WHAT DO YOU MEAN MOISTURE IS 110%!
results of the failure of the geologist to convey the information, or failure of the engineer to consider that information.
Figure 2. Rotational slump in a new highway cut in Cincinnati OH. A series of such slides along this cut initiate in a horizon of lake bed clays. Repeated attempts to remediate the slides by adding gravel cover have failed because the underlying geology has not been taken into account.
2. Proportion of void space to solids A critical property of a soil or rock is the ratio of the volume of void space to that of solids. Geologists work with porosity expressed in percent and sig- nied by in petroleum geology or as a decimal fraction represented by n in hydrogeology (note that phi in geotech- nical engineering is used to represent the internal angle of friction). On the other hand, civil engineers work with void ratio (using e as the symbol) but may also use porosity in the decimal form. Porosity refers to the ratio of pore volume to the total volume of rock or soil, whereas void ratio refers to the ra- tio of the pore volume to the volume of solids alone. The terms are readily con- verted:
n = e/(1e); = n*100% e = n/(1-n)
where n is expressed as a decimal frac- tion.
The potential for confusion is limited because the terms for the two systems of measurement are completely differ- ent.
3. Proportion of water to solids The ratio of the mass of uid to that of solids in a material, the moisture con- tent when the uid is water, has parallel differences between the two disciplines but the vocabulary terms are not dif- ferent and confusion is common. Geolo- gists use the ratio of uid weight to to- tal weight, whereas civil engineers use uid weight to solids weight. In other words, geologists report moisture as a percentage of the weight of the sample as received; engineers report the per-
centage of the dry weight. A sure sign that a report is using the civil engineer- ing denition is the appearance of val- ues higher than 100 %. If we dene for convenience m as the moisture content in the geology sense and w as water or moisture content in the engineering sense, w = m/(1-m) or m = w/(1w) In both the geologic and engineering styles of measurement, the direction of these metrics is the same, the units are the same (dimensionless) and the vari- able used is supercially similar. How- ever, critical differences are present. Each discipline needs to be sensitized to the likelihood of confusion and make it perfectly clear which convention is being used in every report, otherwise the reader will have difculty know- ing which system is being specied. Although it may seem redundant and be resisted by editors, it is our opinion that a good report or paper will include tables with both sets of variables. Consider the infamous Aberfan ow slide. The liquid limit for the material in the waste tip proved to be 26.5% (Bishop, et al., 1969). The moisture
Units
the terms and the units are the same, so unless the denition of the parameter is carefully stated, this value might be regarded as safe.
4. Permeability
Of the many properties of a rock or a soil, arguably the most important is per- meability because it exerts the greatest inuence on the movement of uids and on pore pressures. It is notoriously dif- cult to evaluate permeability because it covers such a wide range and is so variable on a small scale. Litigation in- volving groundwater is likely to turn on widely different estimates of permeabil- ity by the experts on opposing sides. Different schemes of measurement, different units, and subtly different symbols are used by petroleum en- gineers, geotechnical engineers, and hydrogeologists (Table 2). This practice is likely to cause each discipline to ig- nore or misinterpret data generated by the others. In most cases, there should be little danger of confusion if the units are carefully expressed, but there is a tendency to use K and “k” in the oppo- site sense in geotechnical and hydrogeo- logic notation.
Table 2. Measures of the Ability of Soil or Rock to Transmit Fluid Term
Symbols
Measures that depend on the properties of the liquid as well as the solid Hydraulic conductivity
L/T Permeability coefcient L/T
Permeability (petroleum geology and engineering)
Intrinsic permeability (Hydrogeology and Engi- neering)
Effective permeability (per- meability to one uid phase – oil, water, or gas -- when others are also present)
Relative permeability (effec- tive permeability normal- ized to a reference single- phase permeability)
k (Geotechnichal Engineering)
Measures that depend only on the properties of the solid medium darcy k
L2 darcy oil
percent or
decimal oil keo
water kew gas
keg kro = keo/k
water krw = kew/k gas
krg = keg/k
content, when measured as percent of dry weight as is the case for compari- son with the liquid limit, was 23-25%. This number was so close to the liquid limit it should have alerted the authori- ties to the imminent danger of liquefac- tion. However, if the moisture were ex- pressed in the geologic sense as percent of total weight, the value would be only 19-20% and would convey a false sense of security. This emphasizes again that
5. Denition of soil The denition and classication of soils have a great potential for confu- sion as pointed out by Leggett (1953), who suggested that geologists adopt the engineering usage. Since this has not come to pass, clarity about differences in usage is needed. It is helpful to consider soil and soil classications in terms of the purpose of the investigator. We identify ve contrasting approaches to soils that
www.aipg.org K (Geotechnical
Engineering) Ki (hydrogeology)
1 darcy = 9.87x109 cm2 Specic permeability, Synonyms
K (hydrogeology) Field coefcient of per- meability (obsolete)
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