Estimation of Percolation Time

  1. The purpose of this Section and the associated Tables and Charts is to provide assistance to those who must decide on the percolation time(s) to be used in design. Suggested relationships between percolation time, coefficient of permeability and soils of various types are given. it must be emphasized that, particularly for fine grained soils, there is no consistent relationship due to the many factors involved. The following guidance is presented for the soil types outlined in the Unified Soil Classification System (Table 1). In order to assess a particular soil.
    1. Table 2 and Table 3 - Approximate relationship of soil types to permeability and percolation time.
    2. Charts 1 to 14 - Typical grain size distribution curves for soil types in the Unified Soil Classification System.
  2. In Table 2 and Table 3, a range of values of “K” and of “T” are given for various soil descriptions. The principal modifiers which will influence selection of a “T” value within the range given are:
    1. The structure - “massive” fine-grained soils have high values of “T”.
    2. The density - For a given soil higher density produces a higher value of “T”.
    3. The percentage of clay - the higher the percentage the higher the value of “T”.
    4. The mineralogy of the clay portion - The more it “swells” the higher the value of “T”.
    5. The plasticity of the soil - The higher the plasticity index the higher the value of “T”.
    6. Liquid Limit - the higher the liquid limit the higher the value of “T”.
    7. Organic content - The presence of fine organic particles, detectable by colouration and odour, can significantly reduce the permeability and raise the value of “T”.

Table 1 — Unified Soil Classification

Table 1. — Unified Soil Classification: Coarse-Grained Soils
Group SymbolsTypical Names
GWWell-graded gravels, gravel-sand mixtures, little or no fines
GPPoorly-graded gravels, gravel-sand mixtures, little or no fines
GMSilty gravels, gravel-sand-silt mixtures
GCClayey gravels, gravel-sand-clay mixtures
SWWell-graded sands, gravelly sands, little or no fines
SPPoorly-graded sands, gravelly sands, little or no fines
SMSilty sands, sand-silt mixtures
SCClayey sands, sand-clay mixtures
Column 12
Table 1. (continued) — Unified Soil Classification: Fine-Grained Soils
Group SymbolsTypical Names
MLInorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity
CLInorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays
OLOrganic silts and organic silty clays of low plasticity
MHInorganic silts, micoceous or diatomaceous fine sandy or silty soils, elastic silts
CHInorganic clays of high plasticity, fat clays
OHOrganic clays of medium to high plasticity, organic silts
PT (highly organic soils)Peat and other highly organic soils
Column 12

Table 2 — Approximate Relationship of Coarse Grained Soil Types to Permeability and Percolation Time

Soil Type (Unified Soil Classification): Coarse Grained, More than 50% Larger than #200Coefficient of Permeability, K — cm/secPercolation Time, T — mins/cmComment
G.W. — Well graded gravels, gravel-sand mixtures, little or no fines.10−1<1very permeable unacceptable
G.P. — Poorly graded gravels, gravel-sand mixtures, little or no fines.10−1<1very permeable unacceptable
G.M. — Silty gravels, gravel-sand-silt mixtures.10−2–10−44–12Permeable to medium permeable depending on amount of silt.
G.C. — Clayey gravels, gravel-sand-clay mixtures.10−4–10−612–50Important to estimate amount of silt and clay
S.W. — Well graded sands, gravelly sands, little or no fines.10−1–10−42–12medium permeability
S.P. — Poorly graded sands, gravelly sand, little or no fines.10−1–10−32–8medium permeability
S.M. — Silty sands, sand-silt mixtures.10−3–10−58–20medium to low permeability
S.C. — Clayey sands, sand-clay mixtures.10−4–10−612–50medium to low permeability depending on amount of clay
Column 1234

Table 3 — Approximate Relationship of Fine Grained Soil Types to Permeability and Percolation Time

Soil Type (Unified Soil Classification): Fine Grained, More than 50% Passing #200Coefficient of Permeability, K — cm/secPercolation Time, T — mins/cmComment
M.L. — Inorganic silts and very fine sands, rock flour, silty or clayey fine sands, clayey silts with slight plasticity10−5–10−620–50medium to low permeability
C.L. — Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays10−6 and lessover 50unacceptable
O.L. — Organic silts, organic silty clays of low plasticity; liquid limit less than 5010−5 and less20–over 50acceptable depends on clay content
M.H. — Inorganic silts, micareaous or diatomageous fine sandy or silty soils, elastic silts10−6 and lessover 50unacceptable
C.H. — Inorganic clays of medium to high plasticity, organic silts10−7 and lessover 50unacceptable
O.H. — Organic clays of medium to high plasticity, organic silt; liquid limit over 5010−6 and lessover 50unacceptable
Column 1234

Selection of "T" Time From The Above Tabulation

A range of “T” times for each soil type is shown above. Select from within this range by determining if the soil is within the low, middle or high part of the range considering the soil identifiers and soil characteristics. Consider structure, density, colour, prevalence or organics, the clay content and mineralogy, the plasticity index and liquid limit and the functioning of existing systems in similar soils in the area.

Notes:

The following Ministry of the Environment Reports provide further information on the relationship between grain size, coefficient of permeability and percolation time.

  1. Study on the Feasibility of Correlating Percolation Time with Laboratory Permeability - 1975 - Research Report No. S56 by H. T. Chan, PhD., P.Eng.
  2. Study of Conventional Tile Fields in Fine-Grained Soils - 1979 Research Report 74 by H. T. Chan, PhD., P.Eng.

Chart 1 — GW: Well-graded gravels, gravel-sand mixtures

Unified Soil Classification System Chart 1 for GW soils showing grain size distribution curves for well-graded gravel and gravel-sand mixtures with less than 5% finer than 0.074 mm and uniformity coefficient greater than 4.

Chart 2 — GP: Poorly-graded gravels, gravel-sand mixtures

Unified Soil Classification System Chart 2 for GP soils showing grain size distribution curves for poorly-graded gravel and gravel-sand mixtures with less than 5% finer than 0.074 mm and uniformity coefficient less than 4.

Chart 3 — GM: Silty gravels, gravel-sand-silt mixtures

Unified Soil Classification System Chart 3 for GM soils showing grain size distribution curves for silty gravels and gravel-sand-silt mixtures with more than 12% finer than 0.074 mm and plasticity index less than 4 plotting below the A-line.

Chart 4 — GC: Clayey gravels, gravel-sand-clay mixtures

Unified Soil Classification System Chart 4 for GC soils showing grain size distribution curves for clayey gravels and gravel-sand-clay mixtures with more than 12% finer than 0.074 mm and plasticity index greater than 7 plotting above the A-line.

Chart 5 — SW: Well-graded sands, gravelly sands

Unified Soil Classification System Chart 5 for SW soils showing grain size distribution curves for well-graded sands and gravelly sands with little or no fines less than 5% passing 0.074 mm and uniformity coefficient greater than 4.

Chart 6 — SP: Poorly-graded sands, gravelly sands

Unified Soil Classification System Chart 6 for SP soils showing grain size distribution curves for poorly-graded sands and gravelly sands with little or no fines less than 5% passing 0.074 mm and uniformity coefficient less than 6.

Chart 7 — SM: Silty sands, sand-silt mixtures

Unified Soil Classification System Chart 7 for SM soils showing grain size distribution curves for silty sands and sand-silt mixtures with more than 12% finer than 0.074 mm and plasticity index less than 4 plotting below the A-line.

Chart 8 — SC: Clayey sands, sand-clay mixtures

Unified Soil Classification System Chart 8 for SC soils showing grain size distribution curves for clayey sands and sand-clay mixtures with more than 12% finer than 0.074 mm and plasticity index greater than 7 plotting above the A-line.

Chart 9 — ML: Inorganic silts and very fine sands, rock flour, silty or clayey fine sands or clayey silts with slight plasticity

Unified Soil Classification System Chart 9 for ML soils showing grain size distribution curves for inorganic silts, rock flour, silty or clayey fine sands, and clayey fine sands with slight plasticity and liquid limit less than 50.

Chart 10 — CL: Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays

Unified Soil Classification System Chart 10 for CL soils showing grain size distribution curves for inorganic clays of low to medium plasticity including gravelly, sandy, silty, and lean clays with liquid limit less than 50.

Chart 11 — OL: Organic silts, organic silty clays of low plasticity

Unified Soil Classification System Chart 11 for OL soils showing grain size distribution curves for organic silts and organic silty-clays of low plasticity with liquid limit less than 50.

Chart 12 — MH: Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts

Unified Soil Classification System Chart 12 for MH soils showing grain size distribution curves for inorganic silts, micaceous or diatomaceous fine sandy or silty soils, and elastic silts with liquid limit over 50.

Chart 13 — CH: Inorganic clays of high plasticity, fat clays

Unified Soil Classification System Chart 13 for CH soils showing grain size distribution curves for inorganic clays of high plasticity, fat clays, with liquid limit over 50.

Chart 14 — OH: Organic clays of medium to high plasticity, organic silts

Unified Soil Classification System Chart 14 for OH soils showing grain size distribution curves for organic clays of medium to high plasticity and organic silts with liquid limit over 50.