Planning of geotechnical investigations
B.1 Stages of ground investigations in geotechnical design, execution of works and exploitation of the structure
B.2 Selection of ground investigation methods in different stages
|Preliminary investigation||Design investigation||Control investigations|
|Desk study of topographical, historical, geological and hydrogeological maps
Archives of previous construction works and investigations
Preliminary geophysical surveys
Preliminary intrusive investigations
CPT, SS, DP, SE
FVT or SPT
OS TP, PS, OS
|Preliminary choice of foundation method
Preliminary choice of foundation method Preliminary choice of foundation method
|SS, CPT, DP, SR
FVT, SPT, PIL
PS, OS, CS, PMT
|Verification of choice of foundation method and design procedure, control of ground improvement works and stability during construction||PIL, Pile driving tests,
Stress wave measurements
GWC, settlements. Inclinometers
|SS or CPT, DP
FVT, DMT or PMT, BJT
PS, OS, CS, TP
|Check of the soil type
Check of the stiffness
(CPT) Settlements, Inclinometers,
GWC Volume change potential due to water content change
SS, CPT, DP, SR
AS, OS, TP
|CPT, DP, SR
SPT, DMT, PIL
|PIL, Pile driving tests
Stress wave measurements GWC, settlements Inclinometers
SR, CPT, MWD
CS, AS, TP
SPT, PMT, BJT, DMT, PLT
|Check of the soil type
Check of the stiffness (CPT, DP. SPT)
|Pile or shallow foundation||SR, MWD, mapping of discontinuities
RDT, PMT, BJT
|Check inclination and discontinuities in the rock and its surface
Check contact between pile toe/ foundation and rock surface
Verify wafer conditions of How and pressure
BJT Borehole jack test
DP Dynamic probing
SR Soil/rock sounding
SS Static sounding (e,g, weight sounding test, WST)
CPT(U) Cone penetration test (with pore pressure recording)
SPT Standard penetration test
PMT Pressuremeter test
DMT Dilatometer test
FVT Field vane test
PLT Plate load test
MWD Measuring while drilling
SE Seismic measurements
PIL Pile load test.
ROT Rock dilatometer test
PS Piston sampler
CS Core sampler
AS Auger sampler
OS Open sampler
TP Test pit sampling
GW Groundwater measurements
GWO Groundwater measurements with open system
GWC Groundwater measurements with closed system
Soils include naturally deposited and anthropogenic deposits
Surveying and logging are not included in this chart
Laboratory tests are not presented on this table
B.3 examples of recommendations for the spacing and depth of investigations
(1) The following spacing of investigation points should be used as guidance:
- for high-rise and industrial structures, a grid pattern with points at 15 m to 40 m distance;
- for large-area structures, a grid pattern with points at not more than 60 m distance;
- for linear structures (roads, railways, channels, pipelines, dikes, tunnels, retaining walls), a spacing of 20 m to 200 m;
- for special structures (e.g. bridges, stacks, machinery foundations), two to six investigation
- points per foundation;
- for dams and weirs, 25 m to 75 m distance, along relevant sections.
(2) For the investigation depth za the following values should be used as guidance. (The reference level for za is the lowest point of the foundation of the structure or structural element, or the excavation base.) Where more than one alternative is specified for establishing za the one which yields the largest value should be applied.
NOTE For very large or highly complex projects, some of the investigation points generally extend to greater depths than those specified under B.3 (5) to B.3 (13).
(3) Greater investigation depths should always be selected, where unfavourable geological conditions, such as weak or compressible strata below strata of higher bearing capacity, are presumed.
(4) Where structures under B.3 (5) to B.3 (8) and B.3 (13) are built on competent strata, the depth of investigation can be reduced to za = 2 m, unless the geology is indistinct, in which case at least one borehole should be taken down to a minimum of za = 5 m. If a bedrock formation is encountered at the proposed base of the structure, this should be taken as the reference level for za. Otherwise, za refers to the surface of the bedrock formation.
(5) For high-rise structures and civil engineering projects, the larger value of the following conditions should be applied (see Figure B.1 a)):
– za ≥ 6 m;
– za ≥ 3,0bF.
where bF is the smaller side length of the foundation.
(6) For raft foundations and structures with several foundation elements whose effects in deeper strata are superimposed on each other:
za > 1,5·bB
where bB is the smaller side of the structure, (see Fig. B.1 b)).
|a) foundation||b) structure|
(7) Embankments and curlings, the larger value of the following conditions should be met (see Figure B.2):
|a) embankment||b) cutting|
a) For dams;
– 0,8h < za < 1,2h
– za ≥ 6 m
where h is the embankment height.
b) For cuttings:
– za > 2,0 m
– za ≥ 0,4h
where h is the dam height or depth of cutting.
(8) Linear structures, the larger value of the following conditions should be met (see Figure B.3):
|a) road||b) trench|
a) For roads and airfields:
za ≥ 2 m below the proposed formation level.
b) For trenches and pipelines, the larger value of:
– za > 2 m below the invert level;
– za ≥ 1,5bAh
where bAh is the width of excavation.
c) Where relevant, the recommendations for embankments and cuttings should be followed.
(9) For small tunnels and caverns, (see Figure B.4):
bAb < za < 2,0bAb
where bAb is the width of excavation.
The groundwater conditions described in (10) b) should also be taken into account.
(10) Excavations (see Figure B.5).
a) Where the piezometric surface and the groundwater tables are below the excavation base, the larger value of the following conditions should be met:
– za ≥ 0,4h
– za > (t + 2,0) m
t is the embedded length of the support; and
h is the excavation depth.
b) Where the piezometric surface and the groundwater tables are above the excavation base, the larger value of the following conditions should be met:
– za ≥ (1,0H + 2,0)m
– za ≥ (t + 2,0)m
H is the height of the groundwater level above the excavation base; and
t is the embedded length of the support.
If no stratum of low permeability is encountered down to these depths:
za ≥ t + 5 m.
(11) For water-retaining structures, za should be specified as a function of the proposed level of impounded water, the hydrogeological conditions and the construction method.
(12) For cut-off walls (see Figure B.6):
– za ≥ 2 m below the surface of the stratum impermeable to groundwater.
(13) For piles (see Figure B.7), the following three conditions should be met:
– za ≥ 1,0bg
– za ≥ 5,0 m
– za ≥ 3DF
DF is the pile base diameter; and
bg is the smaller side of the rectangle circumscribing the group of piles forming the foundation at the level of the pile base.