Friday, November 30, 2012


In order to write a report that provides required geotechnical information to design a foundation, we need to know how to design the foundation, and what the soils are. Either part by it's self is not enough. We need to know what is to be built and how, in order to provide the proper values, without a bunch of excess. Time is money, in the engineering world. Waste of time cost somebody. When we are working on fixed price, then it costs the engineering company. Hourly rate work is rare in this industry.

I do not like to teach anyone, but those who do not know basic stuff, I find extremely frustrating. The training is all fine, but they seem to be missing the basics. 

Tuesday, November 20, 2012


Grinders, people who grind on designs to reduce cost. You can either take responsibility for the design as you changed it, or build it as per design. Either one, and do not expect that I will be responsible for your modification to reduce cost. That is on you.

So if you do not like my design, find another nigger.

Friday, November 16, 2012

uls sls ws

The relationship between ULS, SLS and working stress is a construct of the collective minds of structural engineers. It has become codified and is just caca el toro in soils terms, but we geotechnical engineers must live with the government decrees, no mater how stupid they are.

Consider the centre of this diagram a number line going from low to high value.
Above ground we start out with the structure loads, and up-scale those values by partial factors of safety, as found in the building code or bridge code. these become the factored loads.

The code says that the factored loads must be less than the factored resistance, which is all fine and good, but in soils terms, the factored soils value is largely theoretical. Here, and any time Standard Penetration is used, the bearing value is empirical working stress set from a bearing value that will give about 1 inch of differential settlement, 2 inches total settlement, aka SLS value. A similar exercise is used for Cone penetration, unconfined values from Shelby tubes, or pocket pen, whether in Qu or C.

Now what happens when we have a mix of values? we need to convert all to allowable design values, and compare, and then pick our report values. We can use a factor of safety to up scale the value to some theoretical unfactored ultimate, and then apply the CFEM factors to come to a factored resistance. But now both the SLS and ULS must be checked, and for most foundations the SLS should govern. But now we are doing something odd. Some use the approach of  up-scaling working stress value by 1.35 or some number between 1.25 and 1.5, the range in load factors, and use that value. 

My preference is to provide ULS and SLS values, and let the structural engineer do twice the work of previous. Some other appear to be calling the old allowable value the factored resistance, which demands larger foundations. The bearing value in some of soils reports of late from others firms seem to be odd in relationship to soil strength and traditional values, which is of concern. Not enough test holes and high factors of safety. Not good engineering. 

Enough said.           

Monday, November 12, 2012

Basement lateral pressure

Basement lateral pressure, backfill pressure, is a tricky subject. Most books say it is Ka times the vertical, and I believe that true, right after construction. Over time I am not so sure. With high plastic clays, dessication - wetting cycles, freeze thaw cycles, for shallow fills, I think the residual stress dissipate, and the wall lateral pressure increasing over time to Ko. This is not what some of the books say. I think this condition takes 30 to 40 years to develop. Are there any long term lateral pressure studies on house basements, high plastic soils, deep frost penetration? none yet found.

After the wall has failed, the lateral repair must be stronger, because prior to failure, there was a diaphragm moment due to the end walls. During failure, the end cracks relieve this moment, hence all the lateral moment must now be taken by the vertical stiffness of the wall.

When we add potential swelling pressure and potential freeze expansion pressures, the lateral pressure becomes a large transient value. The upper bound of the pressure would be Kp; however, Kp exceeds the typical probable pressure. To use Kp for design would be excessive.

There are areas of the wall that have no lateral support, either through wall openings or through no floor diaphragm as dropped floors and stairs parallel to the wall. These may need abutments or counterbalancing exterior stairs or steps. 

Epoxy injection can reach the strength of the previous wall; however, to do that, all the cracks would need to have 100 percent penetration. The other issue is that after the wall cracked, the stresses were relieved. After epoxy injection, the critical failure start point will have changed. This results in loss of effective strength. The corners have loss the deflection necessary to develop the support it had before.

I think the only way is to do a structural design accounting for all the loss of strength, using a Ko = 1.0, plus a live load for all construction equipment, and reconstruct the wall or equivalent repair. Outside stiffeners can be used, but they must account for the actual openings in the wall and diaphragm. Typical spacing may not be adequate. There will now be no (very little) support from the end walls.

Over the years these are the methods I have seen utilized are as follow:

  1. Shotcrete reinforced wall inside
  2. Concrete abutment inside
  3. Reinforced shotcrete wall outside
  4. Wall replacement with proper reinforced design accounting for openings. One required an inside abutment.
  5. PWF inside wall, without plywood
  6. PWF inside wall with an plywood abutment under the beam. All the PWF were inset into the floor to existing foundations. New foundations may also be required. Bottom and top lateral support is required.
  7. Exterior vertical stiffeners, back to back channel bolted through the wall to bring the wall back into line.
All these methods require the wall to be excavated full length and depth, exterior water proofing, good condition weeping tile to provide drainage, and some kind of vertical water drainage is recommended on the exterior wall face.

Thursday, November 8, 2012

failure, who is responsible

An old, (35-40 year) sewage lagoon has "failed". It was built on clay till, (assumed, typical of the area) and affluent penetrated a dike, ran in a field and down a ditch intended for surface water. No geotechnical investigation is available, whether not done, being withheld, or just lost. The original design show how it was to be built, not the way it was. No testing data is available.

Who is responsible?

But we have no information. Piping or erosion, physical damage, muscats, gophers, water level, all unknown. Client is unwilling to pay for an investigation. So what should we do?

Thursday, November 1, 2012

Only Part Residential

What is the real issue in building partly residential buildings? Our stupid building Code. Not really a safety issue, but a code writers ego, and insistence on beginning right without any reason, beyond his own collective ego bullshit.

Now the other side of the coin, the client, a builder is unwilling to spring for a geotechnical investigation, but wants pile designs for a bit of a stairs and roof overhang.

So how do I deal with this. Here is one example.

As this project is not entirely residential, the authority having jurisdiction or their agents are entitled to ask for Alberta Building Code (ABC) A, B, and C schedules. Tully Geotechnical Limited is unwilling to provide ABC A, B, and C schedules without a geotechnical investigation. The designs provide are provided on assuming the project is purely residential, and with assumed soil pile skin friction design parameters. Should the authority having jurisdiction or their agents request ABC A, B, and C schedules, the designs will be cancelled and other arrangements for geotechnical engineering will be required.  

How is that for a weasel clause?  
By the way the design parameters assumed are 15 kPa below 2.5 metres, if they can put in a pile, it will give us that. 

The PWF Solution

Last day I spoke briefly about lateral pressure failures in basement walls. There is a nice solution, and that is a PWF wall inside, if the subfloor is suitable or can be made suitable. A notch is cut into the basement slab, and the wall installed without sheathing. Nice clean solution if the wall is straightened and waterproofing is applied on the outside. This wall must be structurally designed. Excessive lengths of openings can also be an issue, where doors, windows, the like occur together. It must be attached to the floor diaphragm and the floor cut must be grouted. Also an abutting wall maybe required, for lateral support.