2.4 Design investigations

2.4.1 Field investigations General

(1)P In cases where the preliminary investigations do not provide the necessary information to assess the aspects mentioned in 2.3, complementary investigations shall be performed during the design investigation phase.

(2) If relevant, field investigations in the design phase should comprise:

  • drilling and/or excavations (test pits including shafts and headings) for sampling;
  • ground water measurements;
  • field tests.

(3) Examples of the various types of field investigations are:

  • field testing (e.g. CPT, SPT, dynamic probing, WST, pressuremeter tests, dilatometer tests, plate load tests, field vane tests and permeability tests);
  • soil and rock sampling for description of the soil or rock and laboratory tests;
  • groundwater measurements to determine the groundwater table or the pore pressure profile and their fluctuations;
  • geophysical investigations (e.g. seismic profiling, ground penetrating radar, resistivity measurements and down hole logging);
  • large scale tests, for example to determine the bearing capacity or the behaviour directly on prototype elements, such as anchors.

(4) To develop strategies for planning field investigations, Table 2.1 can be used as a guide to the applicability of the field investigations covered in Sections 3 and 4.

NOTE See also B.2.

(5)P Where ground contamination or soil gas is expected, information shall be gathered from the relevant sources. This information shall be taken into account when planning the ground investigation.

Table 2.1 — Simplified overview of the applicability of field investigation methods a) covered by Sections 3 and 4
Field investigation methods a) Possibly obtainable results
Sampling Field tests Groun­dwater measu­rements
Soil Rock
Basic information                                      
Type of soil C1 F1 C1 F1 C2 F2 C2 F2 C3 F3 C3 F3 C2 F1 C3 F3 C3 F3 C2 F2
Type of rock R1 R1 R2 R3c) R3 R2
Extension of layers b) C1 F1 C1 F1 C3 F3 R1 R1 R2 C1 F1 R3 C3 F3 R3 C3 F3 C2 F2 C1 F2 C1 F2 F2 C2 F1
Groundwater level C2 R2 C1 F2 R1 C1 F1
Pore water pressure C2 F2   R2 C1 F2 R1 C1 F1
Geotechnical properties          
Particle size C1 F1 C1 F1 R1 R1 R2 C2 F1
Water content C1 F1 C2 F1 C3 F3 R1 R1 C2 F2
Atterberg limits F1 F1 F2
Density C2 F1 C3 F3 R1 R1 C2 F2 C2 F2 C2 C2     C2 F2
Shear strength C2 F1 R1 C2 F1 C1 F1 C2 F3 C2 F3 C2 F3 C2 F1 C2 F1 R2 C1 F1
Compressibility C2 F1 R1 C1 F2 C1 F1 R1 F1 C2 F2 C2 F2 C2 F2 C2 C2 F1 C1 F1
Permeability C2 F1 R1 C3 F2 F3   C2 F3 C2 F2
Chemical tests C1 F1 C1 F1 R1 R1 C2 F2
a) see sections 3 and 4 for nomenclature
b) in horizontal and vertical direction
c) will depend on pressuremeter type
d)assuming sample is retained
e) soft rock only

R1 High for rock                      R2 Medium for rock                        R3 Low for rock
C1 High for coarse soil*)        C2 Medium for coarse soil            C3 Low for course soil
F1 High for fine soil*)              F2 Medium for fine soil                   F3 Low for fine soil
— not applicable

*)main soil groups "coarse" and "fine" according to ISO 14688-1
NOTE Depending on the ground conditions (such as soil type, groundwater conditions) and the planned design, the selection of investigation methods will vary and may deviate from (his table.

(6)P If ground contamination or soil gas is detected in the course of ground investigations, this shall be reported to the client and the responsible authorities. Field investigation programme

(1)P The field investigation programme shall contain:

  • a plan with the locations of the investigation points including the types of investigation;
  • the depth of the investigations;
  • the types of sample (category, etc.) to be taken including specifications for the number and depth at which they are to be taken;
  • specifications on the groundwater measurement;
  • the types of equipment to be used;
  • the standards to be applied. Locations and depths of the investigation points

(1)P The locations of investigation points and the depths of the investigations shall be selected on the basis of the preliminary investigations as a function of the geological conditions, the dimensions of the structure and the engineering problems involved.

(2) When selecting the locations of investigation points, the following should be observed:

  • the investigation points should be arranged in such a pattern that the stratification can be assessed across the site;
  • the investigation points for a building or structure should be placed at critical points relative to the shape, structural behaviour and expected load distribution (e.g. at the corners of the foundation area);
  • for linear structures, investigation points should be arranged at adequate offsets to the centre line, depending on the overall width of the structure, such as an embankment footprint or a cutting;
  • for structures on or near slopes and steps in the terrain (including excavations), investigation points should also be arranged outside the project area, these being located so that the stability of the slope or cut can be assessed. Where anchorages are installed, due consideration should be given to the likely stresses in their load transfer zone;
  • the investigation points should be arranged so that they do not present a hazard to the structure, the construction work, or the surroundings (e.g. as a result of the changes they may cause to the ground and groundwater conditions);
  • the area considered in the design investigations should extend into the neighbouring area to a distance where no harmful influence on the neighbouring area is expected.
  • for groundwater measuring points, the possibility of using the equipment installed during the ground investigation for continued monitoring during and after the construction period should be considered.

(3) Where ground conditions are relatively uniform or the ground is known to have sufficient strength and stiffness properties, wider spacing or fewer investigation points may be applied. In either case, this choice should be justified by local experience.

(4)P In cases where more than one type of investigation is planned at a certain location (e.g. CPT and piston sampling), the investigation points shall be separated by an appropriate distance.

(5) In the case of a combination of, for example, CPTs and boreholes, the CPTs should be carried out prior to the boreholes. The minimum spacing should then be such that the borehole does not or is considered unlikely to encounter the CPT hole. If the drilling is conducted first, the CPT should be carried out at a horizontal separation of at least 2 m.

(6)P The depth of investigations shall be extended to all strata that will affect the project or are affected by the construction. For dams, weirs and excavations below groundwater level, and where dewatering work is involved, the depth of investigation shall also be selected as a function of the hydrogeological conditions. Slopes and steps in the terrain shall be explored to depths below any potential slip surface.

NOTE For the spacing of investigation points and investigation depths, the values given in B.3 can be used as guidance.

Eurocode 7: Geotechnical design — Part 2: Ground investigation and testing