Soil Exploration for Geotechnical Design According to Eurocode Requirements

Meeting Eurocode requirements can be challenging at first, here we summarized the essentials of the requirements of site investigation. In geotechnical design, if preliminary investigations do not yield sufficient information for proper assessment, it is mandatory to conduct additional investigations during the design phase. Field investigations in this phase should include drilling, excavations, groundwater measurements, and field tests. Examples of field investigation types encompass field testing methods like Cone Penetration Tests (CPT), Standard Penetration Tests (SPT), and dynamic probing; soil and rock sampling for description and laboratory testing; groundwater measurements to determine the groundwater table and pore pressure profiles; geophysical investigations such as seismic profiling and ground-penetrating radar; and large-scale tests to determine the bearing capacity or behavior of prototype elements. Table 2.1 provides a simplified overview of the applicability of various field investigation methods.

Information regarding potential ground contamination or soil gas should be collected from relevant sources and factored into the planning of the ground investigation. If ground contamination or soil gas is discovered during the investigation, it must be reported to the client and the relevant authorities.

Field Investigation Programme

The field investigation program must include a comprehensive plan detailing the location of investigation points, types of investigations to be conducted, depth of investigations, types of samples to be collected, specifications for groundwater measurements, types of equipment to be used, and the standards to be applied.

Locations and Depths of Investigation Points

The selection of investigation point locations and depths should be based on preliminary investigations, taking into account geological conditions, the dimensions of the structure, and engineering challenges. When determining the locations of investigation points, several factors should be considered. The arrangement should facilitate the assessment of stratification across the site. For buildings or structures, points should be placed at critical locations relative to the shape, structural behavior, and load distribution. For linear structures, points should have adequate offsets from the centerline. Structures near slopes or steps in terrain should have points placed outside the project area to assess slope stability. For anchorage locations, potential stresses in the load transfer zone should be considered. Additionally, points should be positioned to avoid posing hazards to the structure, construction process, or surroundings. The investigated area should extend into neighboring areas to ensure that no harmful influence is expected. For groundwater measuring points, the possibility of continued monitoring during and after construction should be considered.

In cases where ground conditions are relatively uniform or have known sufficient strength and stiffness properties, wider spacing or fewer investigation points may be acceptable if justified by local experience. When multiple investigation types are planned at a location, investigation points must be separated by an appropriate distance. For instance, CPTs and boreholes should be conducted with sufficient spacing to avoid interference; if CPTs are performed before boreholes, they should be spaced adequately to prevent encountering the borehole. If drilling is done first, CPTs should be at least two meters horizontally separated from the boreholes.

The depth of investigations should extend to all strata that will affect the project or be affected by construction activities. For dams, weirs, excavations below groundwater level, and dewatering projects, the depth should also consider hydrogeological conditions. Slopes and steps in terrain must be explored to depths below any potential slip surface.

Eurocode provides examples of what the spacing of investigation points should be. Recommendations for the spacing and depth of geotechnical investigation points vary based on the type of structure. For high-rise and industrial buildings, a grid pattern with points spaced 15 to 40 meters apart is suggested. Large-area structures should use a grid with points no more than 60 meters apart. For linear structures like roads, railways, channels, pipelines, dikes, tunnels, and retaining walls, investigation points should be spaced every 20 to 200 meters. Special structures such as bridges, stacks, and machinery foundations require two to six investigation points per foundation. For dams and weirs, points should be placed along relevant sections at intervals ranging from 10 to 75 meters.

the depth of boring can be estimated from the following table

Structure Type	Recommended Investigation Depth (Za)	Notes/Variables
High-rise structures and civil engineering projects	Za ≥ 6 m	Use the larger value
Za ≥ 3 bF	bF = smaller side length of the foundation
Raft foundations and structures with multiple foundation elements	Za ≥ 1.5 bB	bB = smaller side of the structure
Embankments	Za ≥ 6 m	Use the larger value
Za ≥ 0.8 h	h = embankment height
Cuttings	Za ≥ 2.0 m	Use the larger value
Za ≥ 0.4 h	h = depth of cutting
Roads and airfields	Za ≥ 2 m below the proposed formation level
Trenches and pipelines	Za ≥ 2 m below invert level	Use the larger value
Za ≥ 1.5 bAh	bAh = width of excavation
Small tunnels and caverns (groundwater below excavation base)	Za ≥ 0.4 h	Use the larger value
Za ≥ t + 2.0 m	h = excavation depth
	t = embedded length of support
Small tunnels and caverns (groundwater above excavation base)	Za ≥ 1.0 H + 2.0 m	Use the larger value
Za ≥ t + 2.0 m	H = height of groundwater above excavation base
	t = embedded length of support
If no low-permeability stratum is encountered	Za ≥ t + 5 m	Applies to tunnels and caverns
Cut-off walls	Za ≥ 2 m below the surface of the impermeable stratum
Piles	Must satisfy all:	bg = smaller side of the rectangle circumscribing the pile group at pile base level
Za ≥ 1.0 bg	Df = pile base diameter
Za ≥ 5.0 m
Za ≥ 3 Df

Notes:

• Za refers to the investigation depth, measured from the lowest point of the foundation or excavation base.

• When multiple conditions are provided, the largest value should be used for Za.

• For structures built on competent strata, Za can be reduced to 2 m, unless the geology is unclear—in which case, at least one borehole should reach a minimum depth of 5 m.

• If bedrock is encountered at the proposed base level, it serves as the reference level for Za; otherwise, Za is measured from the surface of the bedrock formation.

• Greater investigation depths should be selected if unfavorable geological conditions are suspected, such as weak or compressible layers beneath stronger strata.

Sampling

The categories of sampling and the number of samples to be taken depend on the aim of the investigation, the site's geology, and the complexity of the geotechnical structure. At a minimum, one borehole or trial pit with sampling is required for ground identification and classification. Samples must be obtained from each ground layer that influences the behavior of the structure. Field tests may replace sampling if there is sufficient local experience to correlate them with ground conditions and ensure unambiguous interpretation. Section 3 provides further details on sampling procedures.

Groundwater measurements

Determining the groundwater table or pore water pressures in soils and rocks requires the installation of open or closed groundwater measuring systems. There are two main methods for measuring groundwater pressure:

1. Open Systems: These involve measuring the piezometric groundwater head using an observation well equipped with an open pipe. Open systems are best suited for soils and rocks with relatively high permeability, such as sands, gravels, or highly fissured rocks (aquifers and aquitards). However, in low-permeability soils and rocks, open systems may lead to inaccurate interpretations due to the time lag in filling and emptying the pressure pipe. This time lag can be reduced by using filter tips connected to small-diameter hoses.

2. Closed Systems: These systems measure groundwater pressure directly at a selected point using pressure transducers. Closed systems can be used in all types of soils and rocks but are especially recommended for very low-permeability materials (aquicludes) like clays or low-fissured rocks. They are also advisable when dealing with high Artesian water pressures.

For monitoring very short-term variations or rapid pore water fluctuations, continuous recording with transducers and data loggers should be employed, regardless of the soil or rock type.

When interpreting groundwater measurements, it is important to consider any open water bodies within or near the investigation area. Water levels in nearby wells, the presence of springs, and occurrences of artesian water should also be noted.

The number, location, and depth of measuring stations should be determined based on the purpose of the measurements, topography, stratigraphy, and soil conditions—particularly the permeability of the ground and identified aquifers. For projects involving monitoring—such as groundwater lowering, excavations, fillings, and tunnels—the placement of measuring stations should align with the expected changes to be observed.

To obtain measurements that accurately reflect pore pressure at a specific point within a soil or rock layer, measures must be taken (in accordance with EN ISO 22475-1) to ensure that the measuring point is adequately sealed off from other layers or aquifers.

Planning the number and frequency of readings, as well as the duration of the measuring period, should consider the purpose of the measurements and the time required for stabilization. After an initial period, these criteria may need adjustment based on the actual variations observed in the readings.

When assessing groundwater fluctuations, measurements should be taken at intervals shorter than the natural fluctuations being characterized and over an extended period.

During drilling operations, observing and recording the water level at the end of each day and before resuming drilling the following day can provide valuable insights into groundwater conditions. Any sudden inflow or loss of water during drilling should also be documented, as this information can be useful.

In the initial phases of site investigations, some boreholes might be equipped with open perforated pipes protected with filters. Water level readings from these boreholes in the days that follow offer preliminary indications of groundwater conditions. However, these readings are subject to limitations previously mentioned regarding open systems in low-permeability soils. It's crucial to consider the risks associated with connecting different aquifers and to comply with relevant environmental regulations.

Laboratory Tests

Before establishing a testing program, the expected site stratigraphy should be determined. Relevant strata for design purposes should be selected to specify the type and number of tests required for each. Stratum identification should be based on the geotechnical problem at hand, the complexity of the site, local geology, and the design parameters needed.

Visual Inspection and Preliminary Ground Profile

Samples and trial pits should undergo visual inspection and be compared with drilling logs to establish a preliminary ground profile. Simple manual tests can support the visual inspection of soil samples to identify soil type, consistency, and mechanical behavior. If significant property differences are found within a stratum, the preliminary soil profile should be further subdivided. The quality of the samples should be assessed before laboratory tests, using quality classes defined in the code.

Test Program

The laboratory test program should consider the type of construction, the type of ground and stratigraphy, and the geotechnical parameters needed for design calculations. The programme also depends on the existence of comparable experience. The extent and quality of comparable experience with the specific soil or rock should be determined, and any available field observations on neighboring structures should be utilized. Tests must be conducted on specimens representative of the relevant strata, and classification tests should be used to verify their representativeness.

An iterative process is used to check representativeness. Initially, classification and index tests are performed on many samples to determine the range of index properties of a stratum. Then, the classification and strength index test results of samples used for strength and compressibility tests are compared with all results from the stratum. The need for more advanced testing or additional site investigation should be considered based on geotechnical aspects, soil type, variability, and the computational model used.

Number of Tests

The number of specimens to be tested depends on ground homogeneity, the quality and amount of comparable experience with the ground, and the geotechnical category of the problem. Additional test specimens should be available to account for difficult soil conditions, damaged specimens, and other unforeseen factors. A minimum number of specimens should be investigated depending on the test type. Annexes L to W, and O and P, provide recommended minimum numbers for some test types and can be used to assess whether the investigation was sufficient.

The minimum number of tests can be reduced if the geotechnical design does not require optimization and employs conservative soil parameter values, or if comparable experience or a combination with field information is applicable.

Classification Tests

Soil and rock classification tests are performed to determine the composition and index properties of each stratum. Samples should be selected to ensure even distribution over the area and depth of relevant strata, capturing the range of index properties. The test results are used to determine if further investigation is needed. Table 2.2 presents suitable routine classification tests for ground samples with varying levels of disturbance. These routine tests are typically performed in all phases of the ground investigation.

Tests on Samples

Samples for testing should be selected to cover the range of index properties for each relevant stratum. Reconstituted specimens that mimic the composition, density, and water content of in-situ material may be tested for fill or strata composed of sand or gravel. Table 2.3 lists laboratory tests for determining geotechnical calculation parameters. Suitable routine laboratory tests for rock samples to describe the rock material include geological classification, density or bulk mass density determination, water content determination, porosity determination, uniaxial compression strength determination, and determination of Young's modulus of elasticity and Poisson's ratio, as well as the point load strength index test.

Rock core sample classification usually includes a geological description, core recovery rate, Rock Quality Designation (RQD), degrees of induration, fracture log, weathering, and fissuring. Additional tests for rocks, beyond the routine tests mentioned, may be chosen for different purposes. These include the determination of the density of grains, wave velocity determination, Brazilian tests, shear strength determination of rock and joints, slake durability tests, swelling tests, and abrasion tests. Rock mass properties, including layering and fissuring or discontinuities, may be indirectly investigated by compression and shear strength tests along joints. For weak rocks, complementary tests in the field or large-scale laboratory tests on block samples may be necessary. The Eurocde shows a table of suitable test to obtain certain soil paramters. Most mechanical properties are obtainable by the triaxial test, oedometer test, or shear box test.

Controlling and Monitoring

This section of Eurocode emphasizes the importance of checks and additional tests during construction to ensure that actual ground conditions correspond to those identified during design investigations. It also stresses the need to verify that the properties of the delivered construction materials and executed works align with those assumed or specified in the design.

Mandatory Control Measures

Checking the ground profile during excavation is crucial to confirm the accuracy of the initial site investigation. This involves visually inspecting the exposed soil layers and comparing them to the anticipated stratigraphy based on boreholes and other preliminary investigations. Inspecting the bottom of the excavation serves a similar purpose, allowing for a direct assessment of the foundation's bearing stratum. This step helps identify any unexpected variations in soil properties or the presence of unforeseen features like soft spots or buried obstructions that might impact the structure's stability.

General Control Measures

These measures are not mandatory but can be implemented based on project-specific needs and the level of risk associated with ground conditions. Monitoring groundwater levels or pore pressures and their fluctuations helps assess the impact of construction activities on groundwater conditions. This is particularly important for projects involving excavations below the water table or in areas with fluctuating groundwater levels, as changes in pore pressure can affect the stability of slopes and excavations.

Measuring the behavior of neighboring constructions, services, or civil engineering works offers valuable insights into the potential influence of construction activities on surrounding structures and infrastructure. This monitoring helps detect any unexpected movements or deformations in adjacent structures, which could indicate ground instability or excessive settlement induced by the ongoing construction.

Monitoring the behavior of the actual construction, such as measuring settlement, tilt, or strain in structural elements, helps assess the performance of the structure in response to ground conditions and applied loads. This data allows engineers to verify the design assumptions and detect any potential issues during construction, enabling timely intervention to mitigate risks.

Importance of Documentation and Decision-Making

It is essential to meticulously document the results obtained from all control measures, including both mandatory and general ones. This documentation facilitates a comprehensive evaluation of the ground conditions and construction progress. The findings from these control measures should be compared with the design requirements to identify any discrepancies or potential concerns. Based on the comparison and analysis of the collected data, informed decisions regarding the construction process can be made. This may involve adjustments to the design, implementation of additional ground improvement measures, or modifications to the construction sequence to address unforeseen ground conditions and ensure the long-term safety and performance of the structure.

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Disclaimer: The article summarizes the Eurocode chapter on site investigation with commentary from the author. It is based on the first edition of Eurocode. Always review code requirements that apply in specific regions being Eurocode or any other building codes.