Soils are engineering materials usually formed under random and extremely variable circumstances, which make them rather difficult to characterize for design purposes. Consequently, it has been necessary to standardize laboratory tests to measure the engineering properties of soils with an acceptable rate of accuracy.
The concepts discussed in the theoretical geotechnical engineering courses are reinforced through the direct measurement of soil properties, thus having a direct effect on the student as follows:
1. Better understanding of the differences between soil types through result comparison and analysis.
2. More accurate assessment of the limitations involved when considering the soil-structure interaction in design.
3. Better knowledge of the local soil conditions and the effect of moisture changes and other factors on soil strength.
The Geotechnical Engineering component of the Civil Engineering undergraduate program at PUPR consists of two theoretical and two laboratory courses that are to be carried out in two consecutive terms as follows:
Geotechnical Engineering I (CE 4202) Geotechnical Engineering I Lab (CE 4203)
Geotechnical Engineering II (CE 4204) Geotechnical Engineering II Lab (CE 4205)
The classes meet for two hours twice a week. Safety and test procedure briefs are followed by a Power Point presentation of the test, the students are provided with handouts to follow the test presentations.
The Geotechnical Engineering Laboratory has multiple (usually, four to five) sets of equipment meeting or exceeding industry standards. The laboratory facilities provide enough space for four fully equipped workstations. The laboratory supports the theoretical courses, some elective courses, and research at PUPR.
The maximum enrollment per course section is 16 students. In each academic term (Fall, Winter, and Spring) two or three sections of the CE 4203 course and two sections of the CE 4205 course are offered.
Currently, two instructors teach the Geotechnical Laboratory courses with the assistance of one technician. The qualifications and relevant background data of the staff are shown in the following table:
|Resource ||Employment status|
|Omaira Collazos, PhD of CE || Full time professor and Coordinator of the Geotechnical Engineering Laboratory |
|José A. Martínez, MSCE ||Full time professor |
|Isabel Lorenzana, MEM and BSCE || Assistant to the Geotechnical Engineering Laboratory Coordinator|
|Three-person drilling crew || Contract with Jaca & Sierra Testing Laboratories Inc., Geotechnical Engineers & Services|
|Two undergraduate civil engineering students || Student assistants|
The laboratory facilities are located in a 25 by 35 feet room that houses three working counters with lower storage compartments and a storage room. Another 12 feet by 45 feet room is dedicated to compaction testing and graduate student work. Figures 1 and 2 show the laboratory facilities. Some of the tests and research activities are carried out around the PUPR campus.
Figure 1. Geotechnical Engineering Laboratory facilities
Figure 2. Geotechnical Engineering Laboratory facilities
5. Tests Conducted and Equipment
5.1 Geotechnical Engineering I laboratory (CE 4203)
Following is a description of the tests performed for this course:
5.1.1 Sub-soil Exploration and Sampling
The drilling crew performs two boring by means of the Standard Penetration Test (SPT) at the beginning of the term; the retrieved samples are used for testing throughout the term.
5.1.2 Water Content and Soil Phase Relationships
The students evaluate the relationships between the three phases that make up a partially saturated soil sample by means of direct measurement of volume, total weight, water content, and specific gravity calculations. The results of the tests are then used to solve a geotechnical engineering problem involving earthwork calculations.
5.1.3 Consistency Limits
The students determine the plastic limit, the liquid limit, and the plasticity index of clayey soil samples; the plasticity index value is used to have an idea of the swelling potential of the soil.
5.1.4 Mechanical Grain Size Distribution
The students perform the mechanical grain size distribution of a sandy sample and use the results to determine whether the sample is suitable for use as fine aggregate for a concrete mix (ASTM C-33).
5.1.5 Washed Grain Size Distribution
The students perform a washed grain size distribution of the same sample used for the consistency limits test.
5.1.6 Soil Classification
The results of the consistency and washed grain size distribution tests are combined to classify the fine soil sample as per the Unified and AASHTO systems.
5.1.7 Compaction Test
The maximum dry density and the optimum moisture content of a soil sample are determined by means of a Modified Proctor Test. The trend of the relationship between the water content and the dry density values is established by mixing the soil with a minimum of five different amounts of water.
5.1.8 Field Density
The field density of a sample retrieved from the PUPR campus is determined using the sand cone method; that value is used to determine the degree of compaction of the sample.
5.1.9 Falling Head Permeability Test
The hydraulic conductivity of a sandy sample is determined by means of a falling head test; the result is used to estimate the amount of seepage underneath a concrete dam. The effect of sample handling on void ratio and on the hydraulic conductivity value is discussed. An additional permeability test is conducted on a finer sample to demonstrate the significant (order of magnitude) reduction in hydraulic conductivity for fine soils.
Figures 3 through 10 show the available equipment and the tests performed as part of this course.
Figure 3. Soil sample retrieved by Standard Penetration Tests at PUPR campus
Figure 4. Water content determinations
Figure 5. Consistency limits
Figure 6. Washed grain size distributions
Figure 7. Mechanical grain size distributions
Figure 8. Modified Proctor test
Figure 9. Field dry density
Figure 10. Falling head permeameter
5.2 Geotechnical Engineering II Laboratory (CE 4205)
The following tests are performed as part of this course:
5.2.1 Sub-soil Exploration and Sample – Soil Profile
Soil samples are obtained at the PUPR campus by means of the Standard Penetration Test, (SPT) from two borings to a depth of between 16 and 20 feet, the cohesive nature of the soils allows for high sample recovery yielding good, non-fractured specimens.
Each of the teams of this course is in charge of performing the following tests on one of the specimens: moist and dry unit weight, water content, consistency limits, and washed grain size distribution. The data is shared with the rest of the teams of the other sections after being reviewed by the instructors.
The students prepare a 17 in by 11 in soil profile depicting the variation of the geotechnical properties of the soil with the results from all the teams.
5.2.2 Consolidation test
A saturated fine soil sample, retrieved using a thin wall (Shelby) tube, is subjected to increasing vertical overburden for five days; the teams collect and share the sample deformation data.
An application problem is solved using the test results to estimate the amount and rate of consolidation settlement induced by an axial load on a rectangular footing.
5.2.3 Unconfined Compression Test
A cohesive soil sample obtained at the PUPR campus by means of the Standard Penetration Test (SPT) is subjected to unconfined compression in order to determine its consistency, its modulus of elasticity, and the value of Poisson’s ratio at the peak value.
The test results are used to estimate the immediate/elastic settlement underneath a rigid concrete footing due to axial loading.
5.2.4 Direct Shear Test
A direct shear test is performed on a dry, cohesionless sandy soil sample in order to determine the value of its angle of internal friction, ?.
The unit weight of the soil sample is determined using a cylindrical mold; the results are used to evaluate the overturning moment due to soil pressure on a gravity wall.
5.2.5 Triaxial Compression Test
A triaxial compression test under unconsolidated/undrained (UU) conditions is performed on cohesive soil samples in order to determine its cohesion and internal angle of friction values. The results are used to determine the factor of safety against sliding for a slope.
5.2.6 Hydrometer Test
A hydrometer test is performed on a fine clayey soil sample in conjunction with a washed grain size distribution in order to determine the clay fraction of the sample.
Consistency test results are provided to the students so they can combine with the test results to estimate the swelling potential of the soil sample.
Figures 11 through 21 depict the available equipment and the tests performed as part of this course.
Figure 11. Unconfined compression test on sample retrieved by STP
Figure 12. Determination of sample dimensions
Figure 13. Soil grinder for subsoil evaluation
Figure 14. Reading of consolidation sample deformation
Figure 15. Sample at the end of the consolidation test
Figure 16. Unconfined compression test apparatus
Figure 17. Direct shear test apparatus
Figure 18. Preparation of soil sample for direct shear test
Figure 19. Triaxial compression test apparatus
Figure 20. Triaxial compression test equipment with automatic data acquisition system
Figure 21. Hydrometer test
6. Equipment Maintenance and Calibration
All the major equipment of the laboratory is periodically maintained by the staff and calibrated by external resources. The major equipment includes two triaxial test apparatus, two direct shear apparatus, two unconfined compression machine, three ovens, and two consolidation test stations.