whole production process. The field of soil-selection involves identification of the
distribution of gravel, sand and fines (silt and clay) within a sample. To limit the size of
gravel and remove other large particles, after being first pulverised, soil is passed through a
standardised sieve with 4-6mm openings. An important factor in soil stabilization is the soil’s
cohesion that depends in its fines fraction. Soil selection is often conceived as a once-off
process of testing to confirm the soil passes the criteria for stabilization and to determine the
best ratio of soil to stabiliser. However to maintain soil consistency, it is necessary in practice
to constantly monitor the soil’s properties and compensate for any changes that occur.
The test procedure and the coherent test plan described by Gooding (1993), for preliminary
on-site testing is one of major steps of soil selection. Although the bottle/sedimentation and
less than 2.5% or greater than 9% should be discarded for stabilization unless it can be
modified to achieve adequate cohesion (clay content between 10% and 35% BRU-B2 (1974).
Any soil modified by blending should be tested repeatedly until the attained shrinkage is
between 2.5 and 9%. Data in table 2.2 is a result of field experience in agreement with the
calibrations after VITA (1975) for a low-pressure machine up to 2MPa, and higher-pressure
machine of up to 10MPa after Webb (1988). Linear shrinkage (LS) test results determine the
ratio that allow calculation of the amount of stabilizer to be used as well as the compression
needed. Also agreeing with Webb and Lockwood (1987) recommendations concerning choice
of machine;
• Low shrinkage soils (high sand content) are better stabilized with Portland cement (PC) and compressed by high power (> 4MPa) machines, while
• High shrinkage soils (high clay content) are better-stabilized using lime and low power (to 2MPa) press machines.
Table 2.2 Level of soil shrinkage with recommended compression pressure
(Data using Alcock’s shrink-box - 600x40x40 mm)
Source Measured shrinkage (mm) Shrinkage (%) Recommended cement to soil ratio (C: S) Cement (C %) Remarks Gooding (1993) Hauben & Gullaud (1994) ILO (1987). Norton (1997), UN (1992) VITA (1975) Webb & Lockwood (1987) 6 – 15 1 to 2.5 1:20 4.8
Only for heavy compression above 4MPa provided soil proves to have enough clay to reduce handling breakages
15 – 25 2.5 to 4,17 1:18 5.3 Satisfactory for normal compression up to 4MPa 25 – 35 4.17 to 5.83 1:16 5.9 Best soil for compression as
low as 2MPa
35 – 45 5.83 to 7.5 1:14 6.7 Satisfactory soil for compression as low as 2MPa
45 – 55 7.5 to 9.17 1:12 7.7
Fair soil for compression even lower than 2MPa but of low production pace due to sticking Characteristics (high clay content).
55 – 60 9.17 to 10 1:10 9.1
Poor soil; may need blending to reduce sticking or may need more Cement thus more expensive. Acceptable only when no alternative.
After measurement of fractional distribution of the soil, its linear shrinkage and selection of
appropriate ratio (cement to soil – C: S), the final stage is to produce trial bricks; at least ten
blocks from each soil batch. This is used to verify appropriateness of the soil for stabilisation
using the proposed soil to cement and water to cement ratios (Table 2.3). The following
observations to be made:
• The mixing process: if it is difficult, it indicates too high a clay content in the mix.
The soil requires modification, either by the addition of extra cement or by blending with
sandier soil.
• The rate of breakages on carrying the fresh bricks to their curing place. Too high (>
10%) a rate indicates there is too little clay in the mix.
• Crack developments, warping and any significant shrinkage during the first three days
of curing. If this is too severe, indicates a too-high clay content that may require either sand
blending or addition of extra cement.
• Testing the compressive strength at three, seven and fourteen days to check the
effectiveness of stabiliser (minimum strength after 14 days >1MPa). The test depends on the
availability of a suitably-equipped laboratory and demands of the project Gooding (1993).
The above quality control checks normally will continue for the whole period of production
for every fresh soil batch even if the soil is from one source. Less checking is required if the
soil is prepared all at one time.
2.9.1
SHRINKAGE BOX FOR SOIL TESTING
box dimensions (Table 2.3) are used in different parts of the world. The variation in the
suggested initial moisture content of soil test samples between one researcher and publisher
to another is also confusing, but we can clarify this by defining the two moisture conditions;
Liquid Limit (LL) and Optimum Moisture Content (OMC).
Table 2.3 Linear shrinkage moulds used in different parts of the world S/No Source Box shape Box size in mm
(Internal dimensions) Initial Moisture Content (MC) Where more Applicable 1 BS 1377 (1990) Half round 140 x 25Ø To LL Consistency Laboratory 2 CML-TLM1999 (2000) Half round 140 x 25Ø Within 1% of LL Laboratory 3 California Test 228 (2000) Polygon Tapered Top 127 x 19.05 x 19.05 Base 127 x 17.48 x 17.48 Wetter than LL Laboratory 4 Burroughs (2001) SAA (1977) Half round 250 x 25Ø a) 250 x 25Ø b) 135 x 25Ø it is used only with small soil sample
Near LL At the LL
Laboratory
5 Keefe (2005) Rectangular 600 x 50 x 50 OMC Site
6 Gooding (1993) Houben & Guillaud (1994) Stulz & Mukerji (1993) Adam & Agib (2001)
Rectangular Alcock shrink (box) mould 600 x 40 x 40 Near LL OMC OMC OMC Site 7 Norton (1997) Rectangular a) 600 x 40 x 40 b) 300 x 20 x 20 OMC (Controlled by drop test) Site
8 Wolfskill at el. (1963) Rectangular 127 x 19.05 x 19.05 (5” x ¾” x ¾”)
Slightly wetter than LL
Site and Laboratory
Liquid limit (LL) is moisture content in a mix that allows the mix to start flowing i.e. a change of consistency from plastic to liquid state.
Optimum Moisture Content (OMC) is the moisture content in a cementitious mix that contains enough water for cement to complete its hydration reaction (normally is 0.25 of
Usually the extra water is just enough to enhance densification (Wolfskill et al, 1963) on
compaction “Optimum moisture content at which a specified amount of compaction will
produce its maximum dry-density” (BS 1924-1:1990 clause 2.23).
The free water can be specified and verified by trial mix because of its dependence on various
soil characteristics;
• The type of aggregates (porous or impermeable)
• Shape of aggregates from round to sharp that affect workability of mix
• Type and amount of fines
From the definitions above, it is evident that LL and OMC are two different conditions for
the moisture content in a mix, meant for different purposes. They therefore cannot be
considered to be interchangeably, a wrong assumption used in the work of Keefe (2005),
Houben & Guillaud (1994), Adam & Agib (2001), Norton (1997), Stulz and Mukerji (1993) (Table 2.2). OMC is a proper mix consistency for brick production (Hydraform Manual,
2004) that can be checked by simple field drop test; if the soil ball breaks into few (4-6)
lumps then the water content is right (near to OMC).
However the author agree with BS 1377:1990, Burroughs (2001), Gooding (1993) and
Wolfskill et al. (1963) that the moisture content (Table 2.2) at the start of a linear shrinkage test should be near the LL (“This moisture content is not critical to within a few percent” BS
1377-2: 1990 clause 6.5.4.2 NOTE), with the aim of checking the soil plasticity and getting a rough idea of how much stabiliser is required to modify the soil for safe use in severe