Soil Mechanics - Science topic

Every pass over the field for any type of tillage or by other farm tools can break up soil aggregates and reduce the ability of the soil to hold moisture. Heavily tilled fields may have a good seedbed for planting, but any rainfall after planting may cause the surface to seal, resulting in surface compaction. Tilled soils are more susceptible to compaction than no-till soils. Tillage contributes to the breakdown of soil structure by compressing and breaking soil aggregates, which are necessary for good air and water movement and good root growth. Conservation tillage systems improve infiltration reduce soil compaction. Compaction reduces infiltration and, inevitably, restricts water flow to the root zone. The only way to eliminate soil compaction is not to drive on the soil. Practices such as no-till-, and conservation tillage agriculture can greatly reduce the amount of stress placed on the soil.

You can use some of the packages Wolfram Mathematica, MatLab, Maple.

Dear Waleed,

The matter is not the soil itself. Curing raw soil has no sense unless, you wish to homogenize humidity in it. However, curing the stabilized soils depends on the type of binder that is used for stabilization. If you use chemical binders that require water for reaction, such as hydraulic binders (cement for instance) or pozzolanic binders (lime for instance), then you'll need to keep water in your specimens as long as possible after for reactions to continue and therefore for better efficiency of the stabilization.

Well, once that is clear, the choice of the curing method depends on the researcher's philosophy. Keeping the specimens for a given period after their fabrication in a controlled Temperature and relative humidity environment may rely on the purpose of comparing various admixtures' performances so that the differences could be assessed with no doubt on any impact of their curing conditions. Finally, some scientists may consider that leaving specimens in varying temperature and relative humidity is more realistic. That is to say that these conditions are like those occurring in the construction site where it isn't possible to apply laboratory curing methods.

regards

It depends on the type of application and objective of the improvement. For increasing bearing capacity you may select uniaxial or bi-axial geogrids. On the other hand to improve the slope protection and to increase permeability of soil, one may select woven or non woven geotextile.

Based on the picture, the case in my opinion is relatively easy. From the existing data you can control the shear stability, rolling stability and bearing capacity stability, and what is important is the ability of the material as a retaining wall. If you have doubts about the gravel in front of the retaining wall then in the design it is neglected so that the construction is safer but rather wasteful.

I assume that the foundations are made of hard and compact rock which can be modeled as an elastic material. In this case, there are almost no differences in the response of the dam (static and/or seismic). Another reason could be related to easier modeling and less computational cost.

Personally, I prefer to model also the foundation layer.

Colorado School of Mines

soil structure interaction

Zew Zealand Geomechanics Society Recorded Webinars.

Hi

I would like to suggest International Journal of Geosynthetics and Ground Engineering and Applied and Environmental Soil Science.

Dear Ahmed,

There always exist a shear stress between moving and stationary soil masses due to relative displacement. Localized displacement causes arching. Change of soil strength as well as soil plasticity also the reason of soil arching.

If a statically admissible stress field is

provided support then there will be less failure due to arching.

If the work rate of external load acting on the soil exceeds the rate of internal work rate then arching will not occur.

the soil Classification according to AASHTO is A-7-6 and according to USC systems is CH.

The reason is the one described by Pedro Miguel Vaz Ferreira above. The specific volume is linked to total volumes and definition of volumetric strain is then straightforward. No effect on the CSSM parameters, other than that

I recommended FLAC 3D program

This is a good question.

This is a good question.

That is a good question.

Hello, find the softwares like PLAXIS3D, FLAC3D and also ABAQUS are useful for simulating your problem..ok all the best in your research..

Dear Alireza Akbari

the following tutorials can help you:

Dear Ahmed Hussein Olat,

You should consider the effect of interfaces between the buried structure and peripheral soil on the system interaction.

The elastic media of the soil is represented by a mechanical model, a mathematical model, and a numerical model. The Winkler model is a mechanical model that uses a system of multiple independent linear springs to model the soil. Nevertheless, the Winkler limitations are as follows:

1-the springs are not connected, lack of continuity between the springs, therefore we get no deformation beyond the loading regions.

2- The spring are considered as linear, but the bearing soils is not linear

3- the equal amount of settlement.

It will be better to use non-linear models: Chandra (1979), Boussineq (1985), or an Orthotropic Elastic continuum model (Vlazov model).

G = ρ * (Vs)2 same given by Carlos

Clay loam soil. Theoretically, for maximum compaction there should be an equal proportion of sand, silt and clay fraction, i.e. 33 per cent each. But in nature, it hardly happens. Therefore, try to have proportion within 10 per cent deviation among these fractions to have maximum compaction. Yes type of clay particularly, swelling type is also the factor which determine maximum compaction at given moisture content. Proctor moisture shall be used for achieving highest compaction. May contact further on vkphogat@gmail.com. thanks

Hello Dung Nguyen,

The most frequently applied relationships between void ratio and soil stiffness date back to Hardin and Richart (1963) or Richart et al. (1970). However, what is your intention of using Mohr-Coulomb's model to estimate settlements? M-C is adequate to model soil failure, but not soil deformations.

References:

Hardin, B. O.; Richart, F. E. (1963). Elastic wave velocities in granular soils. Journal of the Soil Mechanics and Foundations Division (ASCE), 89(SM1), pp. 33-65.

Richart, F. E.; Hall, J. R.; Woods, R. D. (1970). Vibrations of soils

and foundations. International Series in Theoretical and Applied

Mechanics, Prentice-Hall, Englewood Cliffs, NJ.

The upward buoyance force should be considered as the external force to act on the immersed object you are dealing with. The effective stress analysis of soils need be considered for the geotechnical resistance, displacement magnitude and rate.

Feel free to get in touch with me for more explanation.

The Air entre value is a pressure AEV given in kPa at a specific volumetric water content θ.

The pressure is defined as AEV= Force (N)/Area (mm2).

The permeability K= distance (cm)/time (Second)

Based on these relations, it is impossible to establish a mathematical correlation between (AEV) and (K). However, a correlation can be established based on experiments by measuring the AEV and K on duplicate specimens for a specific soil material.

Building on some of the previous posts, in my mind "weak rock" refers to the rock's strength. Typically, a boundary strength value will be specified to separate "weak" from "strong" for a given context.

"Soft rock" refers to how a rock responds to weathering, or how abradable the rock is (there tends to be a correlation, but they are different processes). A rock that quickly weathers is "soft." Likewise, a rock that is readily abraded, for example when it forms a stream bed, is "soft." Often, but not always, these "soft" rocks will exhibit slaking behavior when they are cycled through wetting and drying. It is a specific form of disintegration in response to weathering and is associated with clay minerals. Slaking can also result from freezing and thawing or the growth of precipitates, like salt.

Low strength, readily weathered/abraded, and prone to slaking are often attributes that are found together (e.g. a claystone). Likewise, high strength, slow to weather, and not being prone to slaking are often found together (e.g. a quartzite). However, there are exceptions. For example, some sandstones can be relatively strong, but weather relatively quickly without exhibiting slaking behavior.

In compaction curve, the zero air void line which is a line of 100% degree of saturation drawn on the right side of the compaction curve without touching it. this line helps to check that the plotted compaction curve is correct.

Thank you I will check on it.

You said " Now you are applying structural load. So any settlement that happens is predominantly because of the structural load alone even though you back fill it with excavated soil" this means that, after considering the soil as NC, the structural load will be the one causing the settlements , ignoring the weight of the excavated soil, right?

Your explanations are good. Thank you.

Thank you Hani Alharbi

Too many topics :) .. but there is interest nowadays towards bio-stabilization of soil

The increase in density is expected since u adding new material to the soil and that material filled the pores. Water was reduced because this material u added reduced somehow the affinity of the soil water. For example if the soil has clay, something is going pin on mice-scale. for example the available surface area was decreased therefore a decrease in action exchange capacity is expected. this less affinity for water.

Go for PLAXIS and ABAQUS.

Dear Niklas, as you already mentioned various authors [Lacasse & Nadim, Wolff et al., Lumb, JCSS] suggest to use a Gaussian distribution to describe, for instance, the effective friction angle. In principle, however, Lognormal distributions are also a reasonable choice and recommended by other authors [Schultze]. Schneider & Schneider recommend the use of Lognormal distributions in particular for larger coefficients of variation. Additionally, if you favour a Gaussian distribution, you could also model it as a truncated distribution. The hydraulic conductivity k is commonly considered as lognormally distributed.

Dear Ahmed Hussein Olat

The pressuremeter modulus has been related empirically to the elastic modulus of the soil as EM/E = α, in which  α  is termed by Menard as the rheological coefficient and has a value between 0 and 1.

For more details, please refer directly to :

http://www.soil-rock.ir/pm/wp-content/uploads/2017/05/Using-pressuremeter-to-obtain-parameters-to-elastic-plastic-models-for-sands.pdf

Nowadays, the most important advances in the LEM world is the search methods to find the most critical slip surface used together with the LEM approaches. These methods are Metahueristic methods such as Cuckoo Search, Particle Swarm, Simulated annealing from other fields brought to the geotechnical applications. The value of the LEM methods to be combined with the new search methods becomes more important (these are all non-circular methods). Our studies showed that, for complicated models, simplified Bishop method cannot give good results. Janbu method gives the lower-bound for the factor of safety. The best methods that give similar answer to even FEM analysis are Spencer and Morgenstern Price methods. I hope that helps.

Mr.Muneeb,

I feel the main important one in all over the world is population increase and no space available for developing infrastructure relative to increase in population. so the construction activities have taken place as closer to an existing structure. In view of this, the stability analysis of existing structure adjacent to new construction in terms of excavation, embankment, tunneling or pile driving is a serious problem. As a geotechnical engineer we need to come out with design idea to protect the existing structure due to the effect of new construction.

Another important issue is we don't have excellent supporting ground for the new construction. So, in this case we need to develop economical ground improvement technique to support the load. Ground improvement technique alternate to conventional method is catching up among researcher . ok all the best in your research..

Thank you Ahmed Hussein Olat !

That's an interesting piece of work.

However, what I am interested in particularly is the MSE walls.

There are complications in the response of the geogrid interface to the soil and the block in dynamic analyses.

Because of that, I haven't seen research that investigates the dynamic response of an MSE wall.

In industry, I've seen people modelling the whole MSE block (without modelling geogrid and interfaces) using an overly stiff material that at the end would act as an elastic block because of how stiff it is.

I was just wondering if there is anything in guidelines or research that address or recommend this approach.

Bowles' equation is one of the most misunderstood equation as also mistakenly recommended above by Mr. Salem for clays. The two pages in Bowles explains itself. He wanted to give a simple equation based on the settlement based bearing capacity equations for sands. For example, Burland and Burbridge. Those equations calculate the load that corresponds to 25 mm settlement in sand.

Whether your results are reasonable or not will typically depend on a No. of factors:

1. Percentage of Clay and Sand in the sample, more clay implies more cohesion and more sand shall mean a higher friction angle value.

2. Moisture content also has a significant influence on the development of Mohr-Coulumb failure envelope and on the behavior of soil under shear.

3. Testing conditions also have an impact on the results. Often laboratory testing equipments aren't properly calibrated or the lab assistants lack expertise to conduct such tests, these factors have a significant bearing on test outcome.

4. Supervision: A year ago, i conducted DST tests on somewhat similar soil samples having varying proportions of sand and clay, and i found out that the Cohesion and Angle of internal friction values were considerably on the higher side rather unusally. Then my supervisor suggested Triaxial tests to check out influence of capillarity & drainage on results, then i did Consolidated Undrained Triaxial tests to work out the variations between these parameters. The parameters from Triaxial tests showed around 15-20 percent decrease in values when compared with DST values, so it's advised to conduct triaxial tests for better precision. However, if you don't have that much resource availability, you can conduct a No. of trials, work out the variations a

I hope this table answers your question.

There are standard methods for testing soils to consider. Usually soil samples are taken and put into a container of water so the soil is saturated from the bottom, about 24 hours. Unless there is a water table to saturate soils in the field, ability in reaching saturation may vary with the conditions. I remember we had a dry period and wildfire, then a substantial rain, about 3-5 inches. Water was pounded on the surface the next day, but the soil scientist found the soil totally dry about 12-18 inches below the surface. I also learned that an upper clay layer needs to be saturated before the lower sandy layer can effectively gain water and this is related to the tension with which clay holds water as compared to sands. Some soil layers have difficulty becoming saturated, such as well developed forest soils seldom exhibit runoff due to high infiltration and macropores from roots, etc. other soils as hydric soils are saturated frequent enough to exhibit hydric soil indicators due to frequency of high water table. Sampling the soil depth or horizon is probably the most reliable using standard methods to saturate and measure.

I would not use just the high reading in 6 month period, unless at least I reviewed nearby stream gauging stations and the rainfall during the 6 month produced at least a bankfull streamflow event, which occurs generally about every year, and the stream channel is at a level where flooding begins for stable channels that have not aggraded or degraded. I would want to be pretty sure there was a reasonable likelihood of saturation based on evidence like the streams were near or at flooding. The other option might be to set up so the area with the data sensors can be irrigated for long enough so the detectors stabilize after reaching their approximate maximum. Saturating from the surface can apparently leave some air spaces unfilled, but unless the area periodically has a high water table, the air spaces may not be filled easily to saturation by rainfall anyway.

Thanks Alireza Rashiddel

Dear Mustafa

there is an article about "Bearing capacity of strip footings by incorporating a non-associated flow rule in lower bound limit analysis" that found out clearly the increase in the magnitude of bearing capacity factors with an increase in the magnitudes of dilative coefficient (η).You might want to check it out, it may be useful. kind regards

This is one of the toughest questions in rock mechanics. Many 'answers' may be far from the truth - which we will never know anyway. But here goes: what if 2D UDEC-MC or UDEC-BB are allowed/available. Represent the jointing as sketched by an engineering geologist, digitize, and apply scaled JRC, JCS values using block size scaling. Convert the non-linear BB to MC for the joints if UDEC-BB not available. Try to model with some discontinuous jointing, and use numerically glued joints. Can estimate a lower-bound c for the intact bridges/glued joints from a linear tangent to the tensile and UCS Mohr circles, c =1/2(sigma c x sigma t)E0.5: construct and check, also estimate phi. If a continuum analysis is to be performed with linear MC or non-linear HB be aware that degrading c and mobilizing phi is more correct than the usual addition. If the building is very high and you are under higher stress, the more correct curved envelope for the critical intact bridges is found by moving 1 x UCS along the normal stress axis and drawing the critical state max. shear strength Mohr circle, with radius = UCS, therefore max possible shear strength numerically similar to UCS (Singh et al 2011). Sigma 1 (max) = 3 x UCS for stronger more correct curvature than HB. Note also stress-dependent deformation moduli might be relevant for a high building.

Dear Marcelo Llano

You can consider the data result shown in the following paper for Quaternary soils. The liquid limit for the soils has been determined using different methods including the fall-cone method.

Arguably, salts reduce liquid limit of clays.

Does anyone know how natural moisture content effects on porosity/void ratio?

There are some attempts to create artificial soils in a laboratory, mostly cohesive soils. But as far as I am concerned, it seems to be a complicated and requires high technology.

Dear Irakli Skhirtladze

Please see the following link:

I strongly recommend you to read the above recently published article (2020) which discusses both Experimental & Numerical Determination of Creep (Long-Term Deformation) of Sand mixed with Granular Tire Rubber Materials.

Joseph Kaluza-Gilbert J. C. Tarafdar Max Oke Kluger Velusamy Gobinath

Thanks for your useful information William F. Hansen . Actually, I need an idea. If you use these values ( C, Phi of C. C. Block) in any one of your research work, then, let me know.

Soils tend to shrink when they lose moisture. In particular, fine-grained soils are susceptible to shrinkage and the resulting volume change. Shrinkage can cause cracking of soils. Such cracking is related to the shear stress and shear strength of the soil. When the soil reinforces with additives, like husk and straw, its strength increased, and as a result, more resistance to cracking development will be exerted by the soil.

In the direct shear test, the shear stresses and displacements are non-uniformly distributed within the soil specimen. As such, an appropriate height cannot be defined for the calculation of shear strains, in other words, the stress-strain relationships cannot be determined from this test, as a result, any quantity associated to stress-strain curves such as modulus of elasticity cannot be determined.

I'd concur with Hessam's recommendation for Chen (2013). A relatively thick and relatively comprehensive book that addresses a range of aspects of limit analysis in an accessible way.

A book by David and Selvadurai - (Davis, R. O. & Selvadurai (2002) Plasticity and Geomechanics) - is also a good introductory text to the topic.

After these books a dive into the literature in some specific areas that interest you would be a good next step.

Now I am working mainly in the field of Ground Improvement

The key thing about lab tests is consistency and thus comparability. The soil undergoes the same process every time, this means that theoretically you can send the same soil to different laboratories and get the same numerical output. This then means that you can classify the soil and compare it to other soils to get an idea of its behavior. However you are right you should not then carry out design without applying judgement about what the results mean. The site conditions, geological structure, where the samples come from, sampling quality etc all go into the interpretation of the results.

Field tests tend to be more expensive than lab tests, especially large volume field tests. Due to variations in field equipment and field conditions the results from field tests can also be more variable making comparisons between sites more difficult. However you are right, field tests are really useful for ensuring that the results from the lab can be interpreted in relation to what is happening in the field.

Computational models can be carried out... but they will be meaningless without a source of parameters for the grounds behavior (lab or field). Computational models in this respect can be problematic as their sophistication can generate blind faith in the output even where the initial parameters have no basis in reality.

Irrespective of the approach or tools that are used, judgement and interpretation are the most important elements of the process.

Having done a good deal of this kind of work, it is important to understand that this is more than just technical exercise. It is important to understand the standard of care and local practices by which the performance is judged. The American Consulting Engineers Council has an excellent publication on this topic: https://docs.acec.org/pub/18803059-a2fd-2d06-cc39-a6d1dd575265.

It is also important to understand the roles of all parties to the case. Failures are seldom the result of a single error or oversight, but often are the result of a perfect storm of factors involving multiple parties from the initial investigation, design assumptions, owner inputs, contracting limitations, information sharing, construction practices, etc.

Depending on how many observations and variables for each one you have, you can train a model to predict the "risk degree" of different cases or possible scenarios.

I have studied a bit about artificial intelligence, but actually I have not applied it to real data, nevertheless, despite my short expertise in that field, I think that it is highly possible to estimate the "risk degree" for sinkholes if your data is robust enough.

Best regards,

Good question Azm Shaher Al-Homoud .

Type of foundation should be a function of soil conditions and applied load. Recently, we observe also other factors influencing the geotechnical design.

Concerning soil conditions, we may assume that best locations are already occupied and we move our development to "unsuitable places". That is why we should expect further development of deep foundations. It refers also to reinvested lots with remaining of older (demolished) structures. In that case, the reuse of old foundations or debris from demolition works may be an issue imposed by "eco" regulations. I guess that due to political reason (in relatively reach countries, that seem not to have other problems) the last mentioned aspect may also comprise a pressure to develop "low-emission" technologies e.g. substitution of cementous binders with some kind of fly-ash or other materials with neutral carbon footprint. That also imposes a discussion over sustainability of geotechnical technologies and their adequacy for a desired life time of the structure. For now, I have an impression that we tend to provide "immortal" foundations for temporary or short service structures.

Speaking about applied loads, I'd underline the aspect of seismic and climatic impact on tall buildings and structures. That imposes some measures in design of buildings and their foundations, adopted to those conditions. One may notice that the growth of the economy provokes irrational behavior of many investors and designing "original" (tall, deep, shaped in not natural way) structures just for show and fame. That is why so complicated foundation systems must be designed.

Finally, I'd like to point again on the importance of testing. Whenever we observe a progress, we are touching the unknown. In the case of foundations all the risks are multiplied by the consequence factor for the supported structure and the difficulties related to any strengthening or repair methods under the existing structure.

Thomas Makedon Thanks for the answer. It's actually in an earth dam. We can't have another piezometer as a replacement then.

wherever the place of bending is, water will rise up (in case there's no blockage just bend) and then we can compare it with previous readings to know how much water rises or lowered since the last reading.

Here is a link to a public pdf of the 3rd edition:

See chapter 9 in the book "Hydrology of Disasters" By Vijay Singh.

Also see this book chapter:

Formulations gave in both.

Difficult to say. I would suggest that you start with an elastic model, and then move your way down your suggested ladder until you meet a good match. However, ideally, your stress strain curve from lab results should be able to help in deciding your final selection.

Dear Jayatheja Muktinutalapati

The stiffness of the wall, its ability to move and deflect, play a crucial role in soil pressure envelope in uniform soil. In the case of real soil profiles, it may happen that the increased soil internal friction or appearing cohesion provokes the decrease of soil pressure despite of higher vertical stress.

If you consider various methods of vertical soil support calculations (especially for flexible structures) , the soil pressure envelopes are often modified to take into account this phenomena.

From my experience. Usually the total active pressure is corresponding to the theoretical value but its position may vary depending on the above mentioned factors.

Best regards

Dear, Arun

Increment size plays an important role. If you specify too large increments, the load will be described by a broken line and perhaps the maximum amplitude will not be specified. The increment should be set so that the time step falls at the moment of maximum amplitude and about 10 more values between these intervals to obtain a smoother function (the larger the better).

The type of calculation depends on what tasks you need to solve. If you need to take into account the influence of the design's own oscillation patterns during the calculation, then use dynamic explicit. If you want to determine the behavior of the structure for one period of oscillation of the sinusoidal function, use static general. If the oscillation frequency is very high and large deformations and velocities are possible, then dynamic implicit is used.

I uploaded the input file (*.inp). It is compressed.

The first thought is that elastic waves propagation takes place. Corresponding velocities are proportional to sqrt of elastic moduli. See, for example, https://petrowiki.org/Compressional_and_shear_velocities

Glad to know it resolved your issue.