What Are The Testing Apparatus For Site Soil Laboratories?

The usual apparatus suitable for a small soils laboratory on site, to be run by the resident engineer’s staff after proper instruction from a geotechnical engineer, is set out below.

For moisture content determinations
1. Beam balance weighing by 0.01 g divisions.
2. Drying oven, thermostatically controlled. (Not absolutely essential. For rough measurement of moisture content the sample can be dried on a flat tray over a stove.)
3. Six drying trays.

For grading analyses of soils
4. Aset of BS sieves (woven wire) with lid and pan for each different diameter: (a) 300 mm dia – 38, 25, 19, 13, 10 mm. (These can also be used for testing concrete aggregate gradings.) (b) 200 mm dia – 7, 5 and 3 mm, and Nos. 7, 14, 25, 52, 72, 100 and 200.
5. Balance weighing up to 25 kg.
6. Balance capable of weighing up to 7 kg by 1 g divisions.

For in situ density test (sand replacement method)
BS 1377 Part 9:1990 gives four tests of which Test 2.2 is the most useful because it can be used on fine, medium and coarse grained soils. A metal tray with a 200 mm diameter hole cut in it is placed on the formation and material is excavated via the hole.

The volume of the excavation is measured by pouring uniformly graded sand into it whose bulk density has been measured.

Apparatus required (additional to 1, 2, 3 and 6 above):
7. Small tools for excavating hole.
8. A rigid metal tray 500 mm square or larger with a 200 mm diameter hole cut in it.
9. Dried clean sand all passing No. 25 sieve but retained on No. 52 or 100 sieve and suitable airtight containers for storing it. (About 20 kg of this sand will be required initially.)
10. A pouring cylinder (as BS 1377 Part 9 Fig. 4).
11. Acalibrating container 200 mm diameter by 250 mm (as BS 1377 Part 9 Fig. 5).
12. Air-tight containers for the excavated soil.

The method can be applied to larger test holes in soils containing some gravel; the sand being poured in layers from a can with a top spout. A length of hose is attached to the spout with a conical tin shield wired to the lower end, so the sand has only a short standard free fall. Tests to fill measured containers can show the accuracy in ascertaining the bulk density of the sand as poured.
For compaction tests
BS 1377 Part 4: 1990 describes tests using 2.5 or 4.5 kg hammers on soils with or without coarse grains. For the 2.5 kg test on fine and medium grained soils the apparatus required (additional to items 1, 2, 3 and 6 above) is:
13. Compaction mould (BS 1377 Part 4 Fig. 3).
14. 2.5 kg metal ram
15. Palette knife.

16. Glass sheet or metal tray (for mixing in added moisture to sample).
17. 19 mm sieve (from Item 4 above).
18. A 1-litre glass graduated measuring cylinder (for measuring volume of surface-wet material over 19 mm sieved out).

The compaction test and in situ density test are important for earthworks construction. The former indicates the maximum density and optimum moisture content of fill material achieved under ‘standard’ compaction; the latter shows the density of fill achieved.

Specifications often require fill as placed to achieve 90 or 95 per cent of the maximum density obtained under one of the compaction tests stipulated; the 4.5 kg hammer test being used for road construction, the 2.5 kg test for other earthworks.

For road works a CBR test is often essential to check design assumptions for the strength of sub-grade. For accuracy this is normally carried out in the laboratory using standard equipment as set out in BS 1377 but in situ tests can also be done as a ready check on soil strength.

A small unconfined compression testing apparatus for testing the shear strength of 38 mm diameter undisturbed clay samples is a useful addition to the site laboratory in certain circumstances. This machine is cheap, easy-to operate and gives a useful indication of variations of clay strength, as for road making, etc.

The results given by it are not, however, adequate for design purposes. The triaxial compression testing machine would be used for testing soils for design purposes; but this is a sophisticated piece of apparatus, not suitable for site control purposes unless a full-scale soils laboratory has been set up on site under the direction of a properly qualified geotechnical engineer.

It is also useful to have some standard 100 mm diameter sampling tubes on site, to prevent a delay in getting such tubes when an excavation reveals material that needs to be tested.

Provided proper briefing has been given by an experienced geotechnical engineer concerning the techniques of testing to be followed, the foregoing apparatus should permit a useful range of quality control tests to be carried out on site.

Most other tests that might be required, such as consolidation, permeability and triaxial compression tests, must be regarded as advanced laboratory tests to be carried out by trained technical staff.

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