320.1 Preliminary Geotechnical Report (PGR)

Figures

• Insoluble Residue Test lab report example
• Summary for Preliminary Geotechnical Report (Form M-41)
• Guide for Slope Recommendations

This guidance establishes procedures for sampling, laboratory testing and reporting for preliminary geotechnical investigations.

320.1.1 Procedure

The preliminary geotechnical report originates in, and is the responsibility of, the District. Cut classification, preliminary sounding for structures, and the locating of critical foundation areas are considered part of the preliminary geotechnical report. The preliminary geotechnical report shall be conducted essentially in accordance with the AASHTO Manual on Subsurface Investigations, AASHTO R13, “Conducting Geotechnical Subsurface Investigations”, and the Geology and Soils Manual, 1962 Edition, Chapter III, Missouri State Highway Commission. These references are guides and are not intended to preclude the use of ingenuity and judgment.

Description of soils shall be in accordance with MoDOT’s soil and rock classification guide. The Stratigraphic Succession in Missouri by Thomas L. Thompson and subsequent publications of the Missouri DNR, Division of Geology and Land Survey shall be used as guides to stratigraphic identification. Rock quality designation (RQD) and recovery shall be measured wherever applicable in logging rock cores.

320.1.2 Sampling

Samples shall be obtained in accordance with methods listed in AASHTO Manual on Subsurface Investigations and AASHTO R13. Samples of auger cuttings may be obtained from continuous flight augers (CFAs), hollow stem augers (HSAs), standard penetration tests (SPT), Shelby Tube samples or other similar sampling methods. The use of the Cone Penetration Test (CPT) is also an acceptable means of investigation.

320.1.2.1 Soil Classification Samples

Samples shall be obtained, for classification by ASTM D 2487 and AASHTO M145 (except that group index will be calculated by procedures of M 145-49) and for determination of natural moisture content, at intervals of 5 ft. (1.5 m) or less in at least one hole from each cut with a significant amount of soil. Except in extraordinary circumstances, samples should be obtained from multiple (3 or more) representative locations and from each cut where depth of soil is 10 ft. (3 m) or more. Borrow areas other than roadway cuts shall be similarly sampled in detail to define the range of properties of the material to be borrowed. Samples shall also be obtained in areas of pavement construction or reconstruction to a depth of at least 5 ft below the subgrade elevation of the pavement. Samples shall also be obtained within existing embankment areas to confirm existing slopes meet current slope guideline recommendations.

Classification samples, (commonly referred to as Partials) shall be packaged in zippered plastic bags or small canvas sacks with plastic liners. Bags shall be securely packaged for shipment. Sample tags (provided by CO Construction & Materials Laboratory) shall be placed in the bag and on the outside of the bags. Moisture samples shall be sealed in airtight containers for moisture determination and tested in the District Materials Laboratory.

• Note: It is intended that sufficient samples shall be obtained, without sampling bias, so that test results will be reasonably indicative of the dispersion of properties of the soils to be encountered in grading. The number of locations sampled should depend upon the size of the job and soil stratigraphy of the project location. The number of samples at each location in turn should depend upon stratification and upon the depth of material to be excavated and/or left in place as subgrade.

320.1.2.2 Stabilization Samples

Samples shall be submitted for evaluation of lime stabilization, if applicable, in accordance with AASHTO M216, “Lime for Soil Stabilization". Guidance for sampling should also be provided by MoDOT’s Pavement Engineer in Central Office.

320.1.2.3 Fertility Samples

As of July 1, 2022, seeding recommendations shall be as per EPG 805 Seeding for the region that corresponds with the project location, therefore fertility samples are no longer required for each project. Should differing seeding recommendations be requested, the following procedure should be followed to collect a fertility sample.

Composite samples of material, from each soil that will comprise the finished surface layer of the right-of-way to be seeded, shall be sent to the Central Laboratory for fertility tests. The number of samples will depend upon the judgment of the geologist. Samples should weigh about 1 lb (0.5 kg) and be shipped in zippered plastic bags or small canvas sacks with plastic liners. The following information shall accompany the sample:

(a) Type of project: rural, urban, or the approximate percentage of each when a combination.

(b) Project number, location, county, route.

320.1.3 Sample Identification

Sample Identification record shall be entered in AASHTOWARE Project (AWP) and shall accompany all samples shipped to the Laboratory. Sample identification tags shall be included in the sample container (if possible) and on the outside of the sample container.

320.1.4 Quarantine Areas

In quarantine areas, all sampling procedures shall be in accordance with Quarantine Regulations.

320.1.5 Critical Foundation Areas

Requests for investigation of any critical foundation areas located during the preliminary geotechnical report shall be made in accordance with Foundation Investigations. In addition to the request, a phone call to the Geotechnical section should be made to discuss the details and timelines of the project.

320.1.6 Preliminary Geotechnical Report (PGR)

The report shall be in letter form as provided in the PGR standard paragraphs document. Since investigations differ in scope and complexity, the standard paragraphs document is not meant to be a rigid format but as a guide to ensure most topics have been covered. The report should be clear, concise, and in outline form if long or detailed. The report shall contain the following information, as applicable:

(a) Brief description of the project including clearly described location and station limits.

(b) Identification and brief description of soil types and geologic formations to be encountered. Soil type descriptions shall include a note on the geologic origin, i.e., whether glacial, alluvial, residual, or loessial.

(c) Recommendations on handling and classification of excavation, undergrading, and exceptions or additions to standard specifications for moisture control.

(d) Definition, limits, and recommended disposition of unsuitable materials.

(e) Contractor furnished borrow. If the contractor is to furnish borrow, specify those permissible soil types which will provide a suitable subgrade and which are both readily available and compatible with slope recommendations. Avoid specifying a soil type which may have only one practical source (and owner). Sample and report one or more possible borrow sites for each acceptable soil type where the owner expresses an interest in furnishing material. (This does not require a commitment on the part of the owner or the department and the contractor is free to use any other source provided defined criteria are met.)

(f) Steepest slopes allowed for long term stability of cuts and fills including bridge spill slopes. EPG Section 321.1.1 Slope Inclination for Preliminary Geotechnical Report shall be used as a guide in preparing recommendations. Slopes should be no steeper than those shown in Table 321.1 (Guide for Selection of Slope Inclination for Routine Design) and justification should be furnished for flatter or steeper slope recommendations.

(g) Notation of the absence or presence of possible critical foundation areas and reference to special investigations in progress or completed by the Geotechnical Section.

(h) Recommendations for special drainage features or erosion control practices. Water table observations shall be made and reported for soil cuts whenever there is evidence that saturated conditions may exist. Recommendations shall be included in the report for drainage and/or stabilization of slopes and subgrade.

In instances of widespread or non-localized seepage along areas of side hill fill, selective placement of blankets of available pervious material (normally rock from Class C excavation) should be considered to intercept and daylight the seepage to the outside toe of slope. Well-defined, localized seeps and spring flows, however, may be more economically handled with pipe-aggregate underdrains, particularly where project sources of rock or other pervious materials are limited or non-existent. See also paragraph (i) concerning placement of select granular materials for pavement drainage purposes.

(i) All new pavement is to be constructed on two feet of select rock fill when that material is available in suitable quantities on the project site. The preliminary geotechnical report should include a statement concerning whether the quantity of bedrock available on the job site is adequate, of appropriate quality, and located so that it could reasonably be placed as the top two feet (600 mm) of subgrade material. When the quantity of bedrock on a job is less than is necessary, but would be a significant portion of that needed, a general estimate of that proportion should also be given in the preliminary geotechnical report (if cross-sections are provided). Rock fill from sources other than the roadway balance should also be considered if within reasonable proximity to the job.

Note: This information is important in the early stages of project development because the subgrade material to be used will affect the type of pavement selected, the thickness of a flexible pavement, the type of drainage required, and final elevations.

(j) Location and recommended handling of sinkholes, caverns, mines, etc.

(k) A condition survey of the existing pavement when applicable. (Note: A condition survey need not be considered or handled as a preliminary geotechnical report if done for routine resurfacing only. If a resurfacing project includes widening equivalent to at least one lane of pavement the condition survey should be handled as a preliminary geotechnical report including multiple subgrade samples.)

(l) Any other information deemed necessary.

320.1.6.1 Summary for Preliminary Geotechnical Report (Form M-41)

The Summary for Preliminary Geotechnical Report - Form M-41 is to be prepared and attached to the preliminary geotechnical report and shall include the soil samples collected at the project site. Soil samples should be collected for each soil type encountered. Discretion may be exercised as to which soil types justify inclusion of test data. Descriptive and interpretive comments, including the geologic origin of soils, shall be placed in the remarks section (recommendations are to be placed only in the text of the letter). The soil shrinkage factor to be reported on this sheet is the ratio of maximum dry density, determined from the moisture-density relations test to the natural dry density. An estimate of the soil shrinkage factor may be obtained from the Geology and Soils Manual, 1962 Edition, Chapter II, Missouri State Highway Commission. This will be only one consideration for designers in estimating a balance shrinkage factor for the project.

320.1.6.2 Subsurface Logs for Preliminary Geotechnical Report

MoDOT’s electronic boring log software and output formats shall be used for all auger and core drill logs, etc., and shall be included in the PGR. All detailed sample data shall be reported on the logs or as an additional output if not available to include on the boring logs. Core logs shall include the driller's record of run lengths and recovery lengths and the rock quality designation (RQD) in percent.

Prior to final distribution of the PGR, a copy shall be sent to all MoDOT Geologists for internal review. Only one Geologist needs to complete this review prior to final submittal. Following review and any necessary revisions the responsible Geologist shall upload the preliminary geotechnical report to eProjects and tag it with the appropriate Geotechnical metadata. A link to the report shall be sent via email to the following:

Title
Geotechnical Director
District Project Manager
Central Office Geologist
Structural Resource Manager (if Bridge project)

320.1.7 Preliminary Structure Information

When performing a PGR for a project that may include a bridge, box culvert, retaining wall or other structure type, a minimum of one boring should be drilled to top of bedrock. Logs of borings performed for the preliminary structure should be reported in MoDOT’s electronic boring log software. The following questions should be included on the boring log output or provided in paragraph format in the PGR report:

a) Geologic Formations encountered:

b) Are pinnacles, crevices or cavities anticipated?

c) Depth to water table:

d) Is scour anticipated?

e) Is difficulty anticipated in gaining access to the site?

f) Are foundations problems anticipated?

g) Will a special investigation be requested?

h) Other comments:

320.1.8 Guide for Slope Recommendations

Refer to Table 321.1 Guide for Selection of Slope Inclination for Routine Design.

320.1.9 Laboratory Testing Procedures

Soils tests, upon request, are to be performed as shown in this Article. Test results and calculations for all tests shall be recorded on the boring logs or the actual test results included with the final report if they cannot be included on the boring logs.

320.1.9.1 Routine Preliminary Geotechnical Report Tests

The following tests shall be performed on routine preliminary geotechnical report samples.

(a) Liquid Limit - AASHTO T89, Mechanical Method (Alternate).

(b) Plastic Limit and Plasticity Index - AASHTO T90.

(c) Particle Size Analysis - AASHTO T88, except that hydrometer analysis is not required.

(d) Soil Classification - AASHTO M145, except that the group index will be calculated by the procedures of M145-49.

(e) Soil Classification - ASTM D2487, except that the data required for ASTM classification shall be determined by the AASHTO test methods of this section.

(f) Moisture-Density Relations (Compaction Standard) - AASHTO T99, Method C.

320.1.9.2 Tests Specifically Requested

The following tests are considered non-routine and will be performed only upon request.

(a) Shrinkage Factors - AASHTO T92.

(b) Specific Gravity - AASHTO T100.

(c) Field Moisture Equivalent - AASHTO T93.

(d) Moisture Content - ASTM D2216 (Normally this test should be performed in the District).

(e) Fertility - A sample for fertility tests shall be initially prepared according to AASHTO T87. The test portion shall consist of approximately 100 g of material passing the No. 10 (2.00 mm) sieve. Fertility tests of soils shall be performed according to the methods set forth in "Soil Testing in Missouri", September 1973, by the Agronomy Department of the University of Missouri.

(f) Rock Type Identification - Requests for rock type identification, for purposes of cut classification, are to be referred to the Geotechnical Section for determination of specific test requirements. Cores are the preferred form of sample since identification may be influenced by laminations or fissility and since cyclic lithologies could be misrepresented by composite samples. Tests to be performed, as applicable, may include microscopic examination and grain size count, mechanical analysis after degradation, insoluble residue determination and chemical analysis. The test report will indicate where samples may be subject to more than one interpretation of identification.

(g) MoDOT Insoluble Residue (Procedure to Aid in Proper Cut Classification) - The primary purpose of this system of identification is to provide a simple and quick method of cut classification which will be fair to both the state and the contractor. It is based on two accepted tests, the dividing points are justifiable according to authoritative geological authorities, and it is a little more than fair to the contractor.

A representative sample is crushed and treated with hydrochloric acid. If more than 50% of the sample is remaining after treatment with the acid, the remaining sample or insoluble residue shall be used to determine the grain size.

Interpretation

1. If 50% or more of the original weight is lost in solution by treatment with acid, the material will be classified as limestone or dolomite, both of which are Class C material.

2. If less than 50% dissolves, the material will be classed as sand, sandstone, or shale, pending the results of the sieve test.

3. If 50% or more of the insoluble material is larger than #270 sieve, it will be classed as sand or sandstone.

4. If more than 50% of the insoluble residue is smaller than #270 sieve, the material will be classed as shale.

The only thing remaining to be done then is to decide whether the sand is in ledge formation and whether or not it is cemented into sandstone.

This proposed system is not to be construed as an attempt to set up a hard and fast method of classifying excavation materials. It does go a few steps beyond the specifications and reduces the personal equation to some extent. However, judgment will still be one of the primary factors in the final decision. It is possible, for example, when using this method of identification, to test one sample with 51% insoluble residue which contains more than 50% small sized material. It would be classed as shale – Class A, (#2 and #4, above). The next sample might have 49% insoluble material and be identified as Class C excavation (#1, above). For all practical purposes, there would be no difference in the difficulty of excavation.

Although this would be an extreme case, it is a possibility and, if it should occur, certainly judgment would have to be the ruling factor. Similar cases might arise when excavating sandy shale. It must be remembered that no test is any better than the sample tested. Samples submitted for these tests should be taken with utmost care, and the greatest effort be directed toward obtaining material as nearly representative as possible of the disputed excavation materials. If it should be deemed necessary, in order to provide a satisfactory answer, numerous samples should be obtained. The amounts of money usually involved are so great that the cost of multiple tests would be negligible in comparison. Experience has shown that, if a dispute arises over proper classification of excavation materials, a 4 in. core can be examined and classified by the contractor’s representative and by the District Geologist, both of whom should be at the drill site at the same time. In order to avoid any possibility of either influencing the other, the examination should be made by each, separately, while the other occupies himself elsewhere. The respective logs should be compared after the examinations are completed, and the part or parts of the core on which there is disagreement can be removed from the complete core, identified, and sent to Jefferson City for laboratory tests. It is a good idea to have the sample identification sheets initialed by the contractor’s representative.

(h) Laboratory Procedure for Preparing and Classifying Cut Classification Samples

Apparatus:

Hydrochloric Acid (HCl), specific gravity 1.19.

50% Hydrochloric Acid (HCl), by volume

Borosilicate glass beakers, 2000 milliliter

Balance sensitive to within 0.5 percent of the weight (mass) of sample to be weighed

Aspiration system

Glass stirring rod, at least 10 in. long

Standard Sieve #10, #40, #100, #200, and #270

Referenced Documents:

AASHTO Standards:

T87-86 (2008) Dry Preparation of Disturbed Soil and Soil Aggregate Samples for Test M92 (2005) Wire-Cloth Sieves for Testing Purposes

ASTM Standards:

D1140 (2000) Standard Test Methods for Amount of Material in Soils Finer than the No. 200 (75 mm) Sieve

Sample Preparation:

1. The sample as received is split or reduced to yield a representative working sample weighing 3 to 4 lbs.

2. This sample is put through a small jaw type crusher to reduce the maximum particle size to passing #4 sieve.

3. The passing #4 sieve material is split on a sand splitter several times in order to obtain a representative sample weighing approximately 300 grams.

4. The 300 gram sample is then pulverized by means of a power driven rubble pestle, of special design in conjunction with a matching steel bowl or mortar, and/or by means of a burr mill until the total material will pass the #10 sieve. Care shall be exercised to assure that none of the material is degraded to less than individual grain size.

5. The 300 gram sample is then dried in an oven at 230°F. Two representative samples shall be obtained, each to consist of approximately 100 grams. One shall be accurately identified and saved, and the other used for testing.

Insoluble Residue Test:

6. Weigh approximately 100 grams of the appropriate sample in a 2000 milliliter beaker and record to the nearest 0.1 gram.

7. Slowly add 200 milliliters of 50% HCI solution (see note below). As the reaction subsides, add 25 milliliters of concentrated HCI as needed to increase the rate of reaction.

Use extreme caution when adding acid. For classification samples high in carbonates,the reaction may be very vigorous during the initial stages of the reaction. If the sample is in danger of overflowing the beaker, add small amounts of DI water with a wash bottle until the reaction slows.

8. When the addition of concentrated HCI no longer increases the rate of reaction, dilute sample to 1400 milliliters with DI water and stir. Stir after each addition of DI water, as this will assist in expelling any trapped acid in the residue.

9. Let sample settle to the bottom of the beaker for at least 3 hours.

10. After a minimum of 3 hours, carefully siphon liquid from the beaker to an approximate 250 ml mark on the beaker using an aspiration system. Pay close attention not to disturb the sample while performing this procedure. Repeat this at least 2 more times so that all acid has been removed from the sample. During last wash, remove stirring rod and transfer any material adhering to the rod with bottle of DI water.

11. Place beaker in 105 degrees C oven overnight.

12. Take beaker out of oven, cool, and weigh beaker to constant weight. Record the mass of the beaker with residue.

13. Calculate the percentage acid insoluble residue as follows:

Percentage acid insoluble residue = 100 x (weight of insoluble residue / original weight of sample)

If the weight is less than 50 percent of the original weight, the sample is classified as a limestone and further testing is not required.

Sieve Test

14. If more than 50 percent of the original weight remains undissolved at the completion of the addition of HCL, the insoluble residue shall be used to determine the grain size. The sample is placed in a beaker, mixed with a solution of calgon and permitted to stand for 18 hours. This mixture is dispersed with a stirring device and washed over a #270 sieve. The retained portion is dried and a gradation run using the following sieves: #10, #40, #100, #200, and #270.

15. If more than 50% of the insoluble residue passes the #270 sieve, the sample is to be classified as a shale. If less than 50% passes the #270 sieve, the sample is to be classified as a sand.

320.1.10 Geotechnical Laboratory Testing

This guidance establishes procedures for the soil tests routinely performed by the Geotechnical Section.

320.1.10.1 Apparatus

The required equipment is listed in the appropriate method as described in this section.

320.1.10.2 Procedure

(a) Unconfined Compressive Strength - Unconfined compressive strength is to be determined in accordance with AASHTO T208.

(b) Consolidation - Consolidation characteristics of soils are to be determined in accordance with AASHTO T216.

(c) Triaxial Compression - Triaxial strength parameters are to be determined in accordance with AASHTO T234.

(d) Direct Shear - The direct shear test is to be performed in accordance with AASHTO T236.

(e) Swell Test - The swell of soil is determined in accordance with AASHTO T216 except as follows:

(1) Select the restraining load. Restraining loads are to be based on the anticipated load on the soil in place in the subgrade.

(2) Apply the load, record the zero reading and inundate the sample.

(3) Record the increase in height of the sample daily until movement ceases.

(4) Percent swell is the change in height of the sample divided by the original height of the sample.

(f) Constant-Head Permeability - The permeability of granular soils containing not more than 10 percent non-plastic material passing the No. 200 [75 mm] sieve is to be determined in accordance with AASHTO T215.

(g) Falling-Head Permeability - The permeability of granular cohesion less materials consisting predominantly of particles passing the No. 10 (2.00 mm) sieve may also be determined in accordance with "Suggested Method of Test for Permeability of Porous Granular Materials by the Falling-Head Permeameter", Special Procedures for Testing Soil and Rock for Engineering Purposes, ASTM STP 479, 1970, except that:

(1) A steel permeameter with an inside diameter of 4 in. (100 mm) and a height of 4.6 in. (116.8 mm) shall be used.

(2) Soil shall be compacted in the permeameter in accordance with AASHTO T99, Method C.

(h) Permeability of Fine-Grained Soils - Permeability of fine-grained soils is generally to be computed from consolidation test data and may be determined by direct measurements on triaxial shear specimens.

(i) California Bearing Ratio - The California Bearing Ratio (CBR) is to be determined according to AASHTO T193.

320.1.10.3 Report

Test results and calculations for all tests shall be recorded on the boring logs or the actual test results included with the final report if they cannot be included on the boring logs.