C.3. - GEOTECHNICAL REPORT

C.3.1.   General Requirements

These guidelines outline the minimum information that shall be presented and evaluated in a Geotechnical Report. These guidelines are not intended to be a rigid framework of requirements, nor a specific format for all reports. The level of detail and emphasis may vary due to unique or particular conditions or due to the type of project. The Geotechnical Report shall generally show results from the applicable testing. The report shall also include a description of site characteristics, e.g., topography, drainage features, etc.

C.3.2.   Detailed Report Requirements

A.

Required Items. The following items are required to be included in all Geotechnical Reports:

• Site location and description

• Boring location diagram

• Laboratory test reports with evaluations (classification tests) (See Table C-1)

Table C-1. Applicable Geotechnical Laboratory Tests

Applicable Laboratory Tests

Visual classification

Liquid limit - AASHTO T89 or ASTM D4318

Plastic limit - AASHTO T90 or ASTM D4318

Plasticity Index

In-situ moisture content

Percent passing No. 200 sieve - AASHTO T11 or ASTM C117-90

Gradation of granular (sand & gravel) materials - AASHTO T27, ASTM D422 or ASTM C136

AASHTO classification and group index - AASHTO M145

Standard Penetrations Test

Swell Potential Evaluation - ASTM D4546-96

 

• Boring logs, to include but not limited to the following: (1) Date, Strata Elevation, Depth of Boring; (2) Natural moisture content, blow count and dry density of each undisturbed sample; and (3) Water table elevation

• Seasonal variations in soil and groundwater conditions to include the expected seasonal groundwater elevation variation shall be summarized.

• Shallow bedrock depth and description including mitigation requirements if bedrock is within 3 feet of subgrade.

• Percentage of soluble sulfates.

• Backfill and fill evaluation including any additional tests required for trench backfill evaluation, fill evaluation, etc.

• Groundwater evaluation including the elevation of groundwater encountered in each boring at time of drilling and at 24 to 48 hours after drilling. If deemed necessary, the boring may remain open for up to 7 days if required for further assessment.

B.

Recommendations and Mitigation Plans. The report shall include recommendations and mitigation concerning but not limited to the following:

• Allowable soil pressure

• Foundation types

• Groundwater

• Maximum stable slopes (cut, fill, and natural)

• Retaining wall information (if applicable)

• Detention pond design and construction (if applicable)

• Cut and fill criteria (such as compaction, moisture content, benching)

C.

Engineer Seal. All Geotechnical Reports shall be signed and seal by a professional engineer registered or authorized to practice in the State of Colorado.

C.3.3.   Boring Standards

The following represent the minimum boring standards and guidelines for conducting borings for Geotechnical Reports.

A.

Timing of Soil Borings.  

1.

Initial Borings. The information from the initial soil borings shall be summarized in the Geotechnical Report. The entire site shall be sampled for initial testing. This is required to evaluate soil and groundwater conditions and for evaluating roadway locations that may not yet be determined or may change.

2.

Structures. Soil borings for design of transportation structures shall be taken prior to the design of the structure.

3.

Fill for Right-of-Way Grading. Testing shall be provided for all proposed fill material. All proposed fill material shall be approved by The ECM Administrator prior to placement. The material shall meet minimum requirements and be equal to or better than existing conditions. No fill material with a liquid limit greater than 40 and plasticity index greater than 20 shall be used in the upper 2-feet of the pavement subbase without implementing proper mitigation techniques.

B.

Frequency of Borings. The following represent the minimum number of borings that are required based on a typical improvements project. The number of borings may be increased based on the geotechnical engineer's recommendations or at the request of the ECM Administrator.

• A minimum of 2 borings for each project with public improvements shall be performed.

• A minimum 1 boring for each SCS (NRCS) soil type within a development shall be performed.

• A minimum of 1 boring shall be performed for each 10 acres of development up to 100 acres. One additional boring shall be performed for every 25 acres of development above the 100 acres.

C.

Borings for Structures. The boring frequency for transportation structures shall satisfy AASHTO Bridge Design requirements and CDOT Materials Testing requirements.

D.

Depth of Borings. Borings shall be performed to a minimum depth of 20 feet. In areas where the cut depths are expected to exceed 8 feet, borings shall be extended to a minimum of 15 feet below proposed finished grade. Borings shall extend deeper if needed to determine if bedrock or high groundwater levels are design concerns. Samples for structures shall be taken to a minimum depth of 10 feet below the footing elevation. Additional depth may be required for piers or piles.

It should be noted that boring depths will ultimately be determined by the geotechnical engineer based on site conditions. However, when depths different than those presented is performed, documentation as to the difference must be presented in the submitted report.

C.3.4.   Testing

The following represent a listing of the minimum testing requirements and procedures for Geotechnical Reports. The actual numbers and types of tests will be based on the site and the activities being proposed.

A.

Applicable Tests.  

• Visual Classification

• Liquid Limit

• Plastic Limit

• Plasticity Index

• Moisture Content

• Percent Passing 200

• Gradation (Granular Soils)

• AASHTO Classification

• Swell Potential Evaluation

• Percentage of Soluble Sulfates

• Standard Penetration/California Test

• Corrosion Potential Resistivity

B.

Classification Testing. Soils shall be classified visually and representative samples tested to determine the soil properties. Sands and gravel samples shall be analyzed for gradation where needed to comply with classification requirements.

C.3.5.   Soil Mitigation

Mitigation measures for soil problems revealed by the soils investigation shall be included in the Geotechnical Report. The following specific factors shall be addressed:

A.

Mitigation Measures and Approval. All special problems found in soils investigation (e.g., expansion, frost, soluble sulfates, shallow bedrock, heave, groundwater, soil instability, utility backfill, etc.) shall be addressed in the mitigation plans. All proposed mitigation shall be approved by the ECM Administrator.

B.

Swell Criteria. If the average swell is 2.0 percent or greater, the pavement design report must provide mitigation measures. The mitigation measures shall reduce destructive swell potential under the public improvements, including landscaping, to an acceptable level of less than 2.0 percent. The swell test report shall specify sample conditions, surcharge pressures, and other key testing factors.

C.

Swell Mitigation Measures. Some commonly accepted mitigation measures for swell include: (1) over excavation; (2) chemical treatment; and (3) moisture treatments. Other procedures may be proposed for approval by the ECM Administrator. The selected method must work for the full life expectancy of the improvements.

D.

Mitigation of Unstable Subgrade. Some commonly accepted mitigation measures for unstable subgrade include: (1) over-excavation; and (2) chemical treatment. Other procedures may be proposed for approval by The ECM Administrator. The selected method of mitigation must work for the full life expectancy of the improvements.

E.

Mitigation Recommendations.  

1.

Extent of Mitigation. Soil treatment shall extend to the back of curb, or to the back of walk for attached or monolithic walk. For detached walk, separate mitigation procedures may be required. A combination of mitigation methods may be required to sufficiently mitigate a soil problem.

2.

Approval of Chemical Treatment. Mitigation procedures that alter existing soil conditions (such as lime, fly ash, or cement treatment) shall follow an approved mix design process. Additional testing is required to verify that no swell is introduced in the chemical treatment.