failure envelope.
For understanding scientific interpretation,
The shear strength of clayey-sand can be affected by several factors, e.g. gradation, density, moisture content, and the percentage of clay and sand fraction. The addition of clay fraction changes clayey sand mixture gradation. Shear strength components (cohesion and the internal angel of friction) are strongly affected by the particle size distribution of soil mixtures. The larger the sand particles, the higher the friction, and vice versa. Gradation coefficient (Cc and Cu) does not affect the shear strength significantly. The addition of clay fraction in sand mixtures increases the internal angle of friction, and decreases cohesion, at a certain extent where coarse particles still dominates the shear plane. The further of clay addition make coarse particles no longer dominant on the shear plane, resulting in the opposite effects (lower internal friction angle and higher cohesion). Cohesion is more affected by fine materials content rather than coarse particles.
Hope it is helpful for you.
ashish
if your soil profile is various or not homogenous, it can happen. Normally, the shear strength could increase depends on the soil condition. If it is homogenous, it can increase. The terminology on "shear strength increasing with depth (means vertical overburden pressure) is generally ideal for very deep-homogenous soil. You can approve it by conducting CPT test and make a correlation. It seems one of the ways to confirm it. Hopefully, it can help you
Because at higher overburden stress during consolidation results in particle crushing this results in lower friction angle of sand than that of lower consolidation stress during consolidation.
Muhammad Irfan