The Steps of a Geotechnical Evaluation

A geotechnical evaluation is an engineering site assessment that determines what the foundation conditions are and recommends options for foundation design.  It is performed by a geotechnical engineer, and is required for most engineered structures including buildings, bridges, roads, and towers.

The four main components of a geotechnical evaluation are:

  1. Planning
  2. Field Work
  3. Laboratory Analysis
  4. Reporting

Planning

Geotechnical drilling

Geotechnical drilling

Prior to the field work, the geotechnical evaluation must be planned to ensure the right data is collected.  Components of the planning phase include:

  • Obtaining the design information for the site.  The type, size, and characteristics of the structure to be built, including construction sequencing, time of year, and so forth must be known at least to an extent to be able to understand its potential effects on the soil.
  • Determining the number and spacing of boreholes.  Three boreholes is generally considered the minimum because it gives you a basic idea about how the subsurface stratigraphy varies, although single point loads like industrial towers could get away with just one.  For medium sized buildings greater than about 250 m2 and smaller than about 1,000 m2, four boreholes is generally adequate.  And for bridges, two are usually adequate because the bridge footprint is concentrated into two small loading locations (the abutments).
  • A preliminary subsurface model to determine the available foundation options to guide the field program.  For example, if pile foundations are being considered the depth of the boreholes should extend below the expected depth of the piles, to be able to determine a suitable end bearing value.  The depth of the boreholes should extend to a point where the expected stress increase due to the structure is small, say 10% or less.
  • Desktop study and gathering of prior subsurface information obtained by others at or near the site.  The performance of adjacent structures near the site, as well as any past experience is extremely helpful.  Also, a review of geological maps indicates which types of rock and foundation material might be present.

The greater the natural variability of the ground, the greater the extent of the field investigation necessary to obtain an accurate indication of the site conditions.  That is, if the site is in hilly terrain with rock outcrops or other indications of potential subsurface variation, it may be better to err on the side of more boreholes than less.

Field Work

Geotechnical core drilling

Geotechnical core drilling in rock conditions

The field work portion of the geotechnical evaluation usually involves drilling.  The type and size of the drilling rig are determined based on the information obtained during the planning step, above.  There are two main types of drills:

  • Augers are used to sample sands, clays, and silts (granular or non-granular).
    An auger is a round, helical rotating blade that removes the material upward as it turns.  In geotechnical evaluations, grab samples are taking from the auger blade at 500 mm to 1000 mm intervals and logged as to their location (depth) below the ground.  The engineer then leaves the site with a series of grab samples representing the site stratigraphy.
  • Core drills are used to sample rock.
    A core drill is hollow on the inside and brings a cylindrical rock sample to the surface.  Core drilling is necessary when the geotechnical characteristics of the rock must be determined.  Coring in the presence of bedrock is sometimes not necessary if the quality of the rock is not important, for example when the friction capacity of piles is likely to be already sufficient.
pocket penetrometer

A pocket penetrometer testing soil strength

In the case of auger drilling, most drill rigs in North America are outfitted with a Standard Penetration Test (SPT) apparatus, and the engineer will specify SPT tests at roughly 1 – 2 m intervals.  This test determines an “SPT N value” which is reported on the drill hole logs and used in pile and footing design.  Although considerable research shows the SPT test has a low repeatability, and the Cone Penetration Test (CPT) is preferred over SPT, it remains the most widely used strength test for soils in North America.

Also, the pocket penetrometer is a small, handheld device which can measure soil strength, however it should be used only as a qualitative guide in design.

For minor structures where an excavator (backhoe) is readily available, a test pit can provide valuable information and allow the soil to be inspected in its native form.

It is rare that an engineering design of foundations does not require groundwater depth information.  To obtain this, a standpipe is inserted into each borehole and the water table is measured.  To obtain the long term water table elevation (unaffected by the drilling process), the water depth should be measured 1 – 2 weeks later.

Laboratory Analysis

Casagrande cup

A Casagrande Cup is used to measure the liquid limit

Following the field work, a laboratory analysis establishes soil parameters which are used in the design of foundation systems.  The most common laboratory tests used in a geotechnical evaluation are:

  • Moisture content
    Each soil sample is weighed, then dried, then weighed again.  The resulting moisture content is expressed as a percent by weight.
  • Atterberg Limits
    The Atterberg limits are the moisture contents at which the soil changes state.  The two Atterberg limits of importance are the plastic limit (PL) and the liquid limit (LL).  The PL is the interface between plastic and solid.  The LL is the interface between plastic and liquid.  Furthermore, the Plasticity Index (PI) is the difference between the two, that is, the size of the plastic range.  The LL is measured in the lab using a Casagrande cup.  The PL is measured by hand methods.
  • Specific Gravity
    To measure the specific gravity, also known as relative density, the portion of the soil sample smaller than the No. 4 sieve size (4.75 mm) is removed from the sample, and its weight and volume are measured.
  • Rock Quality Designation (RQD)
    For rock samples, the “quality” of the rock is often a factor in the design of foundation systems.  The RQD provides a standardized way of measuring the percentage of the sample that is free of jointing and fractures.  Generally, rock socketed piles or rock anchors should penetrate into rock material with an RQD of at least 90%.

Other laboratory tests frequently used are:

  • Sulphate content
    A soil high in sulphate can attack concrete and cause the aggregate to lose bond with the cement, resulting in severe damage to the concrete over time.
  • Sieve analysis
    To determine the USGS soil classification, a sieve analysis must be performed.
  • Hydrometer test
    This test measures the particle size distribution for fine grained soils (clays and silts) which is used in the design of end bearing for footings and piles.

There are many other laboratory tests that are performed on a case by case basis, hence this should not be taken as a comprehensive list.

Reporting

To conclude the geotechnical evaluation, a report is prepared which communicates the site conditions and foundation design recommendations to the applicable parties.  Geotechnical reports have the following core components:

  • Existing site conditions
    This sections summarizes the results of the field investigation, describes the soil layers encountered and groundwater conditions.  It outlines any other surface features and the results of the desktop and historical analysis.  Drainage and topography is described, along with any issues that it may present to the design and construction of the structure.  The site conditions give the reader a good sense of what was encountered without having to interpret the borehole logs.
  • Geotechnical Recommendations
    borehole log

    A typical borehole log

    This section outlines the design parameters for the site.  It includes the foundation options, such as piles (deep foundations), footings (shallow foundations), rock socketed piles, and so forth.  Design parameters such as soil resistance factors, ultimate static bearing pressures, and base and shaft resistance values are specified for use in foundation design.  Slope stability, asphalt pavement design, and other site specific issues are addressed.  Excavation and backfill recommendations are made.  Frost protection measures are implemented in northern climates, which usually results in a minimum depth of foundation  (to get underneath the frost line). 

  • Borehole logs
    Finally, the borehole logs are usually placed into an appendix.  The logs show the depths and elevations of each soil layer, and show the results of all the tests performed at each level.

Geotechnical evaluations are performed at the beginning of most engineering projects and set the project up for success.  There is no substitute to having a well understood foundation to your structure, since everything else rests upon it.

Good luck with your geotechnical evaluations!

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