Question

What are the subjects (field and laboratory data) that would go as enclosure with the geotechnical report for assisting design purposes?

Answer 1

Sol:-

The geotechnical report is the tool used to communicate the site conditions and design and construction recommendations to the roadway design, bridge design, and construction personnel. Site investigations for transportation projects have the objective of providing specific information on subsurface soil, rock, and water conditions. Interpretation of the site investigation information, by a geotechnical engineer, results in design and construction recommendations that should be presented in a project geotechnical report. The importance of preparing an adequate geotechnical report cannot be overstressed. The information contained in this report is referred to often during the design period, construction period, and frequently after completion of the project (resolving claims). Therefore, the report should be as clear, concise, and accurate. Both an adequate site investigation and a comprehensive geotechnical report are necessary to construct a safe, cost-effective project. Engineers need these reports to conduct an adequate review of geotechnical related features, e.g., earthwork and foundations.

The State or their consultant should prepare “Preliminary” geotechnical reports for submittal to the design team whenever this information will benefit the design process. Early submittal of geotechnical information and recommendations or engineering evaluation of preliminary data may be necessary to establish basic design concepts or design criteria. This is commonly the case on large projects or projects containing complex or difficult geotechnical problems where alignment and/or grade changes may be appropriate based on geotechnical recommendations. The development of a “Final” geotechnical report will not normally be completed until design has progressed to the point where specific recommendations can be made for all of the geotechnical aspects of the work. Final alignment, grade, and geometry will usually have been selected prior to issuance of the final geotechnical report.

While the geotechnical report content and format will vary by project size and highway agency, all geotechnical reports should contain certain basic essential information, including:

• Summary of all subsurface exploration data, including subsurface soil profile, exploration logs, laboratory or in situ test results, and ground water information;

• Interpretation and analysis of the subsurface data;

• Specific engineering recommendations for design;

• Discussion of conditions for solution of anticipated problems; and

• Recommended geotechnical special provisions.

• Data Collection
• Geotechnical Investigation Programme
• Field Investigation and Sampling
• Laboratory Testing
• Geotechnical Investigation Report

The services for the geotechnical investigation will be provided and implemented with strict adherence to international standards and guidelines in order guarantee and assure the highest level of quality for your project.

DATA COLLECTION INCLUDES :-

• Topography maps
• Geology maps and evaluation of regional geology conditions such as earthquake, faults, formation of soils
• Climate and weather data

GEOTECHNICAL INVESTIGATION PROGRAMME

• List of technical standards to be applied (geotechnical site investigation, drilling methods in soil / rock, sampling of soil / rock / groundwater, field testing, geotechnical laboratory testing of soil / rock / groundwater characteristics)
• Number, mapping / location, depth of boreholes
• Soil and rock drilling & sampling methods (Soil Investigations)
• Number of standard penetration tests (SPT)
• Number of field vane shear tests
• Number of disturbed and undisturbed soil samples
• Number of undisturbed rock samples
• Ground water sampling
• Geotechnical laboratory testing programme

FIELD INVESTIGATION AND SAMPLING

Preparation of Field Investigation Works

• Work planning on manpower and equipment with respect to the Programme
• Mobilization and De-mobilization of geotechnical drilling team, supervision personnel and equipment
• Survey and Set-out of borehole coordinates
• Proper Handling, Warehousing and Transport of Samples to the Geotechnical Laboratory

Drilling Works

Drilling works will be carried out by using drilling rigs having the capacity to drill in the sizes and to the termination requirements or depths instructed. We can provide rotary drilling methods into soil and rock strata applying different types of flushing media (depending on the existing / encountered soil conditions). In the particular case of encountering rock layers the appropriate boring / coring method should base on single tube core barrel or double tube core barrel being capable of recovering rock cores.

Soil Sampling & Soil Investigations

The undisturbed samples will be taken at certain intervals to the borehole depth termination and at the change of strata in borehole under application of various sampling methods and respective technical standards. Disturbed samples can be derived from selected recovered cores of standard penetration tests (SPT). For detailed guidance on the suitability of various soil sampling techniques in different types of soil, our geotechnical expert will provide professional advice. Sampling, handling, labeling and transport of samples will be executed in strict adherence with international standards and guidelines.

Undisturbed sampling using the open-tube sample as thin-walled or thick-walled tube samplers can be applied depending on the encountered soil conditions whereas the thick-walled tube sampler is applied for stiff and dense soils and for soils containing coarse particles. For soil types that are difficult to sample, sample-retaining or closure devices are necessary.

Undisturbed sampling using the piston sampler can be applied in low-strength fine soils such as silt and clay, including sensitive clays. It can be used either in boreholes or be pushed directly into the soil.

Rotary Core Drilling and Rock Sampling

In sampling by rotary core drilling, a tube system fitted with a bit at its lower end is rotated and fed into the rock mass by the drill rig via the drill string. This action produces a core sample within the tube system. A flushing medium is normally used. The sampling tool, i.e. the core barrel, can be a single tube, double tube or triple tube with a borehole diameter of 70 mm to 200 mm. The rock coring should be executed minimum 5m into the rock.

For detailed guidance on the suitability of various core drilling and sampling techniques in different types of soil and rock, our geotechnical expert will provide professional advice.

Samples can be obtained by this method as cores/cuttings. The single-tube core barrel only allows core recovery in consolidated formations, whereas double-tube and triple-tube core barrels can be used in all rock formations.

The quality of the rock recovery achieved is determined by applying the following parameters as rock quality designation (RQD), total core recovery (TCR) and solid core recovery (SCR) ratios that have to be recorded and reported for each core run.

Standard Penetration Test (SPT)

Standard Penetration Tests (SPT) shall be performed in boreholes in order to estimate consistency, relative density, and strength-deformation parameters of soils. In addition to this soil samples
obtained from SPT testing are used for classification purposes. SPT testing can also be carried out in weak weathered rock. The SPT testing shall be performed at certain numbers and respective intervals (it is common to apply an interval of 1.5m/test) to the borehole depth termination for all boreholes. The number of blows required to 15cm of penetration or fraction thereof is to be recorded. The first 15cm is to be considered as a seating drive. The number of blows required for the second and the third 15cm penetration is termed as “standard penetration resistance” or the raw “N value”.

Field Vane Shear Test (VST)

Vane Shear Test (VST) is an in-place shear test in which a rod with thin radial vanes at the end is forced into the soil and the resistance to rotation of the rod is determined. VST addresses testing on land and for testing in drill holes or by self drilling or continuous push methods from the ground surface. VST provides an indication of in-situ undrained shear strength of fine- grained clays and silts or other fine geomaterials such as mine tailings, organic muck, and substances where undrained strength determination is required. Knowledge of the nature of the soil in which each vane test is to be made is necessary for assessment of the applicability and interpretation of the test. The test is not applicable for sandy soils which may allow drainage during the test. The test is routinely performed in conjunction with other field and laboratory tests.

Cone Penetration Test (CPT)

Cone penetration test (CPT) can be performed to evaluate effectively the site characterization. It is a valuable method of assessing the subsurface stratigraphy associated with soft materials, discontinuous lenses, organic soil, potentially liquefiable materials (silt, sands and granule gravel), generally excluding bedrock, very dense granular fill and strata containing cobbles and boulders. CPT is performed using a cylindrical penetrometer with a conical tip (cone) penetrating the ground at a constant rate. During the penetration, the forces on the cone and the friction sleeve are measured. CPT and the measured data can be used to evaluate soil conditions and parameters as follows:

• Determination of soil strata variations and characterization of soil types
• Evaluation of ground engineering properties (undrained shear strength, relative density, equivalent standard penetrations test values) may be derived from the measured data as mentioned above
• Piezo-cones (CPTu) can be used to measure the pore water pressure and to assess hydrostatic head, consolidation and permeability characteristics
• Data may be used to analysis the pile load-bearing capacity, foundation bearing capacity, foundation settlement, and liquefaction potential.
• Cone penetration test (CPT) provides a continuous (although indirect) record of ground conditions, avoiding the ground disturbance associated with boring and sampling.
• Remark: Interpretation of CPT results can be enhanced by the use of site specific correlations. Such correlations are obtained by performing laboratory tests on soil samples obtained from the location of the CPT tests.

GEOTECHNICAL LABORATORY TESTING WITHIN GEOTECHNICAL INVESTIGATION

Index property tests (determining of soil classification)

• Moisture content - determination of the moisture content of a soil as a percentage of its oven-dried weight.
• Unit weight - determining the total/moist and dry densities - unit weights - of soil specimens.
• Specific gravity - determination of the specific gravity of soil solids passing the 4.75-mm sieve (No. 4) by means of a water pycnometer.
• Attenberg limits - determination of the liquid limit, plastic limit, and the plasticity index of soils.
• Particle size distribution - quantitative determination of the distribution of particle sizes in soils, distribution of particle sizes larger than 75 μm - retained on the No. 200 sieve - is determined by sieving, while the distribution of particle sizes smaller than 75 μm is determined by a sedimentation process, using a hydrometer.

Engineering properties tests (determining of strength and deformation parameters)

• Unconfined compression strength - determination of the unconfined compressive strength of cohesive soil of the axial load.
• Consolidation test (one-dimensional consolidation properties) - determining the magnitude and rate of consolidation of soil when it is restrained laterally and drained axially while subjected to incrementally applied controlled-stress loading. Data from the consolidation test are used to estimate the magnitude and rate of both differential and total settlement of a structure or earthfill. Estimates of this type are of key importance in the design of engineered structures and the evaluation of their performance.
• Consolidated undrained triaxial compression test - determination of strength and stress-strain relationships of cohesive soil when the specimens are isotropically consolidated and sheared in compression without drainage at a constant rate of axial deformation. Data from the undrained triaxial compression test are used in geotechnical engineering analysis to predict how the material will behave in a larger-scale geotechnical engineering applications. An example would be to predict the stability of the soil on a slope, whether the slope will collapse or whether the soil will support the shear stresses of the slope and remain in place. Undrained triaxial compression tests are used along with other tests to make such engineering predictions.

Rock Strength Properties Tests

• Point load test - determining the point load strength index of rock. This is an index test and is intended to be used to classify rock strength.
• Uniaxial compression strength test - determining unconfined compressive strength of intact rock core specimens. It is a test method for compressive strength and elastic moduli of intact rock core specimens under varying states of stress and temperatures. Unconfined compressive strength of rock is used in many design formulas and is sometimes used as an index property to select the appropriate excavation technique.

Chemical Analysis of Groundwater

• pH value, sulphate content and chloride content tests – determining the aggressiveness of groundwater to concrete and steel structures.

GEOTECHNICAL INVESTIGATION REPORT

The detailed Report will be provided and prepared tailor-made for each project according to the respective project scope of service. The following table of content is a standard table of content that will be adjusted according to the project scope of service:

Executive Summary

• Project description
• Description of the works performed with respect to the geotechnical investigation programme
• Technical standards
• Site conditions

Geotechnical Field Investigation

• Drilling of boreholes
• Soil sampling
• Rock coring and rock sampling
• Standard penetration tests
• Field vane shear tests
• Cone penetration tests
• Final quantity report on field investigation works

Geotechnical Laboratory Testing

• Index properties tests (moisture content tests, unit weight tests, specific gravity tests, atterberg limits tests, particle size distribution tests)
• Engineering properties tests (unconfined compression strength tests, consolidation / oedometer tests, consolidated undrained triaxial compression tests)
• Rock strength properties tests (point load tests, uniaxial compression strength tests)
• Chemical analysis of ground water
• Final quantity report on laboratory tests

Evaluation of Soil Conditions and Soil Parameters

• Soil profile
• Deformation properties of soft clay layers (pre-consolidation pressure and over consolidated ratio, compression, coefficient of primary consolidation index, cecondary consolidation)
• Strength parameters (undrained shear strength, strength parameters from CIU tests, SPT-N values)

Groundwater

• Ground water level measurement
• Results on chemical ground water analysis and evaluation of level of aggressiveness to structures

Conclusions and Recommendation

• Geotechnical soil conditions of the project site
• Recommendation for building foundation methods, soil improvement methods … (acc. to the project scope)

Appendices

• Summary of laboratory testing results on index and engineering properties
• Bore logs and standard penetration test results
• Vane shear test results
• Results of laboratory testing
• Work progress reports and photo documentation
• Daily site record
• Drawings

LIST OF STANDARDS FOR GEOTECHNICAL INVESTIGATION

Geotechnical Planning & Geotechnical Investigation Programme

• Code of Practice for Site Investigation | BS 5930:1999 |
• Vietnamese Code of Practice for Exploratory Drilling in Engineering Geology | 22TCN 259-2000 |
• Vietnamese Standard of Geotechnical Investigation for Marine Structure | 22TCN 260-2000 |

Geotechnical Field Investigation Works

• Standard Guide for Use of Direct Rotary Drilling with Water-Based Drilling Fluid for Geoenvironmental Exploration and the Installation of Subsurface Water-Quality Monitoring Devices | ASTM D5783 |
• Thin-Walled Tube Sampling of Soils for Geotechnical Purposes | ASTM D1587 |
• Standard Practice for Rock Core Drilling and Sampling of Rock for Site Investigation | ASTM D2113-99 |
• Test Method for Penetration Test and Split-Barrel Sampling of Soils | ASTM D1586 | Standard Penetration Test
• Standard Test Method for Electronic Friction Cone and Piezocone Penetration Testing of Soil | ASTM D5778 | Cone Penetration Test
• Standard Test Method for Field Vane Shear Test in Saturated Fine-Grained Soils | ASTM D2573 | Field Vane Shear Test
• Standard Practice for Description and Identification of Soils | ASTM D2488 |

Geotechnical Laboratory Testing

• Standard Practice for Classification of Soils for Engineering Purpose | ASTM D2487 |
• Standard Practice for Dry Preparation of Soil Samples for Particle-Size Analysis and Determination of Soil Constants | ASTM D421 |
• Standard Practice for Particle-Size Analysis of Soils | ASTM D422 | Sieve / Hydrometer Analysis
• Standard Test Method for Liquid Limit, Plastic Limit, and Plasticity Index of Soils | ASTM D4318 | Attenberg Limits
• Standard Test Method for Specific Gravity of Soil Solids by Water Pycnometer | ASTM D854 | Specific Gravity
• Standard Test Methods for Laboratory Determination of Density (Unit Weight) of Soil Specimens | ASTM D7263 | Bulk Density
• Standard Test Method for Laboratory Determination of Water Content of Soil and Rock by Mass | ASTM D2216 | Natural Moisture Content
• Standard Test Methods for One-Dimensional Consolidation Properties of Soils Using Incremental Loading | ASTM D2435 | Oedometer Test
• Standard Test Method for Unconfined Compressive Strength of Cohesive Soils | ASTM D2166 |
• Standard Test Method for Consolidated Undrained Triaxial Compression Test for Cohesive Soils | ASTM D4767 |
• Standard Test Method for Determination of the Point Load Strength Index of Rock and Application to Rock Strength Classifications | ASTM D5731 |
• Standard Test Method for Unconfined Compressive Strength of Intack Rock Core Specimens | ASTM D2938 |
• Standard Test Method for Underground Water Chemical Test – pH Value | ASTM D4972 |
• Standard Test Method for Underground Water Chemical Test – Chloride Content | ASTM D512 |
• Standard Test Method for Underground Water Chemical Test – Sulphate Content | ASTM D516 |