Soil Properties and their Correlations

1

Commonly Measured Properties

The purpose of this chapter is to introduce the more commonly measured properties and give outline descriptions of how they are measured. This will allow engineers and geologists who are specifying test schedules, but who may have little or no experience of soils laboratory work, to have a clear understanding of the procedures used to carry out the tests they are scheduling, along with any problems that might occur. This, in turn, should help them to choose the most appropriate tests and to fully appreciate any problems or shortcomings related to the various test methods when appraising the results. It may also give an appreciation of the complexity of some tests to determine seemingly straightforward properties. For clarity, some details have been omitted; test descriptions are not intended to give definitive procedures or to be of sufficient detail to allow them to be used for actual testing. Such details should be obtained directly from the test standards being used, and will normally be the responsibility of the testing laboratory unless specific variations from the standards are required.

Deeper discussions of the nature and meaning of the various properties, and how they relate to other properties, are given at the beginning of subsequent chapters.

1.1 Moisture Content

Moisture content has a profound effect on many properties, and moisture content determinations are carried out as a routine part of many tests; for example, during the determination of shear strength, compressibility, plasticity and California Bearing Ratio (CBR). An especially common use is during density determinations, where it is used to calculate dry density from measurements of bulk density.

To obtain a moisture content value, a soil specimen is simply heated until dry. By weighing the specimen before and after drying, the weight of dry soil and the weight of water driven off can be obtained, and moisture content is obtained from:

(1.1)

expressed as a percentage.

Note that the definition relates moisture content to the weight of soil solids (dry soil) and not to the total weight of the wet sample. This means that for some soils, such as peat, where the weight of water may exceed the weight of soil solids, the moisture content may exceed 100%.

1.1.1 Test Methods

1.1.1.1 Standard Oven Drying

The standard laboratory procedure is by oven drying a specimen of between 30 g (fine‐grained soils) and 3 kg (coarse‐grained soils) in an open tin or tray at 105‐110 °C for 18‐24hours, or until a stable weight is obtained for at least 4 hours. This temperature is high enough to ensure that all free water is driven off but not so high as to break down the mineral particles within the soil. However, some variation in the method may be required for certain soils, especially those containing gypsum or anthracite (coal), which break down chemically at normal oven temperatures. For such soils, lower temperatures are used, typically 60 °C.

1.1.1.2 Quick Methods

Whist the standard oven drying method is satisfactory for normal ground investigation testing, the length of time taken to dry out the specimen can be a problem for quality control of earthworks, where results are needed quickly. To overcome this problem, a number of quick methods have been developed, some of which are outlined below. In all cases, the quick methods should be calibrated against oven‐drying values for each soil type as not all methods work with all soils.

Microwave oven drying works well for most soil types provided the soil is microwaved for the appropriate times – see, for example, Carter and Bentley (1986). Ceramic or glass dishes that do not absorb microwaves must be used, and some kind of dummy sample should be included that will continue to absorb microwaves after all the water has been driven off to avoid running the microwave with no load, which can damage it. Note that there is a risk that the dummy sample will get very hot.

The ‘Speedy’ moisture tester consists of a sealed cylindrical pressure flask with a pressure gauge mounted at one end. A fixed weight of soil is put into it along with calcium carbide powder, and the flask is shaken. Reaction of the powder with water in the soil produces acetylene gas, creating a pressure that is proportional to the amount of water in the specimen. The pressure gauge is calibrated directly in percentage moisture content. The tester is quick and simple to operate, requiring no specialised knowledge or equipment, and usually gives reasonably accurate results with granular soils but results can be erratic with clay soils. The method should be calibrated against oven‐drying tests for each soil type, and some soils may not give consistent results at all, precluding use of this method.

Field density meters, which measure the transmission or backscatter of radiation through the soil, may also be used to obtain moisture content values. These are an exception to the methods used by the other tests in that the soil is not dried out during testing. The operation of these devices is summarised later in this chapter in the ‘Soil density’ section.

Other methods include heating the specimen over a hot tray of sand placed on a gas burner, mixing the specimen with methylated spirit (a mixture of methyl and ethyl alcohol) then setting it alight. However, these are rarely used now except in remote field locations where only primitive equipment is available and they are not without risks to the tester if not carried out carefully, so are not described here.

1.2 Grading

Grading, otherwise known as particle size distribution or PSD, gives a measure of the sizes and distribution of sizes of the particles that make up a soil. Grading is arguably the most fundamental of all properties, especially for coarse‐grained soils with little or no clay particles.

Particle size distribution is used for a wide variety of assessments, especially where soil is to be used in remoulded form such as fills and embankments, and grading tests are specified in nearly all site investigation test schedules. Uses include: classifying fill materials for design purposes (Appendix A); assessment of permeability and drainage characteristics; and suitability for backfill to pipes. Grading characteristics are more important for coarse‐grained soils (sands and gravels); for fine‐grained soils (silts and clays), plasticity is more indicative of behaviour but, even for these soils, the proportion of coarser material present is important for assessing properties.

1.2.1 Test Methods

There are two main methods of grading soil.

Sieve analysis: Coarse‐grained soils, with soil particles down to 63 µm (fine sand size, defined as below 75 µm in some standards), can be separated out by