Annex B

(informative)

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

Table B.1 — Example of the selection of ground investigation methods in different stages

Preliminary investigation		Design investigation				Control investigations
Desk study of topographical, historical, geological and hydrogeological maps Mineral extraction Aerial photo-interpretation Archives of previous construction works and investigations Site inspection Preliminary geophysical surveys Preliminary intrusive investigations	Fine soil CPT, SS, DP, SE FVT or SPT OS TP, PS, OS GW	Preliminary choice of foundation method Preliminary choice of foundation method Preliminary choice of foundation method	Pile foundation Shallow foundation	SS, CPT, DP, SR FVT, SPT, PIL PS, OS, CS, PMT GWC		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 GWC			Check of the soil type Check of the stiffness (CPT) Settlements, Inclinometers, GWC Volume change potential due to water content change
	Course soil SS, CPT, DP, SR SPT AS, OS, TP GW		Pile foundation Shallow foundation	CPT, DP, SR SPT, DMT, PIL OS, TP GWO			PIL, Pile driving tests Stress wave measurements GWC, settlements Inclinometers
	Rock SR, CPT, MWD PLT CS, AS, TP GW			CPT, DP SPT, PMT, BJT, DMT, PLT OS, TP GWO			Check of the soil type Check of the stiffness (CPT, DP. SPT) Settlements
			Pile or shallow foundation	SR, MWD, mapping of discontinuities RDT, PMT, BJT TP, CS GWO			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
Abbreviations Field testing 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					Sampling PS Piston sampler CS Core sampler AS Auger sampler OS Open sampler TP Test pit sampling Groundwater measurements GW Groundwater measurements GWO Groundwater measurements with open system GWC Groundwater measurements with closed system
Notes: 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 z_a the following values should be used as guidance. (The reference level for z_a 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 z_a 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 z_a = 2 m, unless the geology is indistinct, in which case at least one borehole should be taken down to a minimum of z_a = 5 m. If a bedrock formation is encountered at the proposed base of the structure, this should be taken as the reference level for z_a. Otherwise, z_a 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)):

– z_a ≥ 6 m;

– z_a ≥ 3,0b_F.

where b_F 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:

z_a > 1,5·b_B

where b_B is the smaller side of the structure, (see Fig. B.1 b)).


a) foundation		b) structure

Figure B.1 — High-rise structures, civil engineering projects

(7) Embankments and curlings, the larger value of the following conditions should be met (see Figure B.2):


a) embankment	b) cutting

Figure B.2 — Embankments and cuttings

a) For dams;

– 0,8h < z_a < 1,2h

– z_a ≥ 6 m

where h is the embankment height.

b) For cuttings:

– z_a > 2,0 m

– z_a ≥ 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

Figure B.3 — Linear structures

a) For roads and airfields:

z_a ≥ 2 m below the proposed formation level.

b) For trenches and pipelines, the larger value of:

– z_a > 2 m below the invert level;

– z_a ≥ 1,5b_Ah

where b_Ah 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):

b_Ab < z_a < 2,0b_Ab

where b_Ab is the width of excavation.

The groundwater conditions described in (10) b) should also be taken into account.

Figure B.4 — Tunnels and caverns

(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:

– z_a ≥ 0,4h

– z_a > (t + 2,0) m

where

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:

– z_a ≥ (1,0H + 2,0)m

– z_a ≥ (t + 2,0)m

where

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:

z_a ≥ t + 5 m.

Key

1 groundwater

Figure B.5 — Excavations

(11) For water-retaining structures, z_a 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):

– z_a ≥ 2 m below the surface of the stratum impermeable to groundwater.

Figure B.6 — Cut-off wall

(13) For piles (see Figure B.7), the following three conditions should be met:

– z_a ≥ 1,0b_g

– z_a ≥ 5,0 m

– z_a ≥ 3D_F

where

D_F is the pile base diameter; and

b_g is the smaller side of the rectangle circumscribing the group of piles forming the foundation at the level of the pile base.

Figure B.7 — Pile groups