Annex D

(informative)

Geotechnical and equipment influences on the dynamic probing results (p.I)

D.1 Introduction

D.1.1 General

The following factors can affect the results:

geotechnical influences due to the dependence of the penetration resistance on the shear strength of the soil and the stress level at the depth of penetration;
equipment influences.

In selecting and operating the equipment and in order to avoid misinterpretation of the dynamic probing results, these factors should be considered; also findings from direct investigations (e.g. sampling according prEN ISO 22475-1) should be available.

D.1.2 Geotechnical influences

D.1.2.1 Influence of soil type, soil group and soil characteristics

For coarse-grained soils, apart from density, the grain structure, the grain size distribution, the grain shape and grain roughness, the mineral type, the degree of cementation and the strain condition in the soil can affect the results.

Examples of the influences of soil type, soil group and soil characteristics are given in D.2 to D.4. An example of the influence of the boundary depth is given in D.5.

For fine grained soils, rod friction can have a significant influence on the recorded blow count. The use of drilling mud and water can reduce this effect (see D.3).

D.1.2.2 Influence of the groundwater

Under otherwise equal soil conditions in coarse grained soils, the number of blows is lower below the groundwater level; this is particularly marked for low penetration resistances. Examples of the influence of the groundwater are given in D.6.

Under otherwise equal soil conditions, the number of blows in silty soils may be equal or higher below the groundwater level.

D.1.3 Equipment influences

The following are to be considered as equipment influences on the penetration resistance:

cone diameter;
rod length;
rod deviation;
energy losses within the drive systems.

Examples of some of the equipment influences are given in D.7.

D.2 Examples for results of dynamic probing in coarse-grained soils

Other conditions remaining the same, the following applies:

a) the penetration resistance increases more than linearly with increasing density index of the soil; thus a change in density index, for example as a result of deep compaction, can be detected by dynamic probing;
b) soils with sharp-edged or rough particles possess a higher penetration resistance than soils with round and smooth particles;
c) cobbles and boulders can significantly increase the penetration resistance;
d) particle size distribution (uniformity coefficient and grading curve) influence the penetration resistance;
e) penetration resistance is considerably increased by cementation.

Figure D.1 shows the results of a light dynamic probing test (DPL) in backfilled soil.

Change in penetration resistance with density index ID in a homogeneous backfilled soil

Key

1 Medium and coarse sand
I_D Density index
d Depth

Figure D.1 — Change in penetration resistance with density index I_D in a homogeneous backfilled soil

The tests were made in a test pit in which medium and coarse sand had been placed in layers of different relative densities. The penetration resistance increases sharply with increasing density index of the soil; the indication thus becomes more sensitive.

Figure D.2 shows the increase in penetration resistance when there are thin layers with embedded cobbles. Locally occurring peaks of penetration resistance do not represent a measure of the bearing capacity of the whole layer.

Increase in penetration resistance due to embedded cobbles

Key

1 Coarse silt, fine-sandy with layers of stones
d Depth

Figure D.2 — Increase in penetration resistance due to embedded cobbles

Figure D.3 shows that penetration resistance fluctuates more sharply in coarse-grained soils than in fine grained soils. The range of variation is more pronounced in gravels than in sands.

The absolute variations in penetration resistance obtained with a light dynamic penetrometer (DPL) do not result only from differing relative densities but also from the larger penetration resistance due to displacing or breaking up of embedded larger particles.

Key

1 Silt, sandy
2 Silt
3 Gravel
d Depth

Figure D.3 — Variations in penetration resistance in fine-grained and coarse-grained soils

Figure D.4 shows the effect of cementing of the particles of a sand layer on the penetration resistance to a light dynamic penetrometer (DPL). This type of cementing may remain undetected with borings. The cementing was observed in trial pits.

Increase in penetration resistance in a cemented medium sand

Key

1 Loam
2 Clay
3 Medium sand, cemented
4 Medium sand
d Depth

Figure D.4 — Increase in penetration resistance in a cemented medium sand

D.3 Examples for results from dynamic probing in fine-grained soils

In soft soil types the skin friction along the rod has considerable influence on the penetration resistance. This may mean, for example, that cavities in the subsoil are not recognised as such.

Figure D.5 shows that the standard penetration test (SPT), unlike the result produced by the heavy dynamic penetrometer (DPH), shows virtually the same penetration resistances in clay because here the skin friction along the rod has been eliminated by performing the SPT in a borehole.

Increase in penetration resistance as a result of skin friction along the rod using a heavy dynamic penetrometer compared with the standard penetration test

Key

1 Fill
2 Silt, sandy, gravely
3 Gravel, sandy
4 Clay
d Depth

Figure D.5 — Increase in penetration resistance as a result of skin friction along the rod using a heavy dynamic penetrometer (DPH) compared with the standard penetration test (SPT)

Figure D.6 shows DPM profiles driven with and without the aid of drilling mud. The drilling mud reduces the friction on the drive rods allowing penetration to greater depth. This data has not been corrected for friction using the torque measurements.

Reduction of skin friction due to drilling mud

Key

DPM Dynamic probing medium without drilling mud
DPM ^a Dynamic probing medium with drilling mud
I Crust
II Gravel
III Reworked weathered clay
IV Weathered clay
V Unweathered clay
d Depth

Figure D.6 — Reduction of skin friction due to drilling mud

Figure D.7 shows the DPM data from Figure D.6 corrected using the torque readings to correct for the effect of friction on the rods. The correction reduces the N_10M values at the deeper depths and presents a reproducible depth profile both with and without the drilling mud. All data fall into one band.

Example for the effect of torque measurement correction in a fine-grained soil

Key

DPM Dynamic probing medium without drilling mud
DPM ^aDynamic probing medium with drilling mud
I Crust
II Gravel
III Reworked weathered clay
IV Weathered clay
V Unweathered clay
d Depth

Figure D.7 — Example for the effect of torque measurement correction in a fine-grained soil

Figure D.8 shows the result of a dynamic probing using the light dynamic penetrometer (DPL) in relation to structural changes in a soil in:

a) natural silt and
b) filled, only slightly compacted silt.

Penetration resistance in natural silt and in filled, only slightly compacted silt of nearly the same density

Key

a Natural silt
b Filled, only slightly compacted silt
1 Medium silt, slightly clayey (loess loam)
d Depth

Figure D.8 — Penetration resistance in natural silt and in filled, only slightly compacted silt of nearly the same density

Figure D.9 shows that a decomposed peat has a very low penetration resistance.

Key

1 Silty clay
2 Peat, decomposed
3 Clay, sandy, very silty
d Depth

Figure D.9 — Dynamic probing in decomposed peat

Figure D.10 shows that a fibrous, barely decomposed peat shows high levels of penetration resistance, including skin friction. Similiar effects can be observed in highly organic clays and silts.

Dynamic probing in fibrous, barely decomposed peat

Key

1 Gravel and sand
2 Peat, fibrous
3 Fine sand and silt, slightly clayey
d Depth

Figure D.10 — Dynamic probing in fibrous, barely decomposed peat