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SOILS AND FOUNDATION

SUBSOIL INVESTIGATION
(Geotechnical Engineering Evaluation)

Why do it?

Most lending and insurance agencies require a Geotechnical Engineering Evaluation, including: FHA, VA, HBW, HOW, etc. In addition, most metro area municipalities and counties require them. Everyone wants one... especially the homebuyer. Typically, those who live in areas plagued by expansive soils are aware of their potential damages. As a result, many states have implemented laws requiring a Geotechnical Engineering Evaluation for all new construction (i.e. State of Colorado Senate Bill 13, passed in July, 1984).

The homebuilder especially wants to know what type(s) of foundations and other construction details are likely to be required so that he can figure his cost to build. The land developer needs to know the additional cost due to any soils problems or conditions. This hasn't always been the case. Not too many years ago, for example, you could build in Jefferson County, Colorado (a county with a particularly high incidence of bentonite) without a soil test and use an 8 x 24" wide Conventional Spread Footing.



Field Investigation

On a piece of undeveloped ground, preliminary test holes are generally spaced from 400 to 500 feet apart. For example, on a 160-acre quarter section of land, holes spaced at 400 feet apart would require 49 holes. On the same piece of ground, at 500 foot spacing, only 36 holes would be needed. Keep in mind that -his is a preliminary investigation. Generally, each lot is drilled and sampled after completion of over-lot grading and final lot layout.

Test holes are drilled using continuous flight augers - either solid stem or hollow. The solid stem augers are 4 inches in outside diameter by 5 feet long and are connected together as the hole is drilled. The periphery of the auger is helical in shape so as the auger turns soil, cuttings move up the hole on the helix.

Samples are taken by removing the augers from the hole and inserting a sampling spoon on the end of heavy steel rods. The spoon is then driven into undisturbed soil using a 140 pound hammer dropped from a height of 30 inches. The number of blows that it takes to drive the spoon 12 inches is recorded. This is referred to as the Standard Penetration Test, or SPT. Even though it is a fairly simple, unsophisticated test, It is used throughout the world by ,geotechnical engineers. It is a standard test of the American Society for Testing and Materials (ASTM).

Local engineers use a modification of the standard sampling spoon called the California Barrel, The California Barrel contains brass liners which retain samples of the soils penetrated. The use of brass liners makes it easier to perform various tests on the soil samples recovered. The results of the SPT provide a method of comparing the various materials encountered in the test holes.

Hollow stem augers are a minimum of 6 inches in diameter and, as the name implies, have a hollow stem. This allows taking SPT and obtaining samples without having to remove the augers from the hole each time. This is especially useful on deep holes. And, also where wet or caving holes would prevent obtaining samples or taking a SPT.

A field engineer is on the site when the holes are drilled. He/she logs the holes on a continual basis and records:

  1. Soil type and visual classification
  2. Moisture content, density/stiffness, color
  3. Depth where changes occur
  4. Ground water conditions
  5. SPTs and take undisturbed samples. The samples are sealed immediately to prevent them from drying.

One thing the geotechnical field investigation will not do is determine if the site has been environmentally contaminated in the past. This is a separate type of investigation.



Laboratory Testing

The samples taken in the field are returned to the laboratory for the project engineer's review, classification and determining which samples will be included in the testing program. The samples are classified using the Unified Soils Classification System. Refer to Figure 2. This is the soil classification system used for engineering purposes. It was developed during World War II. There are several other classification systems in use but for different purposes - such as the Soil Conservation Service, American Association of Highway and Transportation Officials, Federal Aviation Administration, etc.

Swell-Consolidation tests are generally only run on samples of clay soils. That includes any sample that contains clay such as a very sandy clay or clayey sands, claystone bedrock, or clayey sandstone bedrock.

The clay minerals in rocks and soils are responsible for their expansion, or "swell," as it is generally called. This swelling is caused by the chemical attraction of water to certain clay minerals. Layers of water molecules can be incorporated between the flat, submicroscopic clay plates. As more water is made available to the clay, more layers of water are added between the plates, and adjacent clay plates are pushed farther apart as shown below:



This pushing apart, or swelling, occurs throughout the mass of soil that is being wetted, and causes increased volume and high swell pressures within the mass. The opposite effect, called shrinkage, may occur if a previously wet swelling clay is dried. Although no large pressures are exerted, shrinkage will cause a volume decrease of the soil mass. These processes of swelling and shrinkage may occur any number of times for a single soil mass. Either swell or shrinkage may cause damage to streets and buildings, but swell accounts for nearly all such damage in Colorado.

All that we have been talking about are swelling soils. Well, there are other kinds of soils even in this area. A few are:

  1. Clean Sands - that will consolidate (settle) under load
  2. Silts - that are hard when dry but will fall apart and consolidate when wetted
  3. Hard Bedrock - that requires blasting to excavate
  4. High water table/soft soils

Tests are made for possible high concentrations of sulfates in the soils. Usually, when there are high concentrations, sulfate crystals will be visible in the samples.

Sulfates combine with ground water to form H2SO4 (hydrogen sulfate) which is an acid. This acid reacts with the cement in concrete and over a period of time, you end up with a pile sand and gravel. Refer to Section on Concrete for a discussion on the types of cement to use in the concrete when there are high sulfate concentrations.



Report

A table of contents for a typical Geotechnical Engineering Evaluation report for a subdivision is shown on Figure 5. The report will generally include the following:

The field and laboratory data will be detailed and summarized on the various figures and tables included in the report.

Individual lot recommendations

Recommendations for individual lots are sometimes summarized on a table in the report, which provides a handy tool for the Job site superintendent.:

Swell RatingPercent Swell
500 psf		Percent Swell
1,000 psf
Low 0 to 3 0 to 2
Moderate 3 to 5 2 to 4
High 5 to 8 4 to 6
Very High 8 to 12 6 to 8
Critical 12 + 8 +

Some additional items that might be covered depending on the scope of the investigation:



Geological Conditions: A Case Study (Denver, CO Metro Area)

The bedrock underlying most of the Denver Metro area is fairly flat, or lying in relatively horizontal layers. As you are all aware, when you drive into the foothills you drive through the Dakota hogback. This is very obvious when you go to Ken Caryl Ranch or Roxborough Park. When you go to Morrison by way of Alameda Parkway, you actually drive over the hogback. Those sandstone and shale beds of the Dakota hogback are not quite vertical but they are dipping (sloping) about 60 or 70 degrees (from the horizontal) to the east.

The hogback was formed during the time the Rocky Mountains were being uplifted. A long time ago. Some of the bedrock to the east of the hogback is also upturned. One of these formations is known as the Pierre Shale. It is composed of a mixture of sandstone, siltstone and claystone. In places, it also contains thin (1 to 2 inches) pure bentonite beds. Some of the claystones have very high swelling potentials. It doesn't form hogbacks because it is less resistant to erosion than the Dakota. It is found on the surface or covered with a mantel of 20 or 30 feet of sand, gravel and clay. Generally, it parallels the hogback to the west.

One of the design considerations when the Pierre Shale is encountered in foundation excavations is lateral pressure. Due to the near vertical attitude of the bedding, it is believed that these beds exert very high pressures laterally (or perpendicular to the bedding) when they become wetted and swell - see Lateral Pressure.


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NOTE: This information is provided for general education only. For detailed information concerning reactive soils, you should always contact a geotechnical firm.

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