Friday, September 28, 2012

Residential Engineered fill

Engineered fill is used to create housing lots from land that was previously unsuitable. Out here, in the high plastic mud plane flats, there are slough areas that hold water most (all) years. The small ones are filled with engineered fill and turned into housing lots. No problems, usually. Bigger sloughs are usually reshaped to fit the lots better. The topsoil and/or marginal material is used to create a damn to control the water and fill is used to bring up the lots.

The primary difference between engineered fill and clean common fill is the amount of compaction and the documentation of the compaction. There is an issue with the extent of the engineered filled provided in the lots, and as the design, size, and location of the houses are not known at the time of lot construction, errors frequently occur as to the required lateral extent of the engineered fill.

After the house is put on the lot, the question becomes, was the rear of the house placed in engineered fill or into common fill? Where settlement of the back wall of the house occurs, it is likely that the engineered portion did not extend far enough into the lot.

But how do we prove the boundary of the engineered fill when the difference is compaction at the time of placement? If they used organic or marginal fill, the difference becomes apparent. But if it is good fill, now what?

Wednesday, September 26, 2012

Assumed Bearing Values

You may save a few dollars by using an assumed bearing value, but you take on the liability and when problem arises, and it all get dumped in the court, you become neglect. If you assume a low value, it costs the client, and if you assume a high value, you get movements.

The issue with foundations is usually to much movement. Often we expect movement, and then you have a choice, build, and allow for the movement, or not build. Some of the time, we can switch foundation types, and reduce movement to an acceptable level.

All foundations move. Movement is a response to changing  load, and to often, changing moisture regimes. It is limiting these movements to an amount that is acceptable that is the trick.

Our building codes are out to lunch in some respects. It calls for foundation design utilising ULS, which should never control in soils, SLS will always govern.  But the building code is written by a bunch of structural engineers, who do not understand soils. They force us soils types to adapt, to take what is right and bend it to fit a wrong code.


Friday, September 21, 2012

Engineered fill

Engineered fill is what we call the process of placing clay to support small buildings such as "traditional" houses. What is being built is far from traditional. And then there is the wet soil issues, where 0.6 metres of dry high plastic clay is placed and crusted over with a packer. The remainder of the fill is placed. Now what?

At the best, with traditional houses, we expect 2 inch of differential movement. Now a few construction problem/errors, tight basement finishes, and what do we have but houses who do not meet the owners expectations, but are within the expectations of the land developer and geotechnical engineers.

Such fun.


Tele-posts, adjustable columns in the basement of most modern homes are to account for differential movement between different foundation types, and they are there for adjustment. They should be adjusted as required to keep the main floor straight. The basement walls must be constructed to allow for adjustment, and not to carry any vertical load to the basement slab. This will cause uplift of the floor, and partition cracking at doorways and other points of stress concentration.

Monday, September 10, 2012

Ice Jacking

Ice jacking is a phenomena that occurs when water is frozen, water is drawn to the freezing front through the soil, and freezes. There is also lateral frost jacking where water ingresses into a crack and freezes, producing large lateral forces. The amount of movement can  become substantial when partial melting and cyclical freezing occurs.

This can occur in a number of situations. The typical one is in old stucco homes with no insulation and vapour barrier in the joist space over the basement. Water condenses and freezes, expands, and produces lateral force. The sun hit it, it melts, runs down, freezes and expands. You get the picture. Beam pockets over basement have a similar issue. Slabs outside have the same effect happening next to houses.

This week I saw a new occurrence of this, inside a unheated insulated garage. Cars drag in snow and it melts, and condensates on the grade beam. On nice days, this melts, runs down into the space between the grade beam and slab an freezes. This provides force, and in this case, after the slab moved a bit, it pushed the grade beam out of alignment.

The solution is not cheap. Heat, to prevent freezing, is one solution. But just insulation with a vapour barrier may be enough. The thermal gradient and water may be there, but these are separated to a point where the thermal capacity is not sufficient to produce frost in quantity to cause a problem. The vapour barrier must be attached to the concrete to prevent moisture ingress with a mastic or like.