Geotechnical Services

Sometimes a geotech report is simply viewed as an item on a checklist that must be completed prior to construction of a project. When properly utilized, a geotech report is a valuable tool used in making decisions regarding land development. A geotech report should identify whether or not rock excavation or groundwater will be factors for the proposed project. The report should determine if the bearing capacity of the soil can support the intended loads. The report should identify any unsuitable materials encountered that may need to be undercut and removed from the project area or possibly off-site. It is possible that information provided in a geotech report may be used to determine that the proposed budget for a project is not feasible. Geotech reports can also be used to screen potential sites during a site selection process. The geotech report should be used by the foundation designer to design the most economical foundations for proposed buildings.

On a typical earthwork project, the owner or contractor employs a geotechnical firm to provide compaction testing services. “Quality Assurance” testing refers to testing conducted on behalf of the owner to assure the work complies with the specifications. “Quality Control” testing refers to testing conducted on behalf of the contractor to verify the work being provided meets the contract requirements. Frequency of testing may be described in the specifications requiring a certain number of tests per lift per area. Alternatively, testing frequency may be determined by the QA/QC firm based on the specific operation. Properly placed and tested fill material is commonly referred to as “structural fill”, “engineered fill” or controlled fill”. A questionable operation with a history of failing tests should usually be tested more frequently than a well performing operation.

ASTM D698 identifies the standard laboratory compaction methods used to determine the relationship between water content and dry unit weight of soils (Proctor curve) compacted in a 4" diameter mold with a 5.5# hammer dropped from a height of 12" producing a compactive effort of 12,400 ft-lb/ft3. This is sometimes referred to as "standard effort". This test is based on a procedure developed by R. R. Proctor (Engineering News Record-September 7, 1933). ASTM D1557 identifies the laboratory compaction methods used to determine the relationship between water content and dry unit weight of soils (Proctor curve) compacted in a 6" diameter mold with a 10# hammer dropped from a height of 18" producing a compactive effort of 56,000 ft-lb/ft3. This test was developed by the U.S. Army Corps of Engineers in 1945. These tests report the target compaction (100%) for soil compacted at optimum moisture. During construction, field compaction tests are performed and the field test result is usually expressed as a fraction of the laboratory test, such as 94.6% compaction.

Special Inspections specified by the International Building Code (IBC) typically include steel inspections. The type of testing and inspection and frequency required for each project is different and is specified by the structural designer. Steel inspections may include: rebar inspections, anchor bolt inspections, structural connection verification, welding inspections, and other inspections of miscellaneous fasteners. Inspection by a Certified Welding Inspector (CWI) certified by the Amercian Welding Society (AWS) ensures the highest level of quality workmanship.

There are many factors that affect the strength and durability of concrete. Properly prepared ready mix concrete delivered to a job site can be significantly altered at the construction site resulting in an unacceptable product. The most common degradation is caused by the addition of too much water to the mix or the addition of too much water for finishing after placement. One gallon of water added to one cubic yard of concrete can reduce its strength by xxx psi. A specific characteristic of concrete is its “slump”. A slump test is performed using a special inverted funnel or slump cone which is filled with concrete to form a twelve inch tall cone. The cone form is removed and the concrete slumps, or sags. The reduced height is referred to as the slump. If the pile of concrete left after the test is eight inches tall, it has a slump of four inches. Concrete with an excessive slump may be susceptible to the aggregate settling in the mix which can result in reduced strength and durability. The entrained air content of the mix can be tested to confirm conformance with the project specifications. Another characteristic that should be measured is the temperature which can indicate premature setting of the mix.

ASTM D4318 identifies the laboratory analysis method used to determine the liquid limit, plastic limit, and plasticity index of soils. The liquid limit, plastic limit, and plasticity index of soils are used extensively with other soil properties to correlate with engineering behavior of soils, including, shrink-well, compressibility, compactibility, shear strength, and hydraulic conductivity.

Concrete strength is determined from samples taken when the pour is made. The sample is molded into six inch diameter cylinders for compression testing. The test specimens are stored on-site for 24-hours then transported to the laboratory where they are cured in a water bath for a specified period of time. When concrete is placed in its plastic state, it has no strength. Most concrete is specified by a 28-day strength which is the strength attained 28-days after the pour. Extra test specimens may be molded and tested sooner than the specified period to predict if the strength will be attained. Typically, specimens with a compressive strength of xxpsi at 7-days will have a compressive strength of 3,000psi at 28-days. Compressive testing is determined by applying an axial load to each test specimen. A six inch diameter specimen with a compressive strength of 3,000psi will fail under an axial load of xxx lbs.

Building foundations are designed with a soil bearing pressure. This is the calculated building load that will be applied to the soil subgrade. No two sites are the same and therefore the soil subgrade prepared to support a building will be different at each site. A soil subgrade bearing capacity determination should be made prior to installation of building foundation footings to confirm the bearing capacity of the soil exceeds the design bearing pressure to be applied by the building. This is typically performed using a dynamic cone penetrometer (DCP) device.

Undercut refers to removal of material that is unsuitable to support the design loads to be applied. Undercut material could be: soil with a high organic content; soil with a high moisture content; or stumps, trash, or other debris. Typically prior to installation of controlled fill, a proof-roll is performed. A proof-roll is performed by driving a fully loaded dump truck over the subject area under the observation of a trained technician or geotechnical engineer. The observer must make the determination if the degree of movement of the soil subgrade is acceptable to support the proposed loads to be applied.

Just as soil can be improperly compacted, so can stone base. Poorly compacted stone base can result in premature pavement failure. Stone base should be tested for proper compaction prior to installation of pavement. “Setting” of stone base is usually accomplished by soaking of the stone layer and compaction with a vibratory roller. When the stone has dried, or set, it is ready for use. One method for determining the in-place density of stone base is the sand cone method (ASTM D 1556). This test requires a small hole to be excavated by hand and a special apparatus is used to fill the void with sand. The excavated stone is weighed, the volume of hole is determined, and the density of the stone is determined. If the properly density is not achieved, the stone can be re-set, compacted, and tested again.

A dynamic cone penetrometer, referred to as a DCP, is a device used to determine the in-situ strength of soil. The device consists of a sliding anvil hammer on a shaft that is driven into the soil to be tested. The number of blows required to penetrate the soil to a given depth is recorded and can be correlated to a standard penetration test or “blow count” obtained from a conventional drill rig. The two greatest benefits of the DCP are low cost and convenience of operation (no drill rig required). The greatest limitation is depth. The DCP is generally used at or near the surface, such as testing the bearing capacity of an excavated footing.

Just as soil can be improperly compacted, so can asphalt pavement. Poorly compacted asphalt pavement can result in premature pavement failure. Asphalt pavement can be tested for proper compaction during the compaction process using a nuclear density gauge. If determination is made in time, before the asphalt has cooled, asphalt pavement may be re-rolled resulting in increased density. An alternate method is to obtain asphalt pavement core samples after placement and test the core samples to determine the density of the pavement. When core samples are used to determine density it is usually too late to correct a deficiency. A failing test may be used to confirm improperly compacted pavement which may be removed, or a payment reduction may sometimes be applied, depending on the project specifications.

Pavements and slabs sometimes fail even though they were properly compacted and testing during construction. One common reason is installation of utility trenches after construction and testing of building pads and pavement areas. All utility trenches in controlled fill should be properly backfilled, compacted and tested.

How steep is too steep? Some jobs require roads to be built on existing steep slopes. The act of grading the roadbed requires building a cut and fill slope which could be steeper than the original grade. Soil characteristics vary with location and therefore what works at one site may not work at another. Slope failure can be catastrophic, costly, and sometimes fatal. All steep slopes proposed on a project should be analyzed by a qualified geotechnical engineer prior to construction.