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Brief Recap of Foundation and Soil

Shallow or spread foundations are employed when stable soil of adequate bearing capacity is sufficient and relatively near the ground surface. They are placed directly below the lowest part of a substructure and transfer building loads directly to the supporting soil by vertical pressure.

Foundations like all structures are built against failure, the design of the foundation system requires professional analysis and design by a qualified structural engineer. When designing anything other than a single-family unit on stable soil, it is advisable to have a geotechnician (engineer) undertake a subsurface investigation in order to determine the type and size of foundation system required for the building design.

Professional advice of a soils engineer experienced in a given locality would most likely be a wise thing to do, since this will mean more options and courses of action for you to consider and that, that advice could preclude the need for even a single test boring at a single-family home construction site. This is especially true when his advice is varifiable with some examination of other structures in the immediate area for evidence of apparent settlement or expansion of the soil, providing that similar conditions of soil, topography, and proposed construction is commonplace.

Soil and Excavating Equipment

The type of soil at the site, determines the type of excavating equipment needed for the job. If the soil is loose and non-cohesive (dry sand or gravel ), excavating can usually be started with a clam-shell bucket and completed with a pay-loader, a tractor with a mounted shovel, or it may be done completely by pay-loader or bulldozer. The use of these two machines require good access in and out of the excavation site. In impassable or restricted locations where such access is not possible, other methods must be used.

Cohesive soils require power equipment; in such cases, a power shovel or pull shovel (backhoe) is employed. Again, a power shovel requires access to and from the excavation, whereas a pull shovel can dig below its own level, but with slight limitations. A bulldozer may also be used for excavating cohesive soils. For very large excavations in soils of either of these types, a scrapper may be deployed.

Soils that are too soft or wet to support machinery traveling over them may be excavated by means of a drag-line. This machine will operate from one or two stable spots at the side of the excavation.

Boulders and broken rock will probably be handled best by a power shovel, though a clam-shell bucket can be used if the pieces are small enough.

When narrow trenches or individual holes are required for footings, or piers, a pull shovel or a trencher is the normal equipment to use.

Trenching

As in all excavating work, trenching excavation requires the right type of equipment. Your first step will be to decide which type of equipment works best, there are four basic types of trenching equipment:

Wheel trencher
Bucket line trencher
backhoe
Drag-line

The wheel trencher is generally used for trenching to depths of 6 feet and under providing there are no obstacles in the path (utility lines, cables, etc.,).

Bucket line trenchers are used to trench at various depths and various widths and is more efficient than the wheel trencher.

The backhoe is versatile and can cut around these obstacle with relative ease, and preferred over other trenchers because, if called for you can cut wider trenching, slope trench walls, and it is more cost effective to move to the site.

If you need a long reach and the material is soft or loose, a drag-line is best. A backhoe is incapable of operation under these conditions due to the unstable ground.

Groundwater

One serious problem regularly encountered during digging operation is the presence of water in the excavation. This may be caused by rain, melting snow, an underground stream, or the fact that the water table in the area is high. The water table is the normal level of groundwater, and if that level is close to the surface, excavating will allow the groundwater to seep and collect in the excavation. Have a good day!!

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Posted in Foundation, Soil, Subsurface Investigation.

Tagged with bulldozer, clamshell bucket, excavating equipment, payloader.

6 comments

By Keith Bray – February 20, 2010

Geotechnicians Prime concern is Cutoff

Groundwater in an excavation or trench is bad news,however, this problem is common, and just, has to be dealt with. ( I'll be back! )

Hydrological Cycle

Moisture vapor in the clouds condense and by changes in temperature fall to the ground as rain snow, sleet, or hail most of this reaches the land surface and the rest is evaporated. there are four ways this water is dispersed;

Evaporation directly from the soil (flyaway)
Removal of soil water by vegetation (transpiration)
Ponds, lakes, streams, etc. (runoff)
Soaks into the soil (cutoff)

Cutoff water that soaks into the soil is of prime concern for the soil engineer,this water percolates downward through the soil pores to replenish the groundwater below. The soil acts as a vast subsurface reservoir from which the swamps, and lakes, and streams are fed during the periods between rainstorms when there is no surface runoff available. The water stored in the soil becomes the source of supply for growth and nourishment of plants, animal life, and humans.

The cutoff portion of precipitation also adversely affect many engineering structures by reducing the bearing capacity of the soil. Many problems in soil engineering involves a study of water seepage quantities through earth dams, levees, and the sides of irrigation ditches, and a study of flow into under-drains and wells. It is highly important, therefore, that an engineer should have a working knowledge of the principles governing the flow and retention of water in the soil and the effect of water upon the strength and stability of this material.

Subsurface Water

Subsurface water is divided into two general classifications, the aeration zone and the saturation zone.In more technical jargon the saturation zone or water table may be defined as the surface at which the pressure in the soil water is equal to atmospheric pressure, and is more commonly none as or termed the water table, aquifer, vadose, phreatic surface, or just groundwater, and is the deepest. The aeration zone; is everything above the water table, and includes the capillary fringe, intermediate belt (which may include one or more perched water zones) and, at the surface, the soil water belt, referred to as the root zone.

Generally, the soil water belt provides moisture vegetibles and plants; the intermediate belt contains moisture in storage - held by molecular forces: the perched ground water, if it occurs, develops from water accumulation either above a relatively impermeable strata or within an unusually permeable lens. Perched water generally occurs after a good rain and is relatively temporary; the capillary fringe contain capillary water originating from the water table.

The cutoff, or soil belt can contain capillary water available from rains or watering; however, unless this moisture is continually restored the soil will eventually desiccate through the effects of gravity, transpiration, and evaporation. In so doing the capillary water is lost. This zone is also the one most critically influencing both foundation design and foundation stability.

The water table can be determined at any time, by digging a test well or boring. As stated, the more shallow zones have the greatest influence on surface structures. Unless the water table is quite shallow, it will have little, if any, material influence on the behavior of foundations of normal residential structures. Further, the surface of the water table (phreatic boundary) will not normally deflect or deform except under certain conditions in the proximity of producing well. In this instance the boundary will draw down or recede. In other words, if the water table is deeper than about 10 feet, the boundary (as well as capillary fringe) is not likely to dome. Should upward deflection or doming occur, (it fluctuates any time in climatic conditions) it would more likely affect foundation than would any draw-down condition. The relative thickness and depth of the various zones depends upon many factors such as soil composition, climate, geology, etc.

If groundwater is found in a test hole, it is extremely important to record its elevation. The registered or recorded depth at which the water table is discovered, is vital information that will be used for project development and design for planners and engineers, and will serve as vital information for all ( contractors ) concerned with the project. This is particularly true, if the material must be excavated below water. If reliable information relative to fluctuation of the water table can be acquired from local residents or by examination of the local terrain, such information should be incorporated in the soil survey notes. Have a good day!!