Posted in Deep Foundations, Geotechnical Engineering

It’s Coming: Geotechnical Analysis One Grain at a Time

In going through some papers, I noticed this one, Friction anlysis of large diameter steel cylinder penetration process using 3D-DEM, whose abstract is as follows:

Large open-ended cylinder piles have been widely used for engineering foundation of port. The penetration process of the large-diameter steel cylinder exhibit complex behaviors, which is difficult to be measured by test and reproduced in numerical models. This study presents a friction analysis of large diameter steel penetration process by using the discrete element method (DEM), which can simulate large deformation and nonlinearity well. Centrifugal model and full-scale model were developed to analyze the sliding friction of the cylinder during installation and the contact force chain of soil particles. The validity of the DEM model was examined by comparing with theoretical values and published studies. Parametric studies were carried out to study the effects of contact parameters on side friction. Simulation results showed that, unlike pile penetration, there is no obvious soil-plug effect during the penetration process of large-diameter steel cylinder. Besides, the inside friction is smaller than the outside friction for large-diameter steel cylinder. What’s more, the computational cost of full-scale model based on the upscale theory was less than the centrifugal model. There is a close relationship between the side friction and micro contact parameters, which provides a reference for the follow-up study of cylinder or pile penetration using DEM.

Friction anlysis of large diameter steel cylinder penetration process using 3D-DEM

Put into simple terms, DEM models the soil by a grain-by-grain analysis of its response to load, which in this case came from 22 m O.D. open-ended pipe piles for the Hong Kong-Zhuhai-Macau Bridge. Believe it or not, I seriously considered using something like this for the STADYN project, but was dissuaded from doing so because of complexity and stability issues. That’s probably for the best, because the program I was in encouraged a “roll your own” approach to computer code, and to be honest at my stage in life and background I wasn’t up to such a development.

One thing that helped the authors of this paper was the fact that the soils they were driving into was cohesionless, without the chemical bonding that comes with cohesive soils. It’s also interesting to note that these piles were vibrated into place; the whole subject of vibratory driving, its performance prediction and static capacity determination, is another long-running subject in this business.

Although the issues of cohesion and verification (always the fun part of geotechincal engineering) need further resolution, DEM is, in my opinion, the ultimate solution of the soil interaction question, and needs to be disseminated further in our industry.

Posted in Deep Foundations

Mary Ellen Large Joins Nicholson Construction —

Geotechnical specialty engineering and construction contractor Nicholson Construction is pleased to announce the addition of industry expert Mary Ellen Large, P.E, D.GE to the team. Ms. Large will be assuming the role of Client Care […]

Mary Ellen Large Joins Nicholson Construction —

Until recently she was Technical Activities Director for the Deep Foundations Institute, a fact which takes some digging to ascertain on their website.

Posted in Deep Foundations, Geotechnical Engineering, Pile Driving Equipment

Reconstructing a Soviet-Era Plastic Model to Predict Vibratory Pile Driving Performance —

The latest in our series of monographs on vibratory pile drivers, this one takes us back to the beginnings of vibratory pile driving in the Soviet Union. It was prepared for the ReSEARCH Dialogues at the University of Tennessee at Chattanooga in April 2021. The vibratory driver that started it all: the Soviet BT-5, used […]

Reconstructing a Soviet-Era Plastic Model to Predict Vibratory Pile Driving Performance —
Posted in Deep Foundations

Video of 2011 Pile Load Test Failure in Singapore —

A static load test was being performed on (presumably) a group of piles at a construction site on Gilstead Road in Singapore in 2011 when the massive pile of concrete blocks toppled over, blocking the […]

Video of 2011 Pile Load Test Failure in Singapore —
Posted in Deep Foundations, Geotechnical Engineering

Vesić’s Method of Estimating the Settlement of Driven Piles and Drilled Shafts

Today for serious projects, if one wants to estimate the settlement of a deep foundation, one uses a “t-z” method of some kind. These methods involve analyzing the pile-soil interaction (and the internal flexibility of the pile) to develop a load-settlement curve similar to one obtained with a static load test. There are several types of algorithms available for this purpose, and they work in different ways.

But what if one wants a “back of the envelope” estimate of the settlement of a deep foundation, or possibly a preliminary load-settlement curve? As is frequently the case with geotechnical engineering, there is more than one way to do this, but probably the simplest–and well represented in practice–is Vesić’s Method, named after Aleksandar Vesić. The method is presented here as shown in NAVFAC DM 7.02, but it appears in other publications and textbooks.

First, we’ll present the method itself:

Let’s break this down: first, it’s necessary to identify some parameters.

  • Ultimate static capacity of the shaft Q'_s . It isn’t necessary to split up the shaft resistance further along the length; that problem will be handled by other factors. Note that this is a primed variable, and not directly entered into the equations. This will be explained shortly. The method handles toe resistance differently; it is necessary to compute the ultimate unit toe resistance q_0 , which is usually a result of whatever method you choose to compute the ultimate static capacity of the pile.
  • Pile geometry, including length L , cross-sectional area A , pile “diameter” B , length of the pile embedded into the soil D and modulus of elasticity of the pile E_s . The method assumes a uniform cross-section and material for the entire pile length.
  • The factor \alpha , which considers the distribution of soil resistance along the shaft. For a uniform stratum, conventional wisdom has been that sandy soils will have a triangular distribution (due to the dependence of shaft resistance from effective stress) and clay soils a uniform distribution, as the resistance is based on the soil cohesion. However, since Burland, that notion has been challenged, and has created something of a controversy. The best resolution, as shown by the current API method and Kolk and van der Velde, is that the adhesion itself varies with effective stress; it’s not really an either/or, but a both/and situation. Unless the soil data indicates otherwise, the simplest way to resolve this is to say that the default value of \alpha = \frac{2}{3} .
  • The variable C_p , given in the table above.

The simplest way to illustrate how this method works is through an example. Consider the case of a 16″ square concrete pile, 125′ long, driven into loose sands. We want to estimate the settlement of the pile under two axial loads: 400 kips and 600 kips. The water table is at the surface. From the TAMWAVE program, we learn the following:

  1. Ultimate shaft resistance Q'_s = 527.95\,kips .
  2. Unit toe resistance q_0 = 238.0\,ksf .
  3. Pile length L = 125' .
  4. Pile cross-sectional area A = 1.778\,ft^2 .
  5. Pile “diameter” B = 1.333\,ft .
  6. Embedment of pile into soil D = 125' (TAMWAVE only analyses groundline piles.)
  7. Modulus of elasticity of pile E_s = 719,910\,ksf .
  8. Shaft distribution factor \alpha = 0.667 .
  9. Settlement coefficient C_p = 0.04 .

With that out of the way, we can proceed; however, at this point we get to the tricky part of this method, namely computing Q_s and Q_p . This is where students get tripped up, but the root problem is how we look at the “capacity” of deep foundations.

Static capacity methods imply that there is “a number” which represents the static capacity of a pile. They also imply that this capacity is divided neatly between shaft and toe resistance. The truth is that neither is the case; a deep foundation is loaded progressively and the resistance is mobilised progressively, and you end up with a load-displacement relationship from which you determine the maximum permissible load of the foundation.

Vesić’s Method replicates that by the way that Q_s and Q_p are computed, and it is as follows:

  • Q_s is either a) equal to the applied load Q_{applied} if Q'_s > Q_{applied} or b) equal to Q'_s if Q'_s > Q_{applied} .
  • Q_p is either a) equal to zero, if Q'_s if Q'_s > Q_{applied} or b) equal to Q_{applied} - Q'_s if Q'_s > Q_{applied} .

With those definitions, the first thing we do is to compute the coefficient C_s . By substitution of the given variables into the equation above, C_s = 0.64 .

Then for the case of Q_{applied} = 400\,kips , since Q'_s > Q_{applied} , Q_s = 400\,kips and Q_p = 0 . Substitution of these values along with those given or computed above yields the following:

  • W_s = 0.026'
  • W_{pp} = 0
  • W_{ps} = 0.009'
  • W_o = 0.026 + 0 + 0.009 = 0.035' = 0.416"

Turning now to the case of Q_{applied} = 600\,kips , since Q'_s < Q_{applied} , Q_s = 527.95\,kips and Q_p = 600 - 527.95 = 72.05\,kips . Substitution of these values along with those given or computed above yields the following:

  • W_s = 0.041'
  • W_{pp} = 0.009'
  • W_{ps} = 0.011'
  • W_o = 0.041 + 0.009 + 0.011 = 0.062' = 0.742"

It should be evident that, if these calculations are repeated for a range of Q_{applied} , a load-settlement curve could be developed, and in fact this has been done and it is documented on this spreadsheet. In addition to Vesić’s Method being shown, the results of TAMWAVE (which uses a numerical t-z method to develop the load-settlement curve) are also included, and the results are shown below.

With this the following should be noted:

  • The results in the range which the two methods share are similar, especially with the “break point” where shaft resistance is maximised and toe resistance starts to kick in.
  • The t-z method which TAMWAVE uses assumes a purely plastic response once the ultimate static capacity is exceeded, i.e. plunging failure. Vesić’s Method does not, and in many cases this is more realistic.
  • The TAMWAVE method has an unloading curve which is not shown here, which Vesić’s Method does not replicate.
  • It is possible to apply a static load test criterion (such as Davisson’s Method) to either or both of these results. It should be expected that the ultimate load derived in this way is different from the ultimate load computed by the static capacity method.

From this we can see that Vesić’s Method is a useful tool to find an initial estimate of the load-settlement characteristics of either driven or bored piles. The difficult part is grasping the method by which Vesić’s Method models progressive mobilisation of the shaft and toe resistance. Doing this, however, can give us a better understand of the whole concept of resistance mobilisation, which is crucial in understand how deep foundations resist and transfer loads.