Atlas Piers of Atlanta, Inc. P.O. Box 3313 Alpharetta, GA 30023 Phone: 770-740-0400 Fax: 770-740-1513 Email: info@atlaspiers.com

The use of manufactured steel foundation products generally requires a prior geotechnical investigation of the subsurface condition of the foundation soils at the site of the proposed project. In addition to the geotechnical investigation, it is necessary to define the structural load requirements and required Factor of Safety for use in the overall design approach. Atlas Systems, Inc. manufactures or supplies three main lines of steel foundation products:

• Resistance Piers for underpinning and repair of residential and commercial buildings, retaining structures and slabs.
• Helical Foundation Piers for new construction and repair of residential and commercial buildings; helical tiebacks used in excavation shoring systems, retaining walls and slope stabilization; and helical anchors used for communication towers, signs, light standards and commercial buildings subjected to wind and earthquake load.
• Rock Anchors for use in providing tension capacity where foundation stratum is competent rock and use of the helical type of anchor/tieback is not feasible. Rock anchors require grouting in place.

DESIGN OR WORKING LOADS

These loads are sometimes referred to as UNFACTORED LOADS and DO NOT INCORPORATE ANY FACTOR OF SAFETY. They may arise from Dead Loads, Live Loads, Snow Loads, and/or Earthquake Loads for bearing (compression) loading conditions; from Dead Loads, Live Loads, Snow Loads and/or Wind Loads for anchor loading conditions, and Earth Pressure, Water Pressure and/or Surcharge Loads (from buildings, etc.) for tieback holding conditions or;

 

ULTIMATE LOADS

Ultimate Loads (sometimes referred to as FULLY FACTORED LOADS) already fully incorporate Factors of Safety for the typical loading conditions described above. Atlas Systems, Inc. recommends to customers and designers that they use a minimum Factor of Safety of 2.0 for permanent loading conditions and 1.5 for any temporary loading condition. This Factor of Safety is applied to the design or working loads as defined above to achieve the ultimate load requirement.

PURPOSE

The primary purpose of the geotechnical investigation is to assist in identifying the key soil strength parameters for design of the steel foundation elements. In addition to the above, such studies are useful for the following reasons:

Resistence Piers:

• To locate the depth of firm bearing stratum for end bearing support of the underpinning pier.
• To establish the location of any weak soil zones in which column stability of the pier shaft must be considered.
• To determine if there are any barriers to installing the pier such as rubble fill, boulders, zones of chert or other similar rock, voids or cavities within the soil mass, any of which might require pre-drilling.
• To do a preliminary evaluation of the corrosion potential of the foundation soils as related to the performance life of the steel pier.

Helical Foundation Piers/Tiebacks/Anchors:

• To locate the depth and thickness of the soil stratum suitable for seating the helical plates of the pier and to determine the necessary soil strength parameters of that stratum.
• To establish the location of weak zones, such as peat type soils, in which column stability of the pier for compression loading situations may need to be considered.
• To locate the depth of the groundwater table (GWT).
• To determine if there are any barriers to installing the piers such as rubble fill, boulders or zones of cemented soils, chert or similar conditions, which might require pre-drilling.
• To do a preliminary evaluation of the corrosion potential of the foundation soils as related to the performance life of the steel pier.

ROCK ANCHORS:

• To establish the depth and location of the rock.
• To determine the type of rock and competence of the rock to support the rock anchor.
• To determine the presence of groundwater.
• To do a preliminary analysis of corrosion potential of the rock and overburden soil for designing a corrosion protection system.

The geotechnical investigation generally consists of four phases:

1. Reconnaissance and Planning
2. Test Boring and Sampling Program
3. Laboratory Testing
4. Geotechnical Reporting

1. Reviewing the structural load requirements and size of the structure and whether the project is new construction or structure repair
2. A review of the general soil and geologic conditions in the proximity of the site
3. A site visit to observe topography and drainage conditions, rock outcrops if present, placement of borings, evidence of soil fill, including rubble and debris and evidence of landslide conditions.

The most common method used in soil exploration is the use of truck mounted continuous flight auger systems together with the use of Standard Penetration Test (SPT) equipment. The continuous flight augers can be either hollow stem or solid stem. The equipment and procedure used are in conformance to ASTM D 1586. Typically a drill truck with a hollow stem auger is used to continuously sample the soil. The advantage of use of the hollow stem auger is to permit the sampler rod for the SPT to be inserted through the auger rather than have to remove the auger stems each time an SPT is conducted. As the auger rotates, the soil moves to the surface and based on visual observation is classified as to type and condition and recorded on a log sheet at the depth of the auger stem. The result of this auger advance process is to establish the vertical sequence of the soil substrata. Generally a sample is recovered and visually classified at each 5-ft. interval of depth or at a change in soil type. This classification continues to the bottom of the boring. During the advance of the auger in the borehole, the driller also records the depth to any observed ground water table (GWT). The presence of a GWT will influence the unit weight and strength characteristics of the soil.

Every recovered sample from the field boring and sampling program is inspected visually and given a visual description as to its color, condition and type. (See Table 2.) In addition to this visual classification, a representative number of samples are selected to conduct the following tests:

• Water Content – measures the amount of moisture in the soil.
• Particle Size Analysis -- measures the distribution of particle sizes within the soil sample.
• Liquid Limit (LL), Plastic Limit (PL) and Plastic Index (PI) – applies to cohesive types of soil and is a measure of the relative stiffness of the soil and potential for expansion.
• Strength Characteristics – in some instances undisturbed soil samples are recovered in the field using a thin wall (Shelby) tube. These recovered samples are tested either in triaxial or direct shear tests to determine directly the friction angle (f) and the cohesion (c) of the soil. For cohesive (clay) soil samples, an unconfined compression (qu) is often conducted. The cohesion of the clay sample is then taken to be one-half of qu.

The geotechnical report provides a summary of the findings of the three phases detailed above and also contains recommendations on options for a foundation together with the recommended soil related design values. Included in this report are the results of the laboratory testing of the soil samples and borings logs providing a visual summary of the vertical profile of foundation soils at the project site. Figure 4 gives the boring log generated from the field exploration program as shown in Figures 1 through 3. A review of this boring log indicates the following:

• The total depth of the boring was 74.3 ft. Except for the upper one-half foot, the soil layers were all lean or lean to fat clay with some variations in color and stiffness down to the depth of 64 ft. At 64 ft., a shale stratum was encountered which was in a highly weathered condition. Shale is a rock but in a weathered condition such as noted on the boring log, it is probable that the helical plates of a pier could be set 1 ft. to 3 ft. into the shale stratum.
• Standard Penetration Tests (SPT) were conducted at each 5-ft. interval of depth down to the bottom of the boring. From the SPT, N column on the boring log, it is noted that the stiffness (or strength) of the lean clay is fairly consistent from depth 30 ft. to 64 ft. (N ranged from 9 to 14). The upper part of this stratum (around 35 ft. to 40 ft. is where the helical plates would be seated).
• Moisture contents were taken on the recovered split spoon samples from the SPT. Again, below about 25 ft., the moisture content of the soil was fairly consistent (ranging between 23-1/2 to 26-1/2 percent). This low variation in moisture content is consistent with the consistent range of N values.
• Liquid Limit and Plastic Limit tests were also conducted on the recovered split spoon samples. The average LL = 45; the average PL = 20, resulting in a PI = 25. These results indicate that the in-situ moisture content of the lean clay (@ 25%) from 30 ft. to 60 ft. is just above the Plastic Limit (20%). As the in-situ moisture content approaches the Plastic Limit, the clay soil will become stiffer (higher cohesion).

As noted in the previous section, Atlas Systems, Inc. provides manufactured steel foundation products that cover the full spectrum of soil and rock conditions. In order for the designer/user to select the proper product for the application the designer/user should note the following items:

• A range is noted based on “N” values where the resistance type of pier will provide the foundation underpinning support in an end-bearing mode. This “N” value is generally above 30 to 35 in cohesionless (sands and gravels) soils and above 35 to 40 in cohesive (clay) soils.
• A range is also indicated for use of the helical bearing (compression) and helical anchor/tieback (tension) piers. As noted on the chart, there are certain conditions for weathered rock and cemented sands where an initial pre-drilling will permit the installation of helical plates under relatively high installing torque (generally above 10,000 ft-lb.).
• The use of anchor/tieback in rock formations is also noted on the chart along with a relative indication of rock type and quality. To assist contractors, an indication of the type of drill is shown based on rock quality as noted by the RQD.

All foundation systems should be designed under the direct supervision of a registered professional engineer knowledgeable in product selections and application.

1. Soil Engineering, Merlin G. Spangler and Richard L. Handy, Intext Educational Publishers, New York.
2. Foundation Analysis and Design, Joseph E. Bowles, McGraw Hill Book Co., New York.
3. An Introduction to Geotechnical Engineering, Robert D. Holtz and William D. Kovacs, Prentice-Hall, New Jersey.