L.4 Preparation of re-compacted specimens

L.4.1 General requirements

(1) Disturbed soil may be re-compacted to form test specimens in accordance with either of the following criteria:

compaction using a specified compactive effort at a specified water content;
achieving a specified dry density at a specified water content.

Table L.2 – Minimum mass for sieving

Largest particle diameter (D) mm	Minimum mass for sieving (MMS) kg
75 63 45 37,5 31,5 22,4 20 16	120 70 25 15 10 4 2 1,5
Largest particle diameter (D) mm	Minimum mass for sieving (MMS) g
11,2 10 8 5,6 4 2,8 < 2	600 500 400 250 200 150 100

Table L.3 – Allowable size of particles for compaction tests

Type of test	Maximum size of particle
Compaction – in one-litre mould – in CBR mould	20 mm 37,5 mm
CBR determination	20 mm

Table L.4 – Mass of soil required for tests on undisturbed samples

Type of test	Specimen dimensions		Minimum mass required g
	Diameter mm	Height mm
Oedometer	50 75 100	20 20 20	90 200 350
Compression ^b – Unconfined – Unconsolidated-undrained – Triaxial compression test	35 38 50 70 100 150	70 76 100 140 200 300	150 200 450 1 200 3 500 12 000
Shear box	Planar size	20 20 150	150 450 30 000
	60 × 60 100 × 100 300 × 300
Density Largest particle size ^a	Largest particle size		125 300 500 1,4 (MMS) ^c
	D = 5,6 mm D = 8 mm D = 10 mm D > 10 mm
^a D is the largest particle diameter in significant proportion (10 %or more by mass). ^b Specimen dimensions and minimum required volume: apply to all three tests ^c MMS is the minimum mass to be taken for sieving, as specified in Table L.2.

(2) Clay soil that is to be re-compacted to form test specimens should not be allowed to dry. If it is necessary to reduce the water content of the soil, this should be done by air drying. If it is necessary to add water in order to increase the water content, the water should be well mixed in and the soil should be allowed to stand in a sealed container for at least 24 hours before use.

(3) The soil should be broken down before re-compaction.

(4) The upper limit of allowable particle sizes depends upon the size of the test specimen to be formed. Particles larger than the sizes given in Table L.5 should be removed before preparing the soil for re-compaction.

(5) The particle size distribution of a re-compacted specimen should be checked before and after compaction.

Table L.5 – Allowable particle size as a function of size of test specimen

Type of test specimen	Maximum size of particle
Oedometer consolidation	H/5^a
Direct shear (shear box)	H/10
Compressive strength (cylindrical specimen with H/d of about 2)	d/5^b
Permeability	d/12
^a H = height of specimen, ^b d = diameter of specimen

L.4.2 Re-compacted sample larger than test specimen

(1) When preparing specimens for oedometer consolidation, direct shear or compressive strength tests, the soil should normally be compacted in the specified manner into a suitable mould that is of a larger size than the desired test specimen. The compacted sample should then be extruded from the mould and the test specimen should be prepared using the procedures described for undisturbed samples.

NOTE The method is not suitable for granular soil.

(2) Specimens for permeability tests may be compacted directly into the mould or container in which the test is performed.

(3) For compaction using a specified effort, the compactive effort applied should normally correspond to that used in one of the two types of compaction tests specified for the compaction test (see 5.10 and Annex R). Compaction should be applied in layers, and the top of each layer should be lightly scarified before adding the next one.

(4) To obtain a specified density, the soil may either be compacted dynamically or compressed under a static load. Weighings and volume measurements should be made after placing each layer to ensure that the desired density will be achieved. Preliminary trials may be desirable to establish the appropriate method.

(5) If clay is present in the soil, the compacted sample should be sealed and stored for a curing period of at least 24 h before extrusion to form test specimens.

L.4.3 Re-compaction of test specimen

(1) For the preparation of small test specimens for direct shear, oedometer or compressive strength, the soil should be tamped, kneaded or compacted into the appropriate mould, ring or tube. A suitable hand rammer, the Harvard compaction apparatus, or a kneading action may be used. Care should be taken to avoid the formation of cavities within the specimen. The exact procedure required to obtain the desired density or compactive effort should first be determined by trial. Details should be recorded so that the procedure can be repeated to provide a number of specimens of consistent properties.

(2) Compaction of cylindrical test specimens of 100 mm diameter or more may be carried out using a compaction rammer. The number of layers and number of blows per layer should be specified.

(3) If clay is present in the soil, the compacted specimen should be sealed and stored for a curing period of at least 24 h before use, to allow for dissipation of excess pore water pressures.

L.4.4 Re-saturation

(1) A re-compacted specimen will invariably be initially unsaturated. Re-saturation will normally be required before testing, and this should be carried out by using one of the recognised saturation methods given in the test procedures for shear strength or compressibility tests. Full saturation should be confirmed by checking the B value, if applicable.

L.4.5 Remoulded test specimen

(1) Remoulding can be achieved by sealing the soil in a plastic bag where it is squeezed and kneaded with the fingers for several minutes. A remoulded test specimen is formed by working the soil into the appropriate mould, e.g. by using a tamping rod. This operation should be carried out as quickly as possible to avoid change of water content, and without entrapping air. The specimen should then be extruded and trimmed.

L.5 Preparation of reconstituted specimens

L.5.1 Preparation of slurry

(1) The soil should be thoroughly mixed with water to form an homogeneous slurry with a water content above the liquid limit. Preparing the slurry should preferably start from the natural water content without drying of the soil. Drying of the soil and grinding it to a powder may change its properties. If necessary, coarser particles can be removed by wet sieving using an appropriate sieve. The mixing water may be either distilled or de-ionized, or of the appropriate chemistry. The slurry should be fluid enough to be poured; a water content about twice the liquid limit is usually satisfactory.

L.5.2 Consolidation

(1) The cell in which the sample is consolidated should be large enough to provide a test specimen, or a sample for trimming, to the required size after consolidation. Provision should be made for drainage of the sample, without allowing soil particles to escape.

(2) After pouring the slurry into the mould, initial consolidation should be applied under the weight of the top plate only, until the specimen ends are stiffened enough to prevent loss of material under further loading. The vertical stress applied for consolidation should be sufficient to enable the sample to be handled when consolidated, and should be maintained for long enough to ensure that consolidation is substantially complete.

L.5.3 Specimen preparation

(1) The consolidated sample should be extruded from the cell and trimmed as necessary for the preparation of a test specimen or specimens.

(2) If one-dimensional consolidation tests are to be performed on the reconstituted soil, they may be carried out in the cell in which it has been consolidated from slurry.