Soil Exploration - Civil Engineering SSC JE (Technical) - Civil Engineering

Soil Sample and Samplers

Soil sampling is a fundamental activity in geotechnical site investigation. Samples recovered from the ground are used to determine engineering properties of the soil in the laboratory. It is important to understand the two broad categories of samples, the purpose of each, and the factors that control sample quality.

Types of soil sample

• Disturbed sample - A sample in which the natural structure and fabric of the soil is partly or fully modified during sampling. Disturbance may alter particle arrangement and pore structure, but the sample should still be representative with respect to the proportions of different size fractions and water content (if preserved). Disturbed samples are acceptable for tests such as grain-size distribution, Atterberg limits, moisture content, and specific gravity.
• Undisturbed sample - A sample in which the natural structure, fabric and in-situ stress condition of the soil are preserved as far as practicable. In practice, perfectly undisturbed samples cannot be obtained; all samples suffer some degree of disturbance. Undisturbed samples are required for tests that depend on structure and fabric, for example consolidation, permeability (in some cases), triaxial and direct shear tests where the in-situ strength and stiffness are needed.

Quality indices of core and tube samples

Several indices are used to judge the quality of a core or tube sample recovered from the field. The main indices are inside clearance (Ci), outside clearance (Co), area ratio (Ar) and recovery ratio (Lr). Each index is defined and discussed below. Preserve all sampling-tube dimensions and note the following symbols when referred to in formulas and explanations.

Symbols
D1 = Inner diameter of cutting tube
D2 = Outer diameter of cutting tube
D3 = Inner diameter of sampling tube (inside bore where core rests)
D4 = Outer diameter of sampling tube

Inside clearance (Ci)

Definition: Inside clearance is the clearance between the inner surface of the sampling tube and the outer surface of the cutting tube or the core sample. It is a small radial gap that allows the sampler to be pushed/withdrawn without excessive friction on the core.

Guidance: Typical acceptable values are small. Empirical guidance from field practice is

• 1 < Ci < 3 (usual units: mm) - a small radial clearance is desirable to reduce rubbing and crushing of the core; too large a clearance increases disturbance and the possibility of the sample rotating or deforming in the tube.

Outside clearance (Co)

Definition: Outside clearance is the clearance between the outer surface of the sampling assembly (cutting tube) and the borehole wall or any guiding casing. It controls the extent to which the cutting edge can penetrate and cut a clean sample.

Guidance:

• 1 < Co < 2 (typical units: mm) - small outside clearance helps obtain a core of correct shape and reduces disturbance at the sample boundary. Excessive outside clearance causes core loss and disturbance of the soil at the interface.

Area ratio (Ar)

Definition (practical explanation): The area ratio is the proportion of the cross-sectional area of the recovered core (useful sample area) to the cross-sectional area of the core barrel or cutting tube, usually expressed as a percentage. Lower area ratio indicates lesser disturbance.

Practical limits:

• Ar < 20% for stiff formations.
• Ar < 10% for soft clays.

These guidance values mean that for very soft soils, the usable core area should be kept even smaller relative to the barrel to reduce bulking and disturbance effects during recovery; selection of an appropriate sampler size and cutting edge is important to meet these limits.

Recovery ratio (Lr)

Definition: Recovery ratio is the ratio of length of sample actually recovered to the length of sample advanced by the sampler (or to the depth of penetration intended for that sampling run). It indicates how much of the sampled length has been retrieved intact.

• Lr = 1 - Good recovery (recovered length equals advanced length).
• Lr < 1 - Compressed or lost sample (some part was sheared off or left in situ).
• Lr > 1 - Swelled sample (expansion in length may occur when soft, highly compressible soils come into the tube and then relieve in-situ stresses).

Recovery ratio is reported as a fraction or percentage. High recovery with low disturbance is the objective; however, some soils (very soft organic silts/clays) may show swelling or extrusion when sampled.

Common samplers and their use

Choice of sampler depends on the soil type, depth, required quality (disturbed or undisturbed) and available equipment. Typical samplers are described briefly below.

• Tube samplers (Shelby tube) - thin-walled seamless tubes pushed into the soil either by static push or with a piston. They are used to obtain relatively undisturbed samples of soft to firm cohesive soils. Use of a piston reduces extrusion and sample disturbance during insertion and withdrawal.
• Thick-walled tube (block) sampler - larger, more robust tube used where a larger sample volume is required for testing; more suitable for stiffer soils but may disturb fabric near the edges.
• Split-spoon sampler - commonly used with standard penetration tests (SPT); provides disturbed samples suitable for classification and index tests but not for strength/stiffness tests requiring undisturbed conditions.
• Piston sampler - sampler with a piston that is held at the tip during penetration and then released to reduce sample disturbance due to extrusion.
• Block samplers and tray samplers - used where undisturbed large samples are needed (for example, for structure observations or certain permeability tests); these are manually excavated and trimmed in the field.

Sampling procedures - key points and precautions

• Selection: Select sampler type, tube diameter and wall thickness suitable to soil type and required tests (undisturbed vs disturbed).
• Cutting edge and clearance: Ensure cutting edge is sharp and inside/outside clearances are within acceptable limits to reduce smearing and compression of the core.
• Piston use for soft soils: Use a piston sampler for very soft clays and silts to limit sample extrusion and disturbance.
• Drive and retrieval: Apply steady push or rotary drive as appropriate; avoid sudden jerks or excessive rotation which increase disturbance.
• Recovery measurement: Immediately measure the recovered length and compute recovery ratio (Lr) and record any visible changes (cracks, smeared faces, water escape).
• Field trimming and protection: Trim the core carefully if needed, but avoid removing or rubbing the surface more than necessary. Seal the ends with wax or caps to preserve moisture content for laboratory testing.
• Labelling and documentation: Label each sample with unique ID, depth interval, date, sampler type, sampler orientation if required (e.g., top/bottom), borehole ID and field observations. Maintain a chain of custody record for critical undisturbed samples.
• Transport and storage: Transport samples in rigid boxes, upright when required, and protect from freezing or excessive drying. Store undisturbed samples in temperature-controlled conditions where possible and test them as soon as practicable.

Application of disturbed and undisturbed samples

• Disturbed samples - used for index properties and classification: particle size distribution, consistency limits (Atterberg limits), organic content, specific gravity and moisture content.
• Undisturbed samples - used for tests sensitive to structure and in-situ stress such as consolidation tests, permeability tests (in some procedures), triaxial shear tests, direct shear tests and strength/stiffness determinations.

Recording and reporting

Field records should always include borehole or sample identification, depths of sampling interval (top and bottom), method of sampling, sampler type and size, recovery length, sample condition (disturbed/undisturbed, soft, stiff, fissured, water loss, roots, etc.), and field observations about layering. Photographs of cores and samples are useful. Report the area ratio, clearances and recovery so laboratory engineers can judge sample reliability when interpreting test results.

Practical notes and common issues

• Soft, sensitive clays are easily disturbed; use piston or thin-walled push samplers and minimise handling.
• Granular soils are difficult to sample undisturbed; laboratory tests for strength may be carried out on samples remolded to known densities or using in-situ tests instead.
• Sample disturbance can bias strength and consolidation results; always report observed disturbance and indices so results can be interpreted accordingly.

GATE Exam Notes:

• Low area ratio → low disturbance
• Piston samplers → soft clays
• Granular soils → undisturbed sampling difficult
• Strength & consolidation tests require undisturbed samples
• Disturbed samples are sufficient for classification tests