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Clays

CLAYS: GOOD STUFFF OR BAD STUFF / FYI

I'm going to get off clays and expansive soils so we can move on to getting the slab-on-grade down, or if you like the foundation walls,so we can get above ground. Not to linger; lets finish with soils after this post or the next post, I hope; and we'll come back ( I'm going to have a lot of coming back to do) and because, you don't really need to worry yourself about this stuff. It's FYI.

(But that's not the reason I want to move on, I'm getting ance. I want to go. And I want to be there at the same time. Maybe I should settle down).

Soils are all covered by the experts and in your site survey or (field exploration) all this is listed for you by an competent site surveyor and engineer. His job ( and I'm going to list several task that are accomplished here) is to obtain the following:

Knowledge of the general topography of the site as it affects foundation design and construction. e.g., surface configuration, adjacent properties, the presence of water sources, ponds, hedges, trees, rock outcrops, etc., and the available access for construction vehicles and materials.
The location of buried utilities such as electric power and telephone cables, water mains, and sewers.
The general geology of the area, with particular references to the main geologic formations underlying the site and the possibility of subsidence from mineral extracting or other causes.
The previous history and the use of the site, including information on any defects or failures of existing or former buildings attributable to foundation conditions.
Any special features such as possibility of earth quakes or climate factors such as flooding, seasonal swelling and shrinkage,permafrost, and soil erosion.
The availability and quality of local construction materials such as concrete aggregates, building and road stone, and water for construction purposes.
For maritime or river structures, information on tidal ranges and river levels, velocity of tidal and river currents, and other hydro-graphic and meteorological data.
A detailed record of the soil and rock strata and (groundwater) conditions within the zones affected by foundation bearing pressures and construction operations, or of any deeper strata affecting the site condition in any way.
Result of laboratory test on soil and rock samples appropriate to the particular foundation design or construction problems.
Results of chemical analysis on soil or groundwater to determine possible deleterious effects of foundation structures.

OK; Clay particles are made up of the finest dirt particles there is. Usually ranging smaller than 1 / 10,000 of an inch in diameter, the peculiar properties that these clays exhibit pose a serious challenge for engineers to rectify, or better; defuse the situation that these clays create.

These soils are a natural impediment to the stability of the foundation and since most residential slab foundations with their transmitted load are designed to be supported by essentially 100% of their total area,as mentioned in the previous post swell-able clays are the contributing factor that cause upheaval or deflection; (a condition exhibiting sufficient deflection from the original construction so to require remedial attention), and the loss of support due to shrinkage can and will result in settlement.

Normal settlement of fill is often active for periods up to 10 years, it is thought to be dependant upon the cycles of precipitation and drought. However it may be possible to nullify soil shrinkage by the introduction of water. Proper moisture controls;effective grade, adequate watering, and sufficient ventilation are all forms of proper moisture control and have been proven to have a successful stabilizing effect.

Keep in mind, sources of unusual amounts of water under a foundation are accumulative and because the foundation covers (protects) this area of bearing soil. Moisture seeping into this confined space via, (broken drain line under the pad, no matter the source), tends to be detrimental. Unusual water might be attributed to subsurface aquifers ( e.g., temporary perched groundwater), surface water (poor drainage), or domestic water (leaks or improper watering) and ponding could also result from a damaged water main.

The mineral montmorillonite is relatively abundant in certain parts of the state. Concerning these areas that are most affected, they are; (Wyoming, Colorado, California, North Dakota, South Dakota, Texas, Alabama, Montana, and Mississippi). These states are affected too, but not as bad; (Utah, Louisiana, Arizona, Florida, Tennessee, Oregon, Washington, and Nebraska). Other clays such as illite, attapulgite, and kaolinite also show swell potential but to a lesser extent.

Movement of soils is often precipitated or exacerbated by the intrusion of water into the soil to the extent that both cohesion and structural strength are threatened or destroyed. California mudslides are a prime example though extreme. Something to think about.

Love to here from you.

Thanks for the comments. Have a good day!!

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Posted in Grade, Planning, Soil, soil characteristics, soil profile.

Tagged with climate factors, earth quakes, expansive soils, field exploration, foundation design and construction, soil errosion.

2 comments

By Keith Bray – February 22, 2010

Basic Construction? / Investigations

What is basic construction, and what is the meaning? That might have been the question for the first post. But keep in mind, safety trumps. You answer this question. To me; " Now that I think about it", there is no such thing, as "Basic Construction". What do you think?

Oh. You can throw a house up; dig a trench, pour the pad, anchor sole plates, throw up stud wall, put the rafterson, Sheath it, sheath the outside walls, do your wiring and plumbing, insulate , install outlet boxes, pull your wires, rock the walls, install utility closet and make connections, rolled felt and shingle the roof, install the doors and windows, brick, block, and flue the chimney, brick the front, place siding on the sides and rear, install the sills, weatherstrip, close the eaves, tile the floor, install plumbing fixtures, put in lights, switches, outlets, cover plates, install kitchen cabinets, counters, sinks, bathtub, Spackle and paint, pour steps and side walks, clean up your mess, look around, ask yourself," did I leave anything out"? "No", Pat yourself on the back, walk out and close the front door, brush the dust off your shoulders, pop your shirt collar, and BAM !! There it is. Then; walk off into the sunset.

The Need for Investigation

For single-family homes and other small construction, a preliminary investigation is made to determine the sites suitability for the proposed structure, and is of prime importance to designers in their information gathering for the design stages of building the foundation, from this information, you should gain the necessary facts, details, and results, about the nature and origin of the existing soils. This initial investigation will verify, if there are other needs (cost is always a factor here) and to what extent, of further investigation or; no further needs necessary, "which is the correct answer".

In some cases, a preliminary investigation can eliminate the expense of extensive soil boring and testing. The preliminary examination usually consist of at least one test boring at the site, the purpose of subsurface exploration (test boring) is primarily to establish three things: (1) obtain soil samples at various depths below the surface for the purpose of laboratory evaluation, (2) to determine the variations of the soil (soil profile) that are present on the site, and (3) to determine the depth at which free water is encountered, that is, the location of the groundwater table.

A truck-mounted drilling rig is employed to drill the test hole from which a sample of relatively undisturbed soil is gathered by using special sampling tools. The two most common are the split spoon sampler and the Shelby tube.

The Shelby tube is a thin-walled cylinder that is pressed down into the soil to the bottom of the test hole and then raised with a core inside. The split spoon sampler is used to test the condition of the soil in the bottom of the test hole and to extract a soil sample for testing. An immediate indication of the soil bearing capacity is established by using a standard weight to drive the spoon into the soil to a predetermined distance (usually a total of 450 mm in three 150 mm increments).

There are other methods, and each particular method depends on the case and requirements.

Other Methods:

Sounding rods
Augers
Test pits
Wash boring's
Rock drilling's
Geophysical instruments

A sounding rod is used to determine the depth below the surface at which rock appears and if the soil resistance is increasing or decreasing as the test proceeds.

An auger is used for bringing up samples from relatively shallow depths in soils that are cohesive enough to be retained in the tool while it is raised to the surface.

The test pitis probably the best method of examining subsurface soil because it is possible by this method, to examine the layers of earth exactly as they exist, soil moisture's are evident, and load test can be made at any desired depth.

Wash boringrequires the use of water; as a result the boring's are in the form of mud. Loose material is forced up between the drill and rod and the outer casing by water pressure and collected through an opening near the top of the casing. This is known as a wet sample.

Rock drilling employs a number of systems; among them are diamond drilling, shot drilling, and churn drilling.

The diamond drill is a diamond studded drill bit attached to a rig. The bit cuts a circular groove, and the core is forced up into a core barrel. When at the desired length, the core is broken off and brought to the surface for inspection.

Shot drillingis similar to diamond drilling except for the bit. It consists of a circular, hollow, hard steel bit with a slot around the bottom to allow circulation of shot. A flow of chilled steel shot is fed through the drill rod to the bit; as the bit turns, the shot cuts the rock and forms a core, which is forced into the core barrel and brought to the surface.

Churn drilling consist of a hard steel chopping bit attached to a drill stem inside a casing. A cable is fastened to a set of weights that raise and drop the bit,so that it strikes the bottom, chipping the rock. Water is forced down the casing, and the resulting slurry is removed.

Two basic types of geophysical instruments are used for shallow subsurface investigation and exploration: refraction seismographs and earth electrical resistivity units.These instruments have been known and used for deep exploration in oil and mineral industries.

To determine the acceptability of the soil for ground-supported slabs and foundations. Such test boring's can be made to determine soil types and the extent, the depth usually necessary to predict the performance of small and large building foundations. Have a good day!!