Soil Investigation

Soil And Foundation

A surface and subsurface investigation looks at a number of factors that include but is not limited to, water drainage, water tables, topography, soil types and characteristics, and other laboratory test that are required to ascertain the fitness and suitability of the soil for support of the proposed structural. dirt6

We made mention of some of these topics earlier, but we should dig a little deeper to uncover some of the hidden effects that take place under different loads and stresses with different soil types; for example; gravel to clay mixtures or silty soils with a high sand or clay mixtures, and how it reacts when these stresses and loads are applied. soil8

Classification and Description

Soils consist of grains; rock fragments and clay particles, with water and gas, either air or condensation, these particles are trapped in the void spaces between the grains. A soil classification scheme is used to separate soils into broad groups each with broad similiar type and behaviour.

The different classification schemes fall under, agricultural: based on how soil supports crops. Geological: based on the age of the deposit or nature of grain. Civil Engineering: based on mechanical behaviour of the soil and many others. The subject of soil is vast in scope and for our purposes we don't need to go into all the technical aspects and breakdown sciences. but we will investigate some basic areas and facts about soil that is of interest and specific to what we, in our endeavors, will lead toward accomplishing our goal. soil6

Soil description is essentially a catalogue of what soil is and describe the soils features. There are several of these schemes available; published in the National Standard, and to some extent reflect the characteristics of most common soils in the region. Consult the relevant standard for the region where you work. soil1

For a concise and in-depth study of soil you can go to or consult:

The United States Department of Agriculture, (USDA). Natural Resources Conservation Services, (NRCS) website.
For Engineering; the Unified Soil Classification System (USCS)
Soil Mechanics; By R.F. CRAIG
Geo-technical Materials in Construction; By Marion P. Rollings, P.E. and Raymond S. Rollings, Jr., P.E.
The Mechanics of Soils and Foundations; By John Atkinson

External loads and water pressure interact with each other to produce a stress that is effective in controlling soil behaviour and as external loads are applied, compression of soil volume occurs and the grains rearrange themselves and the void spaces change from compaction. soil3

Soil shearing is basically frictional so that strength increases with normal stress, and with depth in the ground. Soil stiffness also increases with normal stress and depth. Combining these basic features of soil behaviour leads to the observation that soil strength and stiffness decrease with increasing water pressure and with increasing water content.

Soil compression and distortion are generally not fully recoverable on unloading, so soil is essentially inelastic. This is a consequence of the mechanics of compression by rearrangement of the grains; they do not un-rearrange on unloading. soil5 soil12 soil14

Please leave a comment let me know what you think.

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Posted in Analysis, Soil, soil characteristics.

Tagged with clay particles, soil classification, soil description, soil types, study of soil.

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By Keith Bray – March 2, 2010

Footings and Excavations

Shallow Foundation

Footings

The size and dimension of the footer are based on soil characteristics. Soil and subsoil in their natural state are often sufficiently stable to support light frame construction, the average load bearing capacity for undisturbed soil is 2000 pounds or greater, per square foot. Where building code permit;today modern home builders, in light frame construction build houses using a form type called Earth-forms, simply on an excavated surface, cleared of vegetation, foliage, etc. A trench is dug out according to the footer specifications, say for example the frost-line is at one foot, so you know the footer dimension for a 8 inches thick wall is going to be 16 inches wide by 8 or 12 inches deep, dependant upon soil characteristics, but 8 inches deep is standard thickness, I've heard of 6 inch footers this might be ok for a garage pour but I would never use anything less and neither should you or you'll be asking for trouble. Refer to the blue prints.

Your trench depth would be 1 foot for frost-lineplus 8 or 12 inches for the footer, so your trench depth would be 32 or 36 inches. That's 8 or 12 inches for the footer 12 inches below the frost-line plus one foot of soil to the surface, or grade, so our trench would be 32 to 36 inches at the the highest point of the natural grade. foot7

Then your trencher will dig your earth-form 32 to 36 inches deep by 16 inches wide or more and keep a level dig to complete the earth-form.

From the blue prints the height of your batter board represent the top of the finished foundation wall. If we use the example above it would take two courses of cement block to bring the foundation wall to grade and another two to three courses to the building lines or finished foundation wall. Your underground utilities are marked, staked and stubbed your trench dig will come within two feet of the underground and stubbed utility line and since most of these are buried including the telephone and television cables as well contact with a company representative will alleviate the problem of accidental damage to these lines. The rest will be hand dug to prevent damage by heavy equipment. foot8

Excavations

Some builder's like to put in temporary batter-boards and stake the approximate location of the excavation so the backhoe or bulldozer operator can perform the dig before the exact building lines are set. This is an extra step to the process but it eases the burden from the equipment operator by giving him latitude in movement and should result in greater precision.

Then go back and set the exact location for the building, it's a matter of what is expedient at the time, however, the lines have to be taken down before this operation. Once the lines are down the digger can start digging. Make sure the soil is piled a good distance from the excavation, when you go back to hang your lines, dirt piles won't have to be hand shoveled away from the path of the lines. The operator will dig approximately two-feet or more beyond the building perimeter to allow for pipes, drainage and work space. On a clay or cohesive type soil you'll need slight fer-ring back depending on the depth of the dig and on loose or sandy soil types, you'll cut the slope back at a 45 degree angle to prevent cave-ins. foot11