How to Prepare a Good Geotechnical Test Report

A geotechnical test report is provided at the end of geotechnical investigation or subsurface soil exploration and they are very important in civil engineering works. The purpose of the investigation is to obtain information as outlined below:

1. Determining the nature of the soil at the site and its stratification.
2. Selecting the type and depth of foundation suitable for a given structure.
3. Evaluating the load-bearing capacity of the foundation.
4. Estimating the probable settlement of a structure.
5. Determining potential foundation problems (e.g., expansive soil, collapsible soil, sanitary landfill, etc…).
6. Determining the location of the water table.
7. Determining the depth and nature of bedrock, if and when encountered.
8. Performing some in situ field tests, such as permeability tests, van shear tests, and standard penetration tests.
9. Predicting the lateral earth pressure for structures such as retaining walls, sheet piles, and braced cuts.

Basis for the Report

Most civil engineering works bear on soils and transmit varying amounts of loads on the soil (loads may include: dead loads, live loads, wind and earthquake forces, lateral pressures exerted by the foundation earth on the embedded structural elements, and the effects of dynamic loads) and other forces such as: lateral or uplift forces on the foundation elements due to high water table, swelling pressures on the foundations in expansive soils, heave pressures on foundations in areas subjected to frost heave, negative frictional drag on piles where piles are used in highly compressible soils.

The soil resists these loads by shear actions due to its shear strength. The more the load, the more the shear strength required for the soil to be able to withstand the loads without failure. The shear strength of soil varies from place to place and it is affected by a number of factors that include:

1. Particle sizes of soil
2. Density of soil
3. Water content of soil
4. Strain rate of soil
5. Soil structure
6. Cohesion of soil
7. Frequency and magnitude of loads.

A loaded soil also undergoes settlement which is affected by factors such as:

1. Type of soil
2. Frequency and magnitude of soil.
3. Pore spaces in the soil.
4. Amount of water present in the soil etc

Steps for Geotechnical Investigation

Geotechnical investigation is conducted in three steps:

Step 1: Collection of Preliminary Information

Step 2: Reconnaissance Survey

Step 3: Detailed Site Investigation

Preliminary information consists of the information about the type of structure to be built especially in connection to the expected load, the position of columns (for buildings), length of span for bridges, and applicable code requirements, etc.

In the reconnaissance survey, the engineer visits the site to observe features that can affect the construction works and give some idea about the nature of the soil. Information sought here includes:

1. The general topography of the site, the possible existence of drainage ditches, and other materials present at the site.
2. Evidence of creep of slopes and deep, wide shrinkage cracks at regularly spaced intervals may be indicative of expansive soil.
3. Soil stratification from deep cuts, such as those made for the construction of nearby highways and railroads.
4. The type of vegetation at the site may indicate the nature of the soil.
5. Groundwater levels can be determined by checking nearby wells.
6. The type of construction nearby and the existence of any cracks in walls (indication for settlement) or other problems.
7. The nature of the stratification and physical properties of the soil nearby also can be obtained from any available soil-exploration reports on existing structures.

Information obtained from the reconnaissance survey would give an idea of the type of detailed investigation to be carried out for the soil. The information aids in the:

1. planning (i.e. adopting steps for site investigation and future vision for the site),
2. making test boreholes, and
3. collecting soil samples at desired intervals for visual observation and laboratory tests.

Depending on the nature and complexity of the project and information from the visual inspection of soil through preliminary survey, a few or majority of the following tests may be conducted on the soil in other to classify the soil, and determine its shear properties and settlement characteristics of the soil. These tests include:

1. Classification Tests: Classification tests may include moisture content, Atterberg limits, density, particle density (or specific gravity), and particle size distribution (BS 1377-1:1990).
2. Compaction-Related Tests: Compaction-related tests consist of moisture-density relationship tests (determination of compaction parameters); tests for measurement of the limiting densities of non-cohesive soils; the CBR test, moisture condition test, and the chalk crushing value test (BS 1377-4:1990).
3. Shear Strength Tests: Shear strength tests comprises direct shear box test (for sand), unconfined compressive strength tests, laboratory vane shear test (for very soft soils), ring shear tests, triaxial tests all in total stress terms (BS 1377-7:1990) and consolidated-drained and consolidated-undrained triaxial compression tests for effective shear strength parameters (BS 1377-8:1990)
4. Settlement Tests: Settlement tests consist of consolidation or oedometer tests which can be determined using hydraulic consolidation cells or triaxial cells under isotropic conditions (BS 1377-6:1990).
5. Compressibility, Permeability, and Durability Tests: These tests include soil one-dimensional consolidation tests, permeability tests (constant or falling head method), erodibility and dispersive characteristics of fine-grained soils, frost-heave or frost-thaw or simply durability test (resistance to loss of strength) (BS 1377-5:1990).
6. Insitu Tests: In some situations, in situ tests are also required which are often adjudged more reliable than laboratory tests. Common in situ tests include in-situ density tests (water replacement method, sand replacement method, balloon method, nuclear density meter method etc.); in situ penetration tests (standard penetration test-SPT, static cone penetration test – CPT, dynamic probing (DP) resistance test, pressuremeter test – PMT and flat dilatometer test – DMT); In-situ vertical deformation and strength tests (plate load test, shallow pad maintained-load test, insitu CBR); In-situ corrosivity tests (in-situ apparent resistivity of soil, in-situ redox potential of soil) (BS 1377-9:1990).

Information on the Geotechnical Test Report

At the end of the tests, geotechnical reports prepared typically show the following information outlined below:

The Class of the Soil

Soil classification provides a systematic method of categorizing soils according to their probable engineering behaviuor, and allows engineers access to the accumulated experience of other engineers (Holtz and Kovacs). Soil classification gives the engineer a fair idea of how the soil will behave in the engineering situation because the properties that are usually investigated for classification tests often correlate well with main engineering properties of soil such as maximum dry unit weight, optimum moisture content, CBR value, shear strength parameters, etc. Note that the classification system does not eliminate the need for detailed soil investigation for testing of the main engineering properties. Different soil classification systems exist in soil mechanics/geotechnical engineering. Among all, the USCS and AASHTO systems are the most widely used.

The Bearing Capacity of Soil

The ultimate bearing capacity, or the allowable soil pressure, can be calculated either from bearing capacity theories or from some of the in situ tests. To the students, bearing capacity is commonly determined using Terzaghi bearing capacity theory or the general bearing capacity equation. To professional engineers, the application of field tests for determining bearing capacity is of particular importance since the present practice is to rely more on field tests for determining the bearing capacity or allowable bearing pressure of soil.

Common methods used for bearing capacity computations are the Terzaghi, Meyerhof, and Hansen methods (Equations are outlined below):

a. Terzaghi Bearing Capacity Equation

Based on Terzaghi’s theory, the bearing capacity of a strip footing can be determined using the expression Equation (1).

Where,

Q_ult = ultimate load per unit length of the footing

c = unit cohesion

γ = effective unit weight of soil

B = width of the footing

D_f = depth of foundation

N_c, N_q, and N_y = bearing capacity factors which are functions of the angle of friction, ϕ (the values can be obtained from specialized literature).

When the footing is square,

When the footing is circular,

When the footing is rectangular,

{Note: B = width or diameter; L = length of footing}.

Also, note that the presence of a water table at a depth less than (D_f + B) has a significant effect on the bearing capacity equations and these should be considered in the computation of ultimate bearing capacity values.

b. General Bearing Capacity Equation

Meyerhof developed the ultimate bearing capacity equation which eliminates the need for shape factors inherent in Terzaghi theory by taking into consideration the shape of the footing and the inclination of the load. The formula is expressed below:

Where,

c. Hansen Bearing Capacity Equation

Hansen’s equation is similar to that of Meyerhof except with the addition of two other factors that take care of base tilt and foundation on slopes.

Visit: Bearing Capacity of Soils- Types and Calculations for More

It is good practice to use two methods and compare the value of ultimate bearing capacity, q_u. If the values of q_u do not compare well, use the third method.

d. Bearing Capacity from Field Tests

Two common field tests used for the determination of the bearing capacity of soils are the Standard Penetration Test (SPT) and Cone Penetration Test (CPT). Certain correlations in these tests with the angle of internal friction of the soil, ϕ can be used to determine the bearing of the soil.

The Allowable Settlement of the Soil

The allowable settlement of the soil can be determined from the consolidation (Oedometer) tests in the laboratory and also through the use of some field tests. See Estimation of Foundation Settlement Using SPT Data.

Outline for Geotechnical Test Report

A geotechnical report would normally follow the format outlined below:

Introduction

The introduction gives an overview of the project being carried out, the purpose of the project and the required tests to be carried out for the project.

Sample Preparation

Sample preparation gives detailed information about how the samples for each type of test were carried out, stored, and prepared for the tests. Information in this section gives some confidence in the reliability of the results obtained from the test. There are usually two types of samples:

a. Disturbed Samples: These types of samples are disturbed but representative, and may be used for the following types of laboratory soil tests: Grain size analysis, Determination of liquid and plastic limits, Specific gravity of soil solids, Determination of organic content, and Classification of soil.

The major equipment used to obtain disturbed samples is a split Spoon) which is a steel tube that has an inner diameter of 34.93 mm and an outer diameter of 50.8 mm.

b. Undisturbed Samples: These types of samples are used for the following types of laboratory soil tests: Consolidation test, Hydraulic Conductivity test, Shear Strength tests etc. These samples are more complex and expensive, and it’s suitable for clay, however in sand is very difficult to obtain undisturbed samples. The major equipment used to obtain undisturbed samples is a thin-walled Tube). In a situation where it is impossible to obtain undisturbed soil samples or where there is a need to compare laboratory test results with field test results for bearing capacity tests, field tests such as standard penetration tests, cone penetration tests, etc. may be employed.

Test Conducted

When outlining the tests conducted, the following should be included:

– Purpose of test

– Apparatus used for the test

– Procedure of the test

– Result preparation format

– Results obtained

Conclusion and Recommendation

In the conclusion, the results obtained are outlined and compared to standard values existing in the literature, and then statements are made about the type of soil present at the site, the allowable bearing capacity value obtained, and the recommended foundation type for the structure.

Appendix

Appendix gives information about the tests conducted which are not necessary in the main body of the report.

References Section

There should also be a reference section that points to the materials or sources consulted in the course of preparation of the report.

References

Al-Agha, A.S. (2015). Basics of Foundation Engineering with Solved Problems

Holtz, R.D. and Kovacs, W.D. An Introduction to Geotechnical Engineering. Prentice Hill, Englewood Cliffs, New Jersey, USA.

Murthy, V.N.S. Geotechnical Engineering (Principles and Practices of Soil Mechanics and Foundation Engineering), Marcel Dekker Inc., New York