soil and rock properties
DESCRIPTION
Soil and rock engineering properties workbookTRANSCRIPT
CHAPTER 2
SOIL AND ROCK PROPERTIES
June 2004
These sheets give an outline explanation of which properties are relevant for various types of problem, and which sampling and testing methods are generally suitable. In addition, typical examples of values are given, along with correlations that allow properties to be estimated from other measured values.
Development Need to check Properties needed Insitu tests Laboratory tests Sampling
August 2007
PRO
PERTIES, TESTS A
ND
SAM
PLES NEED
ED FO
R A
PRO
JECT
Clay: Hand vane, UU triaxial: U100 in boreholes, core cutter or U38 inhand penetrometer 3x38mm if not gravelly in trial pits.
Will it stand up? PI for classification CPT in soft ground 100mm multistage if gravelly Disturbed/bulk for PI, grading(short term stability LL, PL, PI (sometimes grading)is critical). Sand, gravel: SPT None normally( but standpipe None, normally
Building foundations Angle of shearing (location of water table useful)(includes, pad may be important)foundations, floor Clay: Oedometer U100 in boreholesslabs, rafts, piles). Coeff. of vol. compress., Core cutter in trial pits
How much will it settle?Sand, gravel: SPT None normally (but standpipe NoneSPT N value (water table location) useful)
Will it shrink/swell?Usually PI, grading. May None LL, PL, PI, grading Disturbed.do swelling potential test. (possibly swelling potential test) (Poss. U100, core cutter for swell pot.)
What pavement thickness?CBR Mexiprobe, insitu CBR CBR Usually bulk samples for lab. CBR
Can take samples in CBR mouldsIs subgrade frost PI, grading, frost heave None LL, PL, PI, grading.
Normally disturbed, bulk samplessusceptible? value. (rarely, frost heave test)
Clay: None, normally CU or CD triaxial U100 in boreholesSlopes (natural, cutting Effective strength stress but location of water table 3x38mm if not gravelly core cutter or 38mm tubes in trial pitsand embankment slopes) Will it stand up? may be critical 100mm multistage if gravellyand retaining walls. (long- term stability) Sand, gravel: SPT Can use slow shear box tests on Normally none, but bulk samples
Angle of shearing Location of water table recompacted samples (need large shear needed if recompacted shear box testsusually critical. box on gravel) required.
Will slopes stand up? All as for slope stability, above.(slope stability failure)If weak subgrade, will it Properties of subgrade soils, all as for short-term foundation stability, above.
Embankments. suffer subgrade failure?(foundation failure)How much will it settle? Consolidation beneath embankment, all as for settlement of foundations, above. Sheet
(consolidation beneath andwithin embankment) values.
Natural moisture content, None LL, PL, PI, grading Bulk (large samples required, especiallyGeneral earthworks. PI, optimum moisture Standard compaction, sometimes with if CBR values taken or sample is very 1
2-1
content. CBR at each compaction point. stony.Seepage problems (Seep- Coefficient of permeability Usually Falling head tests Permeameter testing U100 in clays (core cutter in trial pits) of
age into excavations, from What will be the rate of (vertical, horizontal) in boreholes, standpipes. Permeability from triaxial samples Bulk in sands and gravels.soakaways, through flood flow? Constant head tests if rate Grading, then use Hazen's formula (get 1
protection banks, etc.). of flow too fast. poor correlation).
Shear strength, s or cu
resistance, f
None normally but cv sometimes obtained from field permeability.mv, (+cv for rate of consol.)
Road and car park pavements (see also general earthworks).
parameters, c', f'
resistance, f.
Consolidation within embankment is more tricky, as it depends on embankment materials and compaction - may need to estimate mv
Is the material on site suitable for earthworks?
DENSITY 2-2June 2004 Sheet 1 of 2
Measures:
Used for:
Points to note:
Tests: there is a wide variety of methods, depending on circumstances, as indicated below.
1.
2.
3.
g =
4. The density of compacted samples is taken as part of standard compation tests and CBR tests.
Typical values
Natural density BS compacted densityMaterial Bulk density* Dry density Dry density Opt. m/c
(%)Sand and gravel: very loose 1700-1800 1300-1400
loose 1800-1900 1400-1500medium dense 1900-2100 1500-1800dense 2000-2200 1700-2000very dense 2200-2300 2000-2200
Poorly-graded sands 1700-1900 1300-1500 1500-1800 0-7Well-graded sands 1800-2300 1400-2200 1700-2100 0-8Well-graded sand-gravel mixtures 1900-2300 1500-2200 1800-2200 5-10
Clays: unconsolidated muds 1600-1700 900-1100soft, open-structured 1700-1900 1100-1400typical, normally consolidated 1800-2200 1300-1900boulder clays (Glacial Till) 2000-2400 1700-2200
Compacted sandy clays - - 1800-2200 15-30Tropical red clays 1700-2100 1300-1800 1400-2100 20-40Soil solids 2.65 for quartz; 2.64-2.71 for calcarious sand; and 2.67-2.73 for clay minerals.Notes: 1. * assumes saturated or nearly saturated conditions.
the density of a soil mass or sample. This is usually the natural site density but it may also be the compacted density of fills or embankments, the maximum density obtained from compaction testing, or any other density measurement.
many calculations including ultimate bearing capacity of foundations, settlements due to earth pressures from fills and embankments, and slope stability calculations. It is also needed to estimate earth pressure at depth to select an appropriate mv value from consolidation test results, and to specify appropriate test pressures for consolidation, triaxial and shear box testing.
density may be measured as 'bulk density', which includes the contributions from both the soil solids and any contained water, or as 'dry density', which ignores the contribution of the water. Specifying a dry density does not imply a soil is dry. Density depends on the density of the soil solids, the amount of voids in the soil and (for bulk density) the amount of water contained in it. The theoretical relationships between these values are given in sheet 2.
Insitu tests are carried out for earthworks and highways testing using a nuclear density meter or sand replacement method. Alternatives include the water replacement method and core cutters (see below).
Laboratory tests on clays. Basically, a sample is cut to standard dimensions, typically using a cutter of some kind. This could be a core cutter, the 38mm diameter tubes used to prepare triaxial test specimens, the ring used for oedometer tests or the sample cutter used for small shear box tests.
There are no laboratory tests to measure insitu densities in sands because it is virtually impossible to obtain undisturbed samples. An idea of insitu density in boreholes may be obtained by carrying out maximum and minimum density tests and obtaining relative density, D, from SPT N values. Insitu density, g, is then estimated from:
gmax.gmin
gmax - D(gmax - gmin)
(kg/m3) (kg/m3) (kg/m3)
2. To convert to kN/m3, divide by 1000 and multiply by 9.81 (or, roughly, just divide by 100).
DENSITY 2-2June 2004 Sheet 2 of 2
THEORETICAL RELATIONSHIPS BETWEEN DENSITY, POROSITY AND VOIDS RATIO
The model soil sample
Volumes: Weights:
Gas
Waterwhere
Solid
S is the degree of saturation,= (volume water) / (volume voids)= 0 for a dry soil, 1 for a saturated soil.
By definition:
Voids ratio, e = (volume voids) / (volume solids)
Porosity, n = (volume voids) / (total volume) = e/(1+e)
Hence, re-arranging the above expression e = n/(1-n)
(1)
Density relationships
(2) (1+e)
(where S=1 for a saturated soil) (1+e) (1+e)
(3) (1+e)
Comparing bulk and dry densities from equations (2) and (3) above:
g= =
= (1 + m) (1+e)
i.e.
Maximum theoretical density
Consider a sample of soil in which the soil solids, liquid and gas have been separated out, as shown. For convenience, assume that the sample chosen has a unit volume of solids and a volume of voids, e, as shown in the diagram.
Wg = 0
Ww = density x volume = gw.S.e = m.Ws = m.Gs.gw gw is the density of water
gs is the density of solidsWs = density x volume Gs is the specific gravity of the solids = Gs.gw.1 by definition, Gs = gs/gw so gs = Gs.gw
= Gs.gw
Moisture content, m = (wt. water) / (wt. solids) = gw.S.e/Gs.gw = S.e/Gs
Hence, re-arranging, S.e = m.Gs, and, for a saturated soil (S=1), e = m.Gs
Dry density, gd = (wt. solids)/volume = G s.g w
Bulk density, g = (wt. solids + wt. water)/volume = G s.g w + g w.S.e = (G s.+ S.e)g w
Also, since the weight of water is m.Gs.gw, g = G s(1+ m)g w
G s(1+ m)g w x (1+e) G s(1+ m)g wgd Gs.gw Gs.gw
g = gd(1 + m)
For a given moisture content, the maximum bulk density that can be achieved is when all the air has been driven out of the sample and the soil is saturated.
From equation (2), above, for a saturated soil (S=1), e = m.Gs. Substituting this into equation (2) for dry density gives:
Dry density, gd = G s.g w(1+m.Gs)
This is the maximum theoretical density that can be achieved for a given moisture content, and can be used to plot the zero air voids line on density vs. moisture content plots for standard compaction tests. If any test results indicate densities higher that this, they should be regarded as suspect.
S.e
1
e
1+e
2-3April 2008 Sheet 1 of 2
Measures:
Used for:
Tests: direct: 1. on site, hand penetrometer (stiffer clays) or hand vane (softer clays).2. unconsolidated undrained triaxial (UU) - usually 3x38mm samples if is not gravelly,
- 100mm multistage if clay is gravelly.3. small shear box (quick loading) may be used but this is not usual (unless residual strength is required).
indirect: 1. standard penetration test (SPT) in conjunction with PI (see correlations below).2. dynamic probe (usually heavy dynamic probe HDP) (correlations given on a separate sheet).3. static cone (correlations given on a separate sheet).4. based on correlations with liquidity index (see correlations below).
Correlations:1. Correlation with SPT N values
Stroud and Butler present the following graph with measured values for various soils, and a proposed trend.
In using this correlation it should be remembered that it relates to over-consolidated clays (see comments on sheet 2).
UNDRAINED SHEAR STRENGTH OF CLAY - s or cu
the short term strength of clay soil, before water pressures have had time to re-adjust after loading. Because of this, the combined soil and pore water pressure response to the load can be considered together, so there is no need to separately consider pore water pressures when doing calculations.
calculating foundation bearing pressures and pile bearing capacities. The ground beneath foundations increases in strength as it consolidates, so the short-term response is the most critical (if ultimate bearing capacity failure does not occur soon after a foundation is fully loaded, it is unlikely to occur later).
Reference: Stroud and Butler, The Standard Penetration Test and the Engineering Properties of Glacial Materials, 1975 Conf. of the Midlands Geotechnical Society.
Values of cu may be obtained from the trend line given by Stroud and Butler, but this is significantly higher than many values, especially low plasticity Boulder Clay (Glacial Till). An alternative trend, shown on the graph, is therefore suggested. The Stroud and Butler trend could be used where conservative SPT values are taken (say, lower quartile), but for individual results or average SPT values, the suggested alternative trend would be safer. Given the Boulder Clay values, a value of not more than 5 should be used for this material. If this sheet is viewed on the computer screen, values of cu/N may be obtained by entering PI values in the highlighted box below.
0 10 20 30 40 50 60 702
3
4
5
6
7
8
Stroud and Butler cu-N relationship
Boulder ClayKeuper MarlFlinzUpper LiasLondon ClayKimmeridge ClayBracklesham BedsSunnybrook TillOxford ClayW'ch & R'ding BedsStroud & Butler trendAlternative trend
SPT N values (blows/300mm)
cu/N
(kPa
)
Example: using the recommended trend, for PI = 10 13.3 (Stroud and Butler's trend line)or = 7.0 (Suggested trend line)
If this sheet is viewed on screen, values of cu/N may be obtained by entering PI values in the highlighted box below.
cu/N =
2-3June 2004 Sheet 2 of 2
2. Correlation with liquidity index
References: Skempton and Northey, The Sensitivity of Clays, Geotechnique, vol. 3, 1952Carter and Bentley, Correlations of Soil Properties, Pentech Press, 1991.
where k is a factor usually taken as 1 but may vary between 1 and 2; andand LI is the liquidity index, defined as:
LI = m - PL where m = natural moisture content PI PL = plastic limit
PI = plasticity index
The effects of overconsolidation
• there was originally a much greater depth of overburden but this has been reduced by erosion;• the soil was buried beneath an ice sheet or glacier during a previous ice age (hence the overconsolidation of glacial tills);•
Typical values:
By definition: Very soft = 0 - 20 kPaSoft = 20 - 40 kPaFirm = 40 - 75 kPaStiff = 75 - 150 kPa
Very stiff = 150 - 300 kPaHard = over 300 kPa
UNDRAINED SHEAR STRENGTH OF CLAY - s or cu
Liquidity index values may also be used to estimate undrained shear strength using correlations presented by Skempton and Northey (1952), and by Carter and Bentley (1991), although the values obtained are not always accurate reflections of actual site values. These may be approximated to:
cu = k x 102(1-LI)
The Skempton and Northey correlation was given for re-compacted soils, and is not very precise, especially if used for natural soils (See Carter and Bentley, ref. 2, above), but gives some indication of strength in the absence of more reliable information. Because of this lack of accuracy, it is not recommended that the correlation be used if more reliable data, such as direct measurement or SPT values, is available.
Overconsolidation of clays occurs when the clay has been subjected to higher confining pressures than exist at present. This can occur for a variety of reasons:
previous drying, or partial drying, of the soil has resulted in reduced pore water pressures (including the meniscus effect which can produce very high negative water pressures), resulting in a corresponding increase in inter-particle (effective) stress, which has a similar effect to increasing overburden - this effect often results in alluvial deposits with a firm or stiff crust underlain by soft material.
EFFECTIVE STRESS STRENGTH PARAMETERS OF CLAYS 2-4April 2008 Sheet 1 of 1
Measures:
Used for:
Tests:direct: consolidated drained triaxial (CD)
consolidated undrained triaxial with pore pressure measurement (CU)- usually 3 x 38mm samples if soil is not gravelly,- 100mm multistage if soil is gravelly.
indirect: Plasticity index (PI), used with standard correlations (see below)
Correlations
The effective angle of shearing resistance may be estimated from plasticity index as indicated below.
1. Reference: BS 8002 : 1994, Earth Retaining Structures, BSI. PITable 2 of the Standard gives the correlation shown on the right. 15 30
20 2825 2730 2540 22
2. 50 2060 15
the strength parameters c' (effective cohesion) and f' (effective angle of shearing resistance), which give the response of the soil skeleton to stress. Groundwater pressures are considered separately in effective stress calculations.
problems where long-term conditions are (or may be) more critical, such as slope stability and retaining wall problems. Peak values are normally used except where movement has already taken place, in which case residual values are used.
small shear box (with a slow rate of loading) may be used, especially where residual strength parameters are required.
f'crit
It should be noted that this gives the critical angle of shearing resistance, which will be valid for both normally- and over-consolidated soils, but will be slightly lower than the peak value.
Reference: Gibson, Experimental Determination of the True Cohesion and True Angle of Internal Friction in Clays, Proc. of the Third Int. Conf. on Soil Mechanics and Foundation Engineering, Zurich, 1953.
Values may be obtained from the graph below. The residual values should be true for clay in any state of consolidation but peak values will be affected by the consolidation history (see 2-3 sheet 2), and the correlation cannot take account of previous overconsolidation; nor does it give any indication of a possible effective cohesion value.
0 10 20 30 40 50 60 70 80 90 1000.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Relationship between PI and drained angle of shear strength
Plasticity index (%)
An
gle
of
shea
rin
g r
esis
tan
ce (
deg
)
fresid = true angle of inter-nal frictionfres = true angle of in-ternal friction
fd = drained angle of shearing resistance
Example: for PI = 20 28.9 23.2fd = and fres =
0 10 20 30 40 50 60 70 80 90 1000.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
Relationship between PI and drained angle of shear strength
Plasticity index (%)
An
gle
of
shea
rin
g r
esis
tan
ce (
deg
)
fresid = true angle of inter-nal frictionfres = true angle of in-ternal friction
fd = drained angle of shearing resistance
SHEAR STRENGTH OF SAND AND GRAVEL 2-5April 2008 Sheet 1 of 1
Measures:
Used for:
Tests:direct:
indirect: 1. standard penetration tests (SPTs) in boreholes (see correlation below).2. dynamic probe (usually heavy dynamic probe HDP) (correlations given on a separate sheet)3. static cone (correlations given on a separate sheet)
Correlations:
(See Tomlinson, etc., for handy reference.)
SPT PHIN VALUE (DEG)
0 274 28
10 3015 3220 3325 3530 3635 3740 3945 4050 4155 4260 43
Example: for SPT N value = 18.5 32.7 º
Comparing descriptions of relative density:
Description N value rangeVery loose 0 4 27 28Loose 4 10 28 30Medium dense 10 30 30 36
the strength parameter f (angle of shearing resistance), which give the response of the soil skeleton to stress. Since sands and gravels have a high permeability, pore water pressures due to loading are dissipated rapidly and total and effective stresses are equal. Also, there is normally no cementing or chemical bonding of the grains so shear strength is purely frictional, with no cohesion. Groundwater pressures are considered separately in stress calculations.
long and short term problems including ultimate bearing capacity of foundations, slope stability and retaining wall problems. Peak values are normally used except where movement has already taken place, in which case residual values are used.
shear box tests can be carried out on remoulded samples but no direct measurement of the materials in their insitu state is possible because undisturbed samples cannot be obtained by normal site investigation methods. For sands, a small shear box (60mm square) is suitable but for gravel a larger 300mm square box (expensive), is needed.
Reference: Peck, Hanson and Thornburn, Foundation Engineering, 2nd ed., 1974, John Wiley and Sons.
The table and graph below, based on a correlation by Peck, Hanson and Thornburn, may be used to obtain φ values from N values. Corrections for depth may be (cautiously) applied to N values as described in 2-7. It is recommended that, if an SPT correction factor is used, the Peck, Hanson and Thornburn curve be chosen.
The table and graph reproduce Peck, Hanson and Thornburn's graph using the correlation, φ =
25.5 + 0.36N + 0.0014N2, obtained by curve fitting
f =
f range (º)
0 5 10 15 20 25 30 35 40 45 50 55 6026
28
30
32
34
36
38
40
42
44
CORRELATION OF PHI AND SPT N VALUE
SPT N value
Angle
of s
hear
ing re
sista
nce,
phi
- deg
Dense 30 50 36 41Very dense 50 + 41 +
SHEAR STRENGTH OF WEAK ROCKS 2-6June 2004 Sheet 1 of 1
Strength/Shear Approx. consistency
strength (kPa) N value description Grade Breakability Penetration Scratch
60,000
Rock
Difficult to break Cannot beStrong A against solid object scratched with
with a hammer. a knife.
25,000780
20,000
Can be broken Can just beModerately against solid scratched
10,000600 strong B object with with a knife.
a hammer.
6,250430
6,000 400Moderately weak
Broken in handC by hitting with
hammer.
2,500240
2,000 200 Broken by leaning NoD on sample with penetration
hammer. with knife.
1,000Weak
PenetrationE Broken by hand. to about 2mm
100 with knife.
600 60090
F Easily broken
Wea
ther
ed to
soi
l
by hand.
30050
20040 Penetrated by
Very stiff thumb nail and about 15mm
15030 with knife.
10020
StiffIndented by
thumb.
7515
60Firm
10
408
Soft
20 204
Very soft
References/notes:1) Geol. Soc. Working Party Report (1970) and CP2004 (1972) 2) Grades and shear strengths for rock refer to intact specimens.
The table below summarises strengths for weak rocks, including rocks weathered to clay, based on descriptions, simple tests and SPT N values.
Scratched with knife. Can just be scratched
with fingernail.
Very weak or
hard
Penetration to about 5mm with
knife.
Penetrated by thumb with
effort.
Easily penetrated by thumb.
except that the designation 'hard' has been given a separate However, the N value is an insitu test and includes some effectidentity which is analogous to very weak for materials of the discontinuities. For cohesive soils, the correlation betweenclassified as rock. N values and insitu strength assumed is that given by Stroud
(1974) for clays of low plasticity.
CORRECTION FACTORS FOR STANDARD PENETRATION TESTS 2-7June 2004 Sheet 1 of 1
Correction for overburden pressure
Overburden pressure, P - kPa (use bulk density above water table, submerged density below) 70
Peck, Hanson and Thornburn 1.12Skempton (fine grained) Factor = 2/(1+0.01P) = 1.18Skempton (coarse grained) Factor = 3/(2+0.01P) = 1.11Skempton (overconsolidated) Factor = 1.7/(0.7+0.01P) = 1.21
Correction for silts
Various researchers have shown that SPT N values tend to underestimate relative density at shallow depth, and have suggested correction factors for estimating relative density and shear strength parameters. Correction factors are also recommended by some for SPTs carried out in silts and fine sands below the water table, where the build-up of pore water pressures during the test is said to give unrealistically high N values. The application and values of correction factors for SPT N values is discussed below.
Overburden correction factors that have been proposed by Skempton and by Peck, Hanson and Thornburn are given in the graph below. It can be seen that they are broadly similar.
There is not complete agreement about the validity of applying depth correction factors, but if they are applied, the Skempton or the Peck, Hanson and Thornburn factors given in the graph above may be used. The formulae for these are given in the table below, which may be used to calculate factors if the manual is used on-screen. BS 8002 recommends factors by Thornburn, which are also shown.
The use of factors is recommended by Tomlinson(1) but, since the effect for shallow foundations is to increase calculated bearing capacity considerably, they should be applied with caution, especially the Thornburn values recommended in BS 8002, which are higher than those given by most other researchers.
Factor = 0.77*log10(2000/P) =
In silts and fine sands, which have a fairly low permeability compared with gravels and coarse sands, pore water pressures can build up during driving of the sampler, increasing soil resistance. Tomlinson(1) and Terzaghi and Peck(2) recommend that for N values exceeding 15, the following correction factor should be applied:
Ncorrected = 15 + ½(N - 15)
0.00 0.50 1.00 1.50 2.00 2.50 3.000
50
100
150
200
250
300
350
Peck,Hanson,ThornburnSkempton - fine grainedSkempton - coarse grainedSkempton-overconsol.Thornburn
Correction factor (corrected N value = measured N value x factor)
Ove
rbur
den
pres
sure
(kP
a)
This correction is in addition to any overburden correction factors that are applied.
(1) Tomlinson, M. J., Foundation Design and Construction, Longman, 5th ed., 1986.(2) Terzaghi, K and R. B. Peck, Soil Mechanics in Engineering Practice, Wiley, 2nd ed., 1967.
INTERPRETING DYNAMIC CONE TEST RESULTS 2-8June 2004 Sheet 1 of 1
GRANULAR SOILS (sands and gravels)
SPT = M(p) * F(p) * A(s) * P(s) x probe blowsM(s) * F(s) * A(p) * P(p)
where
M(p) = mass or weight of the probe (= 50kg for DPH*)
M(s) = mass or weight of the SPT monkey (= 65kg)
F(p) = fall of the monkey for the probe (= 0.5m for DPH) P(p) = penetration per blow count for the probe (= 100mm)
F(s) = fall of the monkey for the SPT (= 0.76m) P(s) = penetration for the blow count for the SPT (= 300mm)
Thus, for a DPH:SPT N value (blows/300mm) ≈ 2 x DPH probe blows (blows/100mm)
COHESIVE SOILS (clays)
Basic cone details (given information)Mass of hammer, M (kg) (= 50kg for DPH) 50Mass of anvil + guiding rods, M" (kg) (= 18kg for DPH) 18Mass of driving rods, R (kg/m) (= 6kg/m for DPH) 6Height of fall of hammer, h (m) (= 0.5m for DPH) 0.5Depth of penetration over blow count, L (= 0.1m for dynamic cones, =0.3m for SPT) 0.1Diameter of cone (mm) (=43.7mm for DPH) 43.7
0.0015
Individual test data (given information)Depth of test (m) 5Number of blows per 100mm (N) 3
1.2
CalculationsStep 1: calculate average penetration per blow (e = L/N) 0.033Step 2: calculate mass of anvil + extension rods + guiding rods (= M" + (R x test depth)) 48
49052502
Step 5: Calculate cohesion (kPa):104114
3518
Dynamic probes come in a variety of cone sizes, hammer weights and hammer drops, but modern probes are generally the Heavy probe (DPH) and the Super-Heavy (DPSH) probe. The Super-Heavy probe has the same dimensions as an SPT, and interpretation is the same as for SPT data, taking the number of blows for each 300mm penetration. For other size probes, the correlations used below may be used to interpret results.
For probes in granular soils, the equivalent SPT values are simply calculated on the proportionate hammer energy, probe area and driving length.
A(p) = end area of the probe (= p x 43.72/4 = 1500mm2 for DPH)
A(s) = end area of the probe (= p x 502/4 = 1960mm2 for SPT)
Estimates of the shear strength of clays may be made from dynamic probe tests using correlations given in a paper, Dynamic Probing and its use in Clay Soils, by A. P. Butcher, K. McElmeel and J. J. Powell (all of BRE), published in Advances in Site Investigation Practice, March 1995, by Thomas Telford. The method is given in the example below. If this sheet is viewed on the computer screen it may be used to calculate shear strength values by typing the appropriate values in the highlighted boxes.
Hence end area of cone (m2) ((=p x D2/4)/106)
Sensitivity of clay, S, if known (sensitivity = insitu shear strength / remoulded shear strength)
Step 3:calculate unit point resistance, rd (rd = (M.g.h)/(1000.A.e))Step 4: calculate dynamic point resistance, qd (qd = M/(M+M").rd)
Any clay of specified sensitivity (= 0.045(qd/S) + 10)Stiff clay (= qd/22)Soft clay (= (qd/170) + 20)
Step 6: calculate equivalent SPT value for clay (NSPT = 8N - 6)
This method may also be used to estimate shear strength results from SPT N values, as a check on values obtained by Stroud and Butler's correlations, especially for normally consolidated clays. However, since the results are dependent on clay sensitivity, which is not generally known, or on classification of the clay into 'stiff' or 'soft', the shear strength values obtained by this method should be viewed with some caution.
INTERPRETING STATIC CONE TEST RESULTS 2-9June 2004 Sheet 1 of 2
THE STATIC CONE AND TEST RESULTS
SOIL CLASSIFICATION
The most commonly-used static cone comprises a 34.7mm diameter 60º cone which is pushed into the ground at a constant rate. Variations exist, and reference should be made to standard texts on cone penetration testing for details. The booklet, Guide to cone penetration testing on shore and near shore, produced by Fugro, gives a useful quick reference. Both cone tip resistance and sleeve friction are measured as the cone is pushed in, and results are given as a graph, as indicated on the example shown below.
A variation of the method, suitable for saturated clays, is the piezocone test, which measures pore water pressures separately from soil pressures. Using a piezocone, penetration may be stopped and a pore pressure dissipation test carried out, to measure the fall-off of pore water pressure with time. This can be used to obtain compressibility properties. The test can be time-consuming, therefore expensive.
Results may be used in three ways: to identify the types of soil present; to estimate geotechnical parameters; and to produce designs directly from the cone data.
The type of soil present may be identified by a combination of the cone resistance and the friction ratio (the ratio of sleeve resistance to cone resistance, expressed as stresses - MPa). Several correlations have been published, notably by Meigh(1) and Lunne et al(2). The chart opposite shows that given by Meigh.
This interpretation is normally carried out by the specialist site investigation contractor carrying out the testing.
(1) Meigh A. C., Cone penetration testing: methods and interpretation, Butterworths, London, 1987, ISBN 0 408 02446 1.(2) Lunne T, P. K. Robertson and J. J. M. Powell, Cone penetration testing in geotechnical practice, Spon, London, ISBN 0 419 23750 X.
INTERPRETING STATIC CONE TEST RESULTSJune 2004 Sheet 2 of
ESTIMATING BASIC SOIL PARAMETERS
Undrained shear strength of clay
where
= 20 for overconsolidated clay (e.g. London Clay)
Coefficient of volume compressibility of clay
where
Relative density of sand and gravel
Equivalent SPT values
Static cone test results may be used to give approximate estimates of some basic soil parameters. The following correlations are suggested in the catalogue produced by Fugro(1).
A preliminary estimate may be obtained from a correlation given by Fugro(1):
cu = qc/Nk qc = measured cone resistanceNk = 17 or 18 for normally consolidated clay soils
A rough approximation can be obtained for normally consolidated clays and lightly overconsolidated clays and silts up to firm consistency (qc less than about 1.2MPa), using a correlation by Meigh(2):
mv = 1/aqc a = a coefficient, between 2 and 8, which depends on the overconsolidation ratio.
qc = measured cone resistance
Compressibility properties may by more accurately determined from pore pressure dissipation tests. Appropriate texts should be consulted for test methods and interpretation of results.
A rough estimate of relative density may by obtained from the chart on the right, based on the work of Robertson and Campanella(3).
A number of studies have been carried out on CPT-SPT correlations. The chart on the left shows a correlation by Robertson et al(3).
(3) Robertson K. P. and R. G. Campanella, Interpretation of cone penetration tests, Canadian Geotechnical Journal, 1983.
2-92
= 20 for overconsolidated clay (e.g. London Clay)
Static cone test results may be used to give approximate estimates of some basic soil parameters. The following correlations are
A rough approximation can be obtained for normally consolidated clays and lightly overconsolidated clays and silts up to firm consistency
= a coefficient, between 2 and 8, which depends on the overconsolidation ratio.
A number of studies have been carried out on CPT-SPT correlations. The chart on the left shows a correlation by
CALIFORNIA BEARING RATIO 2-10April 2008 Sheet 1 of 1
Typical values
High water table Low water tableSoil PI (%) Construction conditions Construction conditions
Poor Average Good Poor Average GoodThin Thick Thin Thick Thin Thick Thin Thick Thin Thick Thin Thick
Heavy clay 70 1.5 2 2 2 2 2 1.5 2 2 2 2 2.560 1.5 2 2 2 2 2.5 1.5 2 2 2 2 2.550 1.5 2 2 2.5 2 2.5 2 2 2 2.5 2 2.540 2 2.5 2.5 3 2.5 3 2.5 2.5 3 3 3 3.5
Silty clay 30 2.5 35 3 4 3.5 5 3 3.5 4 4 4 6Sandy clay 20 2.5 4 4 5 4.5 7 3 4 5 6 6 8
10 1.5 3.5 3 6 3.5 7 2.5 4 4.5 7 6 >8Silt* - 1 1 1 1 2 2 1 1 1 1 1 1Sand (poorly graded) - 20Sand (well graded) - 40Sandy gravel (well graded) - 60*estimated assuming some probablilty of material saturatingNotes: 1. A high water table is 300mm below formation or sub-formation
2. A low water table is 1000mm below formation or sub-formation3. A thick layered construction is a depth to subgrade of 1200mm4. A thin layered construction is a depth to subgrade of 300mm
CBR and shear strength
where c is the soil cohesion
B is the foundation width
CBR = ≈ 0.1c6900
Other correlations
California Bearing Ratio (CBR) values may be obtained from insitu or laboratory testing or may be assessed from dynamic probe measurements. Approximate values may be estimated from the material type and other tests. This sheet gives typical CBR values for various material types, and correlations between CBR and other properties, so that values can be estimated.
The table below gives values recommended in Highways Agency Advice Note (73/06) Design Guidance for Road Pavement Foundations (linked to HD26/06), which also gives correlations for CBR, vane strength, plate bearing value and dynamic cone resistance against stiffness, and should be consulted for geotechnical work related to pavements.
The CBR test can be thought of as a bearing capacity problem in miniature, the plunger acting as a foundation. Terzaghi's bearing capacity equation for a circular foundation is:
qu = 1.2cNc + p0Nq + 0.3gBNg
p0 is the effective overburden pressure at founding depthg is the bulk density
Nc, Nq and Ng are Terzaghi's bearing capacity factors.
For a saturated clay in undrained conditions, the angle of shearing resistance is zero, which gives Nc = 5.14 (2+p), Nq = 1 and Ng = 0. Thus, the third term in the bearing capacity equation disappears and, since the overburden pressure is equal only to the light pressure exerted by the surcharge weights used in the test, the second term also approximates to zero, giving,
qu = 6.2c
Using SI units, the CBR is 100% when the plunger load is 6900kN/m2 (for 2.5mm penetration), giving,
qu x 100
A large number of correlations have been proposed to relate CBR to soil type and to other soil properties. Some of these have been reviewed in Correlations of soil properties by Carter and Bentley (Pentech Press, now Wiley, 1991).
COMPRESSIBILITY AND STIFFNESS RELATIONSHIPS 2-11March 2009 Sheet 1 of 3
COEFFICIENT OF VOLUME COMPRESSIBILITY, COMPRESSION INDEX AND YOUNG'S MODULUS
The coefficient of compressibility is defined as shown below:
de . 1=
dh . 1dp dp
whereand de is the change of voids ratio due to a change in pressure, dp.or, whereand dh is the change in thickness due to a change in pressure dp.
1 .E
=
.
Coefficient of compressibility and coefficient of volume compressibility
de Thus:dp
TYPICAL VALUES OF COEFFICIENT OF VOLUME COMPRESSIBILITY
Type of clayDescriptive Coefficient of volume
termVery low
< 0.05compressibility
Stiff boulder clays, marlsLow
0.05 - 0.1compressibility
Medium0.1 - 0.3
compressibility
Normally consolidated alluvial clays such as estuarine deposits, and sensitive clays.High
0.3 - 1.5
The coefficient of volume compressibility, mv, gives a measure of the amount of compression that can be expected within a clay due to consolidation under loading. Since the stress-strain characteristics of soil are not linear, mv varies with loading, and the value used in settlement calculations should reflect the anticipated load range (typically overburden pressure initially, and overburden plus foundation pressure finally).
mv = 1+e1 h1
e1 is the initial voids ratio
h1 is the initial sample or strata thickness
For settlement calculations, a value of Young's modulus, E, is sometimes required (for calculations using elastic solutions, or finite element modelling, for instance). It can be seen from the equation above that mv is something like 1/E, except that in the consolidation test the specimen is laterally constrained, so compression of the specimen depends on Poisson's ratio, n. Thus, the relationship between mv and E is actually:
mv =(1+n)(1-2n)
(1-n)
The value of mv tends to decrease with increasing pressures, and to give a measure of compressibility that is less dependent on pressure ranges, the compression index, Cc, is sometimes quoted, where:
Cc =de e1 - e2
d(log p) log(p2/p1)
where e1 and e2 are the initial and final voids ratios, corresponding to initial and final pressures p1 and p2.
Comparing this with the expression for mv, it can be seen that mv can be obtained from Cc by:
mv =Cc log (p2/p1)
1 + e1 p2 -p1
The commonly-used coefficient of volume compressibility, mv, should not be confused with the (now little-used) coefficient of compressibility, ev, which is defined in terms of the change of voids ratio (rather than the change of volume) with pressure:
ev = ev = mv.(1 + e)
Rough estimates of values of mv for common UK clays may obtained from the table below.
compressibility (m2/MN)
Hard, heavily overconsolidated glacial boulder clays, stiff weathered rocks (e.g. completely weathered mudstone) and hard clays.
Firm clays, glacial outwash clays, lake deposits, weathered marls, firm boulder clays, and normally consolidated clays at depth.
Normally consolidated alluvial clays such as estuarine deposits, and sensitive clays.compressibility
0.3 - 1.5
Highly organic alluvial clays and peats.Very high
>1.5compressibility
COMPRESSIBILITY AND STIFFNESS RELATIONSHIPS 2-11March 2009 Sheet 2 of 3
EFFECTS OF OVERCONSOLIDATION ON COMPRESSIBILITY
CORRELATIONS BETWEEN COMPRESSIBILITY OR STIFFNESS AND OTHER PROPERTIES
Coefficient of volume compressibility
1.35 PI where PI is the plasticity index (%)e is the voids ratio
PI = 20 e = 0.7 25 kPa 0.2762
PI = 50N = 3
0.00077
Compression index
Typical values of compression index are given in the table below. It may also be estimated from:
(Skempton 1944) Soilor Normally consolidated medium sensitive clays 0.2 - 0.5
Organic silt and silty clays 1.5 - 4.0Sensitive clays 1 - 4
Generally, clays become stiffer at higher pressures; that is, mv decreases. However, in overconsolidated clay (see 2-3 for explanation of overconsolidation), the clay will be stiffer than for a normally-consolidated clay up to the overconsolidation pressure, then will become less stiff (mv and Cc will increase), as it follows the virgin compression curve - see sketch on right. This can lead to underestimates of settlements if mv values for pressures below the overconsolidation pressure are used for pressures extending beyond it. However, this problem can be avoided by always ensuring that the mv value chosen in calculations reflects the pressure range that will be experienced by the soil in the ground.
A large number of correlations exist between compressibility or stiffness and other properties, some of which are given below. These should be used with caution, as there is generally a poor correlation between compressibility and other properties.
For normally consolidated clays, mv (in m2/MN) may be estimated from the relationship:
mv =2.3 (1+e).sv
sv is the effective vertical overburden pressure (kPa)For instance, for (insert values as required):
σv = then mv = m2/MN
For overconsolidated clays, stiff insensitive clays and soft rocks, mv (in m2/MN) may be estimated from values given by Stroud(1):
For instance, for (insert numbers as required):
then mv = m2/MN
(1) Stroud, M. A., The standard penetration test in insensitive clays and soft rocks, Proc. Europ. Symp. Pen. Testing, Stockholm, 1974.
This correlation should not be used for normally consolidated soils or sensitive clays, for which it may give unrealistically low values.
Cc = 0.007(LL-7) Cc
Cc = 0.01m
Virgin compression curve
Reconsolidation curve
Overconsolidation pressure
Consolidation pressure
Com
pres
sion
0 10 20 30 40 50 60
400
450
500
550
600
650
700
Plasticity index - %
1/(
mv
.N)
where LL = liquid limit (%) and m = moisture content (%). Organic clays > 4Peats 10 - 15
These correlations tend to give excessively high values of Cc (and mv), and should not be used if better estimates are available.
COMPRESSIBILITY AND STIFFNESS RELATIONSHIPS 2-11March 2009 Sheet 3 of 3
Stiffness of clays
Plasticityindex (%)
0 250010 170020 1200
For London Clay, the following relationships are commonly used: 30 80040+ 500
where
Stiffness of granular soils
Soil SPT correlations * CPT correlations *
E = 500(N + 15)
Sand (normally consolidated)
Sand (saturated) 250(N + 15)Sand (overconsolidated) 18,000 + 750N
E = 1,200(N + 6)Gravelly sand and gravel
Clayey sand and silty sandE = 320(N + 15)E = 300(N + 6)
* all values of E are in kPa.
Stroud and Butler (1975) present values of drained vertical stiffness, E'v, related to SPT N values and plasticity index. The table on the right is based on their suggested design line.
E'v/N
E'v = 300cu
E'v and Euv are the drained and undrained Young's moduli, respectively, and cu is the undrained shear strength.Euv = 500cu
Values are usually estimated from SPT N values or CPT cone values. A compendium of values, correlated by Bowles, (Foundation Analysis and Design) is given in the table below:
E = 2 - 4 qc
E = (15,000 - 22,000)logeN E = (1 + Dr2)qc
E = (35,000 - 50,000)log10NE = 6 - 30 qc
E = 600(N + 6) for N £ 15E = 300(N + 6) + 200N for N ³ 15
E = 3 - 6 qc
E = 1 - 2 qc
Dr = rel. density; qc = cone resistance
SECONDARY COMPRESSION 2-12June 2004 Sheet 1 of 1
de or dh/h =d(log t) d(log t) 1 + e
where de is the change in voids ratio, e, over time t; or dh is the change in thickness over a material of initial thickness h, over time t.
SoilOrganic silts 0.035-0.06Amorphous and fibrous peat 0.035-0.085Post glacial Swedish clay 0.05-0.07Organic clays and silts 0.025-0.055Varved clay 0.03-0.06River silt 0.04-0.075
Calculations of secondary compression are obtained by re-arranging the equations above:
or1+e
Secondary compression is volume change that takes place at constant effective stress - creep movements that occur after consolidation settlement has ceased. It is estimated from the coefficient of secondary compression, Ca, or the modified coefficient of secondary compression, Cae, defined as:
Ca = Cae = Ca
The process is not well understood, and the definitions of the coefficients take no account of the magnitude of the original loading that triggered it. Consequently, estimates are very approximate unless based on some field data, which is not usually available because of the length of time required to monitor secondary compression movements. The coefficient of secondary compression can also be estimated from extended consolidation tests, carried out at the stress ranges anticipated on site.
The coefficient of secondary compression is sometimes assumed to be related to the compression index, Cc - some typical values given by researchers are given in the table on the right. However, these are necessarily approximate since the correlation does not take account of the stress range.
Ca/Cc
Values may also be estimated from a relationship proposed by Mesri(1), shown on the graph on the left. Again, values are vague as the correlation does not take account of stress ranges imposed on the ground.
rc = Cae.H.log10(t2/t1) rc = C a. H.log10(t2/t1)
Where H is the thickness of the compressible deposit. For the purpose of the calculations the start time, t1, is assumed to be when primary consolidation has reached 90%, and the end time, t2, is the assumed life-span of the project.
PERMEABILITY 2-13January 2008 Sheet 1 of 1
Measures:
Used for:
Tests:direct:
indirect: estimated from grading curves using Hazen's formula.
Typical values
Typical values, related to soil type, may be roughly estimated from the table below.
Coefficient of permeability, m/s (log scale)
1
Permeability description:
Practically impermeable Very low Low Medium High
Drainage conditions:
Practically impermeable Poor Good
Soil types:
Clean gravels
The Hazen formula
The coefficient of permeability may be estimated from the Hazen formula:
whereand
Permeability and infiltration rate
the resistance to flow of water through soils. The coefficient of permeability represents the rate of flow through 1m2 of soil under a pressure gradient of 1m head per m length, along the flow direction.
estimating rates of flow through soil, typically for seepage losses, pumping capacity requirements or estimates of contaminant movement.
laboratory constant head or falling head permeability tests - good control over flow conditions, so the relevant permeability formula can be applied with confidence, but test specimens are too small to be representative of the soil macro-structure.
field constant head or falling head tests in boreholes, standpipes, pits or trenches - flow geometry is uncertain, so permeability formula assumptions may not be entirely true, but the test includes the soil macro-structure.
10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1
Homogeneous clays below the zone of
weathering
Silts, fine sands, silty sands, glacial clay stratified clays Clean sands, sand
and gravel mixturesFissured and weathered clays and clays modified by
the effects of vegetation
k = C1D102 C1 is a factor of between 0.01 and 0.015 for k in m/s
D10 is the effective size (the 10% size), in mm.
This may be used to give a rough indication of permeability but it does not take into account the full grading, density or particle shape, so accuracy is limited.
There is no fixed relationship between the coefficient of permeability, k, and the infiltration rate, f, as defined by BRE and measured by the BRE soakaway test (BRE Digest 365 - see Geotechnical Manual Appendix 9B). However, comparison of flow rates using the infiltration rate formula with those using an equivalent permeability formula indicates that for water depths of between 1 and 3 times the average pit side length (½[length+width]), infiltration rate, f, is approximately equal to the coefficient of permeability, so that for practical purposes, the two values may be used interchangeably.
UNCONFINED COMPRESSIVE STRENGTH AND POINT LOAD INDEX 2-14October 2004 Sheet 1 of 1
The unconfined compressive strength of rocks can be estimated from point load index, using the relationship:
Size correlation chart for point load index by Broch and Franklin (1972)
sc = Kp.Is where Kp is normally taken as 24 but may vary from 15 to 50.and Is is the point load index (units are MPa)
When using this formula, Is should be corrected for size to the standard 54mm core diameter. This is done by reference to the chart below.
(The arrows shown on the chart indicate the method of use to give the correction factor from one core size to another - in the example, a point load index of 5 taken on a 35mm diameter core becomes 4 when corrected to a 50mm core.
CALORIFIC VALUE AND MASS LOSS ON IGNITION 2-15October 2004 Sheet 1 of 1
Susceptibility to combustion
Possible recommended precautions if susceptible
The following may be used as guidance for report text if the material appears susceptible to combustion.
• Inert material should also be used to backfill trenches to utilities, especially electrical cables, which may generate heat.
Mass loss on ignition
When dealing with some materials, especially colliery discard material, the susceptibility to combustion needs to be considered. ICRCL Guidance Note 61/84(12) states that material with calorific values below 2000kJ/kg is considered to be unlikely to burn, whilst material with values greater than 10,000kJ/kg are certainly combustible. Material with calorific value between 2000kJ/kg and 10,000kJ/kg should be considered potentially combustible.
In view of the above, it is recommended that precautions be taken to prevent heat sources reaching the tip material and to minimise the potential for it to burn. Suggested precautions are given in ICRCL Guidance Note 61/84(12) and Building Research Establishment Information Paper P2/87(13). These could include the following.
• The potential for combustion can be reduced by reducing the available air within the material (i.e. reducing the voids ratio). This can be achieved in areas to be filled with tip material by ensuring the fill is well-compacted. In areas where levels are to be reduced, exposing insitu tip material to the surface, insitu densities should be checked, and if it is found that the air voids content exceeds about 10% then consideration should be given to the use of deep dynamic compaction (heavy tamping).
• The area should be covered in a layer of inert material to keep surface heat sources away from it. This should be at least 1m thick in areas scheduled for development, to allow for the possibility of subsequent activity reducing this cover, but a thinner layer may be used in areas to be kept as open fields. The material used as covering should be non-combustible and chemically compatible with the proposed development, but otherwise there are no special restrictions on the material that can be used. Cover provided by foundations and pavements could also be considered to form part of the cover.
Mass loss on ignition tests are sometimes carried out as a cheaper alternative to calorific value tests. From information collated by WYGE Cardiff, there appears to be a strong correlation between mass loss on ignition and calorific value, as shown in the chart below, which may be used to estimate calorific values from mass loss on ignition values.
0 5 10 15 20 25 30 35 40 45 500
1000
2000
3000
4000
5000
6000
Bedwas tip
Linear (Bedwas tip)
Wingfield tip
Mass loss on ignition (%)
Cal
orifi
c va
lue
(kJ/
kg)
0 5 10 15 20 25 30 35 40 45 500
1000
2000
3000
4000
5000
6000
Bedwas tip
Linear (Bedwas tip)
Wingfield tip
Mass loss on ignition (%)
Cal
orifi
c va
lue
(kJ/
kg)
SPARESheet 1 of
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