12 SOIL SAMPLING
(1)P The aim of soil sampling is to obtain samples for soil identification and for laboratory testing to determine geotechnical properties of the ground.
(2) Important soil properties needed in geotechnical design are strength and deformation properties. In the laboratory these properties can be obtained reliably only from high quality undisturbed samples, representative of each soil layer. From granular materials it is possible to obtain undisturbed samples only using special methods not covered in this section. Strength and deformation properties of these soils are usually supported by in-situ tests and confirmed by disturbed samples. Soil classification properties may be obtained also from disturbed samples.
(3) Commonly used laboratory tests for soil identification and determination of geotechnical properties of soils are covered in ENV 1997-2
12.2 Categories and concepts
12.2.1 Categories of sampling methods
(1) There are three categories of sampling methods:
- category A sampling methods;
- category B sampling methods;
- category C sampling methods.
(2) By using category A sampling methods, the intention is to obtain samples in which no or only slight disturbance of the soil structure has occurred during the sampling procedure or in handling of the samples. The water content and the void ratio of the soil correspond to that in situ. No change in constituents or in chemical composition of the soil has occurred.
(3) By using category B sampling methods, samples contain all the constituents of the in situ soil in their original proportions and the soil has retained its natural water content. The general arrangement of the different soil layers or components can be identified. The structure of the soil has been disturbed.
(4) By using category C sampling methods, the soil's structure in the sample has been totally changed. The general arrangement of the different soil layers or components has been changed so that the in situ layers cannot be identified accurately. The water content of the sample may not represent the natural water content of the soil layer sampled.
12.2.2 Area ratio and inside clearance of the sample tube
(1)P The area ratio Ca (%) of the sample tube is defined by the following relation:
D1 is the inside diameter of the cutting shoe;
D2 is the greatest outside diameter of the cutting shoe.
(2) The area ratio is one of the factors that determine the mechanical disturbance of the soil, being the ratio of the volume of soil displaced by the sampler tube in proportion to the volume of the sample.
(3)P The minimum length of the tube where the area ratio shall be applied is two times the outside diameter of the tube or cutting shoe, see figure 12.1.
(4) Thin-walled samplers have a sample tube with the area ratio equal or less than 15 % and thick-walled open-tube more than 15 %.
(5)P The inside clearance ratio Ci (%) of the sample tube is defined as:
D1 is the inside diameter of the cutting shoe;
D3 is the inside diameter of the sample tube or liner.
(6) The inside clearance ratio is one of the factors that determine the mechanical disturbance of the soil caused by the friction on the inside wall of sample tube or of the liner.
12.2.3 Techniques for sampling
(1) Techniques for obtaining soil samples can be divided in four groups defined as follows:
- drive sampling, in which a tube or a split-tube sampler having a sharp cutting edge at its lower end is forced into the ground either by a static thrust (by pushing), by a dynamic impact or by percussion. Drive samplers are usually open tube samplers or piston samplers. Drive samplers are mostly used as category A or B sampling methods;
- rotary core sampling, in which a tube with a cutter at its lower end is rotated into the ground, thereby producing a core sample. Rotary core samplers can have a single, double or triple tube rotary core barrel with or without a liner. Rotary core sampling is usually used as category B sampling methods, in some cases as category A sampling methods. Rotary core sampling can be replaced by vibrocoring;
- auger sampling with hand or mechanical augers. Augers are usually used as category C and rarely as category B sampling methods;
- block sampling made by hand cutting from a trial pit, shaft or heading or at depth using specially made block samplers with cutting procedure. Block sampling is mostly used as category A sampling methods.
12.2.4 Quality classes of soil samples related to sampling categories
(1) According to 2.3 of ENV 1997-2 soil samples for laboratory tests are classified in five quality classes with respect to the soil properties that are assumed to remain unchanged during sampling and handling, and the category of sampling that may be used. The classes are described in table 12.1.