Extrapolation Beyond the Testhole Area

Extrapolation beyond the testhole area is dangerous and must be totally at the clients risk. A situation has arisen where the client want advise on a site adjacent to a building for a parking area. It was not included in the original investigation.  Any additional advice should be an extra.

The first recommendation should be testholes are required to provide the requested recommendations. We only know the soils at the testhole locations, and there may be only soils classifications that are relevant. Soil varies rapidly across a site.  Fluvial deposits vary rapidly across the direction of water flow, less along the flow, and the flow is constantly changing, so the direction is changing. As sediment is placed, the channel is filled in. When old braiding and oxbows occur, peat will grow. So when we find peat, is it a channel or bog? How uniform do you expect the geology to be?

We expect that there is peat on this site. With peat, they need to be aware that movement, settlement, will occur, and that the settlement can be substantial. Grades are important when settlement is expected, and a 1.5 metre pad over the peat is require, remember that peat is associated with water, so what do we really have?

Next, as fill will be required, we will need to confirm the clay is suitable, and is consistent with the clay assumed in the fill design. Settlement of any remaining peat will occur. How much settlement? Rule of thumb 50% +_ 25% of fill depth. OK. Another one for deep muskegs-- 1/2 the depth of the fill placed. Peat is fibrosis, shearing is often not the issue, just compression of the peat. Fabric or grid can also spread the load better, but does nothing for settlement. Low weight fill has been utilized a few times. Artificial foam has been proposed to be floated, but I have never seen. 

One more thing to think about.
   

Routine Buildings, required number of testholes for preliminary investigation.

Ultimately, the purpose of any geotechnical investigation is to obtain information about the unknown conditions that lie below the surface. Often the geotechnical investigation is the first information on the site, often before even the sight survey. We are exploring the unknown. After exploration, we only know the conditions at the testhole locations, but that can imply a variation between the testholes. We can have tremendous variation in short distances both vertically and horizontally.

The information required is to provide input into the type of foundation, design parameters, and other issues that can be expected. Two large “unexpected” costs often encountered are unsuitable fills and poor foundation conditions, requiring expensive foundations. The more information we provide, the better the owner can estimate the cost and performance before construction is started.

With fills, additional testhole may be required to examine the extent and depth of the fills. Environmental cleanup can be dangerous due to fill settlement downdrag on piles. Redevelopment sites often have old fill over old utilities. Old fill compaction was often poor, and old fills often contain garbage.

Heavy buildings, high retaining structures, elevators, and similar are not included in this section: heavy buildings may have special requirements. This is intended for one story industrial buildings, three storey or so apartments, motels, and other buildings, but may apply to others in a general way. Areas with complex geology may require additional testholes. Thalwegs, stream erosion channels, occur in bedrock and clay till formations. Native buried peat moss formations occur in geologically recent river and delta deposits.

The testholes are intended to provide an indication of the vertical and horizontal variability of the strength and settlement characteristic of the various geological strata to sufficient depth to support the expected loads. One testhole tell the conditions only where the testhole was at: we have no clue as to the variation across site. One testhole is suitable for lift stations, or similar very small structure.

Two testholes are suitable for linear structures, 100 to 200 metre long sewer, roadways;  50 metre short height retaining walls, add one per each additional 200 metres of roadway or sewer, 1 per each 50 metres retaining wall.

Three testholes is the minimum required to describe a plain in geometry; however, the contact between strata is seldom plainer, and especially water lain strata. Three testholes is suitable for buildings to 500 square metres, add 1 per 250 m2 to 1000, 1 per 500 thereafter. Personally, I would not build a house without 4 testholes.

Should something odd or expensive to deal with be found, additional testholes for clarification may be required. Old coal mines, undocumented environmental cleanups, any industrial use, or redevelopment site can be hazardous.  

Cognative Dissonance

Cognative Dissonance – psychological conflict resulting from incongruous beliefs and attitudes held simultaneously.

In engineering, it may be expressed in other terms.

Example, Beliefs

1. Engineering is important; to do construction correctly is valued; I make my meager living as a “Professional Engineer”.
2. Construction should be conducted as per the Alberta Building Code. 
3. Some cases the Alberta Building Code is grossly wrong.
4. Now what? We are forced to create an “artificial value” for an Ultimate Limit State soil strength, that has no physical significant, and calibrate it to the desired outcome of the building code.

Most foundation should be designed for the Service Limit State. In 40 years, I have not run across a condition that I would employ a Ultimate Limit States soil strength for design. Many of the methods of evaluation of the soil strata strength do not generate a ultimate value, largely due to the time required in clays, and the gross amount of movement that occur in such tests.

Pile load tests are as close to ultimate as we would ever come, and usually those are limited to something like 50 millimetres of movement before it is declared failed. The true pile capacity may be 1.01 to 2.0 times that amount, for discussion.

For a typical project there is a wide range of soil strengths, and all this soil data must be reduced to something that is manageable for design. That is more of an art than a science, since the reliability and variation of the data, the amount of data, the geology of the formations, and the type of foundation all play major roles in the final value. After we go through all that, then we have to fudge a number so that the final result will be correct, due to government insistence on a system of design that matches the structural concepts of how materials should behave, not reality.

Is there a Full Moon?

I received a call from a structural engineer, and he was looking for a way to find geotechnical information, the strength of the soil, on a specific site, on a particular pile type, without conducting a geotechnical investigation.

I suggested a load test, a procedure that will cost about twice what a geotechnical investigation would, but he could do it himself. He went away, happy.

Damn Lawyers

I was asked to do an inspection by a lawyer but they were only willing to pay if I could provide a specific outcome from that inspection. What do lawyers think engineers are? Prostitutes? Magicians? 

I declined the opportunity. 

Shrinking Clay

A problem that I see frequently in Edmonton is clay shrinkage below foundation causing substantial damage to houses, sometimes destroying the value of the house.

The typical situation is as follows:

1.      The house was built on wet high plastic clay. Much of Edmonton has high plastic clay at and below foundation level. 10 to 20% of the land was wet before development into housing.

2.      The home owners (Or the city, developers) plant trees that become large, and suck water from the soil. In dry years, there roots go to the wetter soil, often below houses. The roots extend under the exterior foundations and desiccate an amount of soil. Typically values may be 3 meters of soil shrink 3% causing a 90 millimetres of settlement. If the interior foundations are unaffected, there is a noticeable slope toward the tree, usually with cracking.

3.      Now you have a problem, without a good solution. The first part of the solution is to remove the trees. Next is to underpin, and thirdly, to level the house and repair the other damage.  As the desiccated material thickens, the depth required for underpinning increases. A geotechnical investigation is required to answer the question of how deep, and what about downdrag.

4.      We geotechnical engineers think trees are nice, but in high plastic soils, they should be at least twice the height of the tree away from the houses. That makes a city treeless.

5.      The city likes to plant elm trees along the roadways because they dry out the subgrade and improve the subgrade strength, reducing the pavement failure due to loadings. The down side is the houses on the other side of the trees suffer. Is the City of Edmonton responsible for the damage there trees cause to the private properties? How about your trees and the neighbor’s house? What about the neighbor’s trees and your house?

6.      Shrinkage is only reversible after the virgin cycle; the first time there is some nonreversible shrinkage. Most of Edmonton upper clays have been desiccated to some depth, the desiccated crust, typically 2 to 3 metres; most house foundations are in or below this. Often the desiccation results in thickening of the desiccated crust.  

7.      Watering of the trees and downspouts helps but not much. Many of the large trees will use over a cubic metre of water per day. Some 2 or 3 cubic metres.

8.      Downspouts create the other side, swelling of that clay which have been desiccated. Downspouts at trees may cause a yo-yo foundation.
9.    Now one more complication, add water after shrinkage has occurred, and we get swelling. Now swelling is pressure sensitive. The exterior foundations, interior foundation and floor slab all produce different contact pressure on the soil, so different amounts of swelling causes differential elevations across the floor. What fun the kids have chasing their marbles to the walls usually. Time   to adjust your tela-posts to keep your main floor level. If you have bearing walls in the basement, perhaps you will need to install adjustable posts. Allow for future movements by providing access to the post and slip joints in the walls or above the ceiling. 

Alberta ABC Building Code Schedules

Ultimately, the Alberta ABC Building Code Schedules require a geotechnical engineer to make the statement that the project has been designed and constructed in accordance with the Alberta Building Code, the Geotechnical Investigation, and good engineering practices. In addition, there may be a requirement to agree with the recommendations of the geotechnical report, at least to not disagree with the report.

We will not tolerate the owner, structural engineer, or contractor withholding information required to verify that design complies with the Alberta Building Code, and provide ABC schedules. The Alberta Building code requires that all designs be conducted by ULS and checked with SLS, and the more conservative solution be adopted.

To this end we require the actual pile loads, live and dead, a site plan to verify the building is at the same location and elevation as the geotechnical investigation, and also the geotechnical investigation must generally comply with the Canadian Foundation Engineering Manual as good engineering practices.