TRANSPORT RESEARCH LABORATORY Department of Transport RESEARCH REPORT 361 COMPACTION MONITORING DEVICES FOR EARTHWORKS by R A Snowdon Crown Copyright 1992. The views expressed in this Report are not necessarily those of the Department of Transport. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged. Ground Engineering Division Structures Group Transport Research Laboratory Crowthorne, Berkshire, RG11 6AU 1992 ISSN 0266-5247
Transcript
TRANSPORT RESEARCH LABORATORY Department of Transport
RESEARCH REPORT 361
COMPACTION MONITORING DEVICES FOR EARTHWORKS
by R A Snowdon
Crown Copyright 1992. The views expressed in this Report are not necessarily those of the Department of Transport. Extracts from the text may be reproduced, except for commercial purposes, provided the source is acknowledged.
Ground Engineering Division Structures Group Transport Research Laboratory Crowthorne, Berkshire, RG11 6AU 1992
ISSN 0266-5247
CONTENTS
Abstract
1. Introduction
2. UK Compaction requirements
3. Methods studied
3.1 Density Measurement
3.2 Strength Measurement
3.2.1 Clegg Impact Soil Tester (CIST)
3.2.2 TRL Dynamic Cone Penetrometer (DCP)
3.2.3 Strength monitoring
3.3 Compaction Meters
3.3.1 Bomag Terrameter
3.3.2 Dynapac CCS-RA
3.4 Tachographs
4. Conclusions
5. Acknowledgements
6. References
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The Transport Research Laboratory is no longer an Executive Agency of the Department of Transport as ownership was transferred to a subsidiary of the Transport Research Foundation on I st April 1996.
This report has been reproduced by permission of the Controller of HMSO. The views expressed in this publication are not necessarily those of the Department of Transport.
COMPACTION MONITORING DEVICES FOR EARTHWORKS
ABSTRACT
A number of methods and devices has been investigated to establish their possible suitability and application for monitoring the state of compaction of earthworks, within the requirements of the DTp Specification for Highway Works (1986), Series 600 Earthworks, and for the reinstatement of trench backfills. This report summarises research carried out in the compaction bays at TRL using the nuclear moisture/density gauge, Clegg impact soil tester, TRL dynamic cone penetrometer, Bomag Terrameter and Dynapac compaction meters mounted on vibratory rollers. Comparisons were made between the outputs from the monitoring devices and the dry density and air voids of compacted layers of cohesive and granular fill materials.
The general conclusion was that density measurement was the only acceptable method and that none of the strength measuring devices were acceptable at the present time, except for the Bomag Terrameter attached to a vibratory roller when used on granular fills. However, compaction meters and tachographs might be worth consideration, if hard outputs were required for compli- ance records or Quality Assurance purposes.
1. INTRODUCTION
A series of studies has been carried out to review the possible suitability and application of a selection of methods of assessing the state of compaction of earthworks in the UK.
If any move were to be made from the method compac- tion specification, currently in use for general earthwork fills, to an end product specification, then it would be essential that the test method used to verify such an end product requirement could indicate that a satisfactory state of compaction had been achieved.
The methods studied in the TRL soil compaction bays which could be applicable to earthworks, and to the reinstatement of trench backfills, included in-situ density measurements and soil strength measuring devices, both hand operated and mounted on vibratory rollers.
2. UK COMPACTION REQUIREMENTS
The prevention of high air void contents occurring within compacted fill materials is crucial if long term weakening and settlement of pavements and structures is to be avoided. It is generally accepted that the best measures of an acceptable state of compaction are the dry density and air void content values achieved. However, direct
assessment of these under field conditions is not always an easy task. Because of this, and other operational considerations, compaction specifications are frequently written in terms of the method to be used, although end product specifications are used in particular circum- stances.
The DTp Specification for Highway Works (DTp 1986a, 1986b) defines the requirements for the compaction of general fills for earthworks in the UK. This is based on method specification, the method being such that it will produce an air void content of 10 per cent or less within a defined moisture content range. Within 600mm of the pavement structure, extra compactive effort is required, such that an air void content of 5 per cent or less will be produced. Some selected fills i.e. fills to structures, pulverised fuel ash and stabilised materials are required to achieve a relative compaction value: i.e. the specifica- tion for these is end product.
Unfortunately, the strength of a compacted material is not directly related to its state of compaction: for this report the term strength is loosely used to include bearing capacity and resistance to penetration and impact. A good example of typical compaction curves and their associated California Bearing Ratio values (CBR) versus moisture content is shown in Figure 1. It can be seen that a high strength (CBR) is obtained at the drier end of the moisture content range and reduces as the moisture content increases. The dry density, however, increases as the moisture content approaches the optimum value and then decreases running parallel to the zero air voids line as the moisture content increases. Thus it is quite possible to obtain a high strength at a dry density which equates to a relatively poor state of compaction, and at which settlement and weakening may well occur in the longer term.
3. METHODS STUDIED
The methods used for evaluating states of compaction tend to fall into two categories. The first, and primary method, is the measurement of dry density and air voids; this forms the basis of both the end product and method compaction specifications presented in the SHW (1986). Secondly, there is a number of methods and devices for measuring some indicator of the strength of the com- pacted soil.
3.1 DENSITY MEASUREMENT
As stated above, dry density and air voids are the primary measurements used for determining if an acceptable state of compaction has been achieved for earthwork fills. If the method specification in the SHW (1986) is correctly applied it will produce an acceptable state of compaction based on air voids: further details are given in Clause NG612 of the SHW (1986).
1.6
-8
£3
1.5
1.4
1.3
! ! I
Q J
100
80
60
40
20
$ 10
n~ ,.~ 8
d 6
Fig.1
1.7
f Density C.B.R.
O • Mod. A.A.S.H.O.
[ ] • Intermediate
A • B.S.
ISO-C.B.R. lines
1 ! I 1 I ! I I I 0 4 8 12 18 20 24 28 32 35
Moisture content (per cent)
Compaction and California Bearing Ratio relations for black sandy clay soil (O'Reilly, Russam and Wil l iams,1968)
2
In-situ dry density measurements are required to verify that an end product specification, based upon laboratory compaction tests to BS1377:1990, has been met. The sand replacement method to BS1377:1990 is preferred but can be time consuming. An alternative method is the nuclear moisture/density gauge which is also included in BS1377:1990.
Nuclear gauges operate by measuring the residual neutron and gamma radiation emanating from two separate sources after passing through, or being re- flected from, the compacted layer of soil. These are normally referred to as the direct transmission and backscatter modes respectively. The neutron source and both the neutron and gamma detectors are located within the body of the gauge. The gamma radiation source is in the tip of a steel rod which can be lowered into a pre- pared hole in the material for use in the direct transmis- sion mode, or retained in the body of the gauge for use in the backscatter mode. The gamma radiation count is converted by a microprocessor in the gauge into the soil bulk density, and the neutron count, which measures the hydrogen concentration, is related to the moisture density.
Bulk density can be determined either in the backscatter or direct transmission mode: the former has a depth of
penetration of about 50 to 75mm and the latter has a depth of measurement which can be set to between 50 and 300mm. The moisture density can only be measured in the backscatter mode and the depth sampled, which depends on the moisture of the soil, is about 150mm. For all of the TRL studies the direct transmission method was used for bulk density as it allowed the average density through the compacted layer to be measured and was, therefore, directly comparable to the sand replacement method. As the moisture density does not normally vary significantly within the depth of a compacted layer, the backscatter mode will give results which are representa- tive of the whole layer.
A Troxler 3411B and a Humboldt 5001 nuclear moisture/ density gauge have been used to determine the state of compaction of cohesive and granular soils during studies of the performance of compaction plant at TRL. Measure- ments of bulk and moisture density were made using both the sand replacement method and the nuclear gauge to establish calibrations for the different soil types. The accuracy of the gauges were assessed on the basis of these comparative results. The Troxler gauge and a typical calibration are shown in Figures 2 and 3 respec- tively. Correlations obtained between the nuclear gauge and sand replacement tests are given in Table 1.
7 J
Fig.2 The Troxler 3411B nuclear gauge
;2
Neg.No.CR462/86/9
3
2.2
A c o
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w
E
"Z3
C
2.1
2.0
1.9
1.8
1.7
1.6
1.5
1.4
/ 95% confidence limit
/Oo/o
/ o / /
/
I I I I I I I 1.5 1.6 1.7 1.8 1.9 2.0 2.1
Nuclear gauge bulk density (Mg/m 3)
Fig.3 Ca l i b ra t i on of bu l k dens i t y measurements made by nuclear gauge and sand rep lacement methods in heavy clay
2.2
When used in trenches, the gauge manufacturers state that the moisture density reading is affected if the gauge is used within 600mm of a trench wall which has a moisture density greater than 0.24Mg/m 3. To allow for this, a trench correction factor has to be established to take into account the effect of moisture in the surrounding material on the count measured by the gauge. Also, the width of the gauge restricts its use in trenches less than about 260mm wide.
Studies of the nuclear method on the surface of com- pacted soils (Servais and York, 1990) and in trenches have shown that the nuclear method is reliable, provided that calibrations are established against sand replace- ment densities. Nuclear methods also significantly reduce the testing time required.
3.2 STRENGTH MEASUREMENT
3.2.1 Clegg Impact Soil Tester (CIST)
In 1976, Clegg introduced an impact tester for use as a rapid method of evaluating the strength of pavement base courses (Clegg, 1976). Since then a number of papers has been published describing the advantages and disadvantages of the device when used for compac- tion control on a variety of soils. Some recent publications are by Todres (1986), Forbes (1986), Freer-Hewish (1982), Punch (1981) and Winter and Selby (1991). The general conclusions reached were that the Clegg Impact Soil Tester correlated with CBR but did not correlate with standard density tests. However, in some individual cases i.e. granular materials on the dry side of the optimum moisture content, correlations were obtained with density.
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TABLE 1
Regression equations for bulk and moisture densities
Soil
Wellgraded sand
Uniformly gradedfine sand
Gravel-sand- clay
Sandy clay
Heavy clay
No oftests Regression equation
54 y = 1.264x- 0.470
24 y = 1.366x- 0.613
56 y = 1.158x- 0.307
30 y = 1.388x- 0.777
32 y = 1.282x- 0.562
Correlation coefficient
O.984
O.987
0.980
0.984
0.969
Wellgraded sand
Uniformly graded fine sand
Gravel-sand- clay
Sandy clay
Heavy clay
54 ym = 0 .896x + 0.020
24 Ym = 0-862xm + 0.014
54 Ym = 0 .851x + 0.022
30 Ym = 0'872xm + 0.028
32 ym = 1.125x m - 0.048
0.982
0.950
0.987
0.962
0.953
Where y = bulk density from sand replacement method x = bulk density measured by nuclear gauge
and Ym = moisture density from sand replacement method x m = moisture density measured by nuclear gauge
The device, shown in Figure 4, is essentially similar to a BS1377 4.5kg laboratory compaction test rammer with an accelerometer incorporated in the handle. The 50mm diameter hammer is dropped through a height of 450mm and the retardation as it strikes the soil surface is meas- ured by the accelerometer. A display on the hand unit indicates the peak deceleration value in units of ten gravities. For the tests an average of the fourth and fifth Impact Values (IV) obtained at each sample point was recorded.
It must be noted that two known versions of the CIST are available, an Australian manufactured device and an Italian version. Both have similar mechanical specifica- tions but produce different results, i.e. the IVs are different and therefore, are not interchangeable.
The IV results obtained during testing of a variety of compaction plant at TRL indicated that the CIST was affected by too many variables to be used for monitoring the achievement of a satisfactory state of compaction, as
defined in Clause NG612 of the Specification for Highway Works (1986): a set of typical results is shown in Figure 5. The primary reason is that the CIST is a strength dependent device and that strength, in turn, is dependent upon a combination of density, soil type and moisture content (see Figure 1). It may, therefore, be argued that if a series of calibrations between IV and dry density at constant moisture contents for a specific backfill material were obtained, then, by measuring both the IV and moisture content on site the dry density could be determined. Unfortunately, results obtained during the study show that this approach would not be acceptable since the type of compaction plant used and the com- pacted layer thickness also have a significant effect on the IV recorded, as illustrated in Figures 6 and 7.
It appears that the CIST should only be used for the purpose it was devised for, i.e. the in-situ evaluation of granular base course strength, and that any application of the CIST to control or monitor dry density and state of compaction for earthworks should not be considered.
5
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. . . . . . .
• ~ , :
N e g . N o . C R 3 3 5 / 9 2 / 4 4
Fig.4 The Clegg Impact Soil Tester
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1.9
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Zero air voids
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1 4 Limits of acceptabil i ty n r l
I 0 1 8 I I I I •17 I I I I
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36 • • 30 • 27
18
7 8 9 10 11
Moisture content (per cent)
Fig.5 Clegg Impac t Values f r o m a n a r r o w t rench v i b r a t o r y
ro l ler on a 300 mm layer o f grave l -sand-c lay
IV
impact value
\
3 .2 .2 T R L D y n a m i c C o n e P e n e t r o m e t e r ( D C P )
A short study into the suitability of the TRL Dynamic Cone Penetrometer for compaction control on general earthwork fills was also carried out in the soil bays. The DCP, shown in Figure 8, was designed for rapid in-situ measurement of the structural properties of road pave- ments; correlations have been established between the DCP and CBR by Kleyn (1975) and Smith and Pratt (1983). The DCP value is defined as the vertical penetra- tion achieved by the penetrometer per blow of an 8kg mass falling 575mm onto an anvil driving the penetrometer rod into the soil. Typical results, obtained on gravel-sand-clay, presented in Figure 9 illustrate that the DCP value was also influenced by both the dry density and moisture content of the fill material and was, therefore, unsuitable for monitoring the state of compac- tion for earthwork fills.
3 . 2 . 3 S t r e n g t h m o n i t o r i n g
As all strength monitoring devices, such as those above, are sensitive to changes in a combination of both density and moisture content, they are unsuitable for assessing the state of compaction in terms of density or air voids.
3.3 COMPACTION METERS
Instrumentation, in the form of transducers and onboard computers, mounted on vibratory rollers is being ac- cepted more widely for general use in Europe as a means of compaction control. In Sweden, for instance, the use of such compaction meters is to be included in their national specifications for Quality Assurance, compaction control and documentation of the results obtained (Lindh, 1989; Thurner, 1991 ; Thurner and Sandstrom, 1989). Prior to this, such meters have mainly been used on specific sites such as airfields and dams,
7
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1.7
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10 percentairvoids ~ m
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\ ~ V~ibrating plate
G ravel-sand-clay 7 per cent moisture content
150 mm layer thickness
1.6 I ! ! 0 10 20 30
Clegg Impact Value
Fig.6 Effect of compaction plant type on Impact Value
40
30
m r~ > r~ ~ 20 E
10
Type 1 MC (].9%
Air voids 19.1%
J o ~ c l a y
0
Fig.7
Well graded Sand MC 9.4% o •
Air voids ~ A,,~ ' ' ' ' ' ~ A Sandy-Clay
MC 14.6% Air voids 25.7%
I I I 1 oo 200 300
Compacted layer thickness (mm)
Effect of layer thickness on Impact Value at constant states of compaction
whilst compacting sandy gravel or rock fill materials (Forssblad, 1980; Floss et al, 1983; Schwab et al, 1983).
3.3 .1 B o m a g T e r r a m e t e r
For the study at TRL a Bomag Terrameter BTM attached to a BW213D self-propelled vibratory roller, with 3000kg mass per metre width of roll, was used as shown in Figures 10 and 11. The Terrameter BTM consists of three basic components;
(i) a transducer bolted to the vibrating roller drum shaft to measure the change in rebound acceleration during compaction,
(ii) an onboard computer, located in the driver's cab, for processing and storing information received from the transducer,
(iii) a printer, display unit and control panel, providing hard copy of the output for documentation and information for the operator on compaction progress.
In operation, the transducer monitors the rebound acceleration of the vibrating roller resulting from the changing material conditions during the compaction process. Data recorded from the transducer, called
omega values, are stored in the computer for comparison with successive passes. When an increase in omega of less than 10 per cent is recorded for two consecutive passes in the same direction then an indicator light shows the operator that compaction is effectively com- plete, i.e. any further passes would not be economic. A print out provides a graph of omega value against distance travelled, the maximum, minimum and average omega values, the percentage increase in omega value for two consecutive passes, the frequency of vibration and speed of travel. A typical print out is shown in Figure 12.
An initial analysis of all the results showed that the range of omega values recorded varied greatly. Individual values obtained on the cohesive fills only reached about 35 whilst those on the granular fills ranged between 65 and 740, depending upon the state of compaction and moisture content. A typical set of results on a 200mm thick layer of gravel-sand-clay are given in Table 2. It was obvious that the actual omega value was unsuitable as an indicator of the state of compaction for general earthworks as it depended upon both the dry density and moisture content. The rate of increase in omega value was therefore used as the basis for comparison with the state of compaction achieved. The <10 per cent increase in omega value, as used by the manufacturer, was adopted as the criterion for analysing the results.
8
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Neg.No. 0R764/91/1
Fig.8 Dynamic Cone Penetrometer in use
Z >-
2.3
2 . 2 - -
21 -
2 0 - -
1 9 - -
1 8 - -
1 7 - -
~6 --
15 0
6 2
I I I 20 40 60
6 7 73 77 8 2 ~ 7 9.U
. . . . . . . . . . . . . ° ° o ° ° ° ,
' , . ~ ( ) , S t L l ' e C O r a t + ' r ~ t I D ! ; ' { : { 7 '~ t~
Fig.9 DCP Values for gravel-sand-clay against dry density
80
9.8
\ ~~ :~o~- ,~ , , - -~ .~ /
Fig.lO Bomag Terrameter BTM control panel
o
&¢ \
:)I ° B O M R G ' ~ ° ~ 7:" 0 0
0 ;: . ~ : > , 0
0 0
Fig.11 Neg.No CR885'9012
BTM system attached to the BW213D
10
B O M A G T E R R A M E T E R . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
P A S S N O . 3 F O R .
DATE : REV 04
OMEGA MAX. : 63B D OMEGA MIN. : 309 MEAN OMEGA VALUE : 456 OMEGA INCREASE = 8,0 Z FREQUENCY : 29,7 Hz MEAN TRAVEL SPEED = 2,0 km/h TRACK LENGTH = 12 m
Fig.12 Printout from the Terrameter on a 200 mm layer of Gravel-Sand-Clay
It was also seen that the rate of change in the omega value did indicate when the roller had reached its limit of useful compactive effort on the particular soil under test. This did not, however, ensure that an acceptable state of compaction had been achieved.
The conclusions reached in the study were as follows.
1. The actual rebound acceleration value could not be considered as a suitable indicator of the state of compac- tion of general earthwork fills.
2. Neither the <10 per cent increase in omega value criterion nor the Terrameter system would be suitable for compaction control on cohesive soils in the UK.
3. On granular soils the Terrameter did indicate when the roller had reached its limit of useful compactive effort. This occurred at moisture contents lower than those given in Clause NG612 (DTp, 1986b) and an acceptable state of compaction had not been achieved.
4. At moisture contents meeting the requirements for compliance with Table 6/4, SHW (1986), a <10 per cent
increase in omega value between consecutive passes did indicate achievement of a satisfactory state of compac- tion on the granular soils tested. Indeed the Terrameter was slightly conservative, as it required a further pass for confirmation of achievement.
5. There does not appear to be any potential for increased productivity by using this type of device. Any possible advantage gained in reducing the number of passes required, as defined in the SHW method compac- tion, at higher moisture contents or increased layer thicknesses was negated by the minimum number of passes the Terrameter needed to operate.
6. The skill of the operator in controlling the roller and the Terrameter was important in ensuring a representa- tive and accurate output.
7. Hard copy of relevant information, provided via the Terrameter printer, may provide useful documentation for inclusion in a Quality Assurance scheme.
3.3.2 Dynapac CCS-RA
A similar study was carried out using the Dynapac CCS- RA compaction monitoring system attached to a Dynapac CA251PD self-propelled vibratory tamping roller. The system used a similar method of measurement to the Bomag Terrameter but differed in the way the information was presented to the operator. A computer was mounted in the operator's cab coupled to the processing unit. Activation of the computer by the driver processed the information received from an accelerometer, distance sensor and site reference codes. The resulting Compac- tion Meter Value (CMV) was displayed on the meter, either numerically or in a diagram form on the built-in computer screen for any selected location, or time period. A typical print out, obtained by connecting the CCS-RA computer to a line printer, is shown in Figure 13.
CMV readings obtained on the heavy clay and sandy clay soils are shown plotted against the corresponding in-situ measurements of dry density in Figure 14. Results that correspond to a satisfactory state of compaction, i.e. an air void content of 10% or less, are highlighted. The CMVs were very low, between 3 and 6 on a scale of 0 to 150 and did not appear to be significantly influenced by soil type, state of compaction or moisture content. With both soils, the same CMV reading was recorded on the first or second pass as on the sixteenth, although the measured state of compaction increased with every pass. It was not possible with either soil, at the moisture contents used for the tests, to discern any trend of increasing CMV with successive passes that could be used to indicate when a specified state of compaction was being achieved.
Relations obtained between CMV and dry density for the well-graded sand and the gravel-sand-clay soils are shown in Figure 15. These show that, whereas the CMVs did increase with an increasing state of compaction, a different range of values was obtained for each selected moisture content. With the well-graded sand, a satisfac- tory state of compaction was attained at a CMV of about
11
TABLE 2
Omega, dry density and air void values measured on a 200ram layer of gravel-sand-clay
, START REV ........ CMV ........... SPEED (k~lh) ....... FREQ (rpm) .... ! TRACK NAME DATE TIME LENGTH POS. EItSE MEAN MIN ~X D~ MEAN MIN ~X MF~kN MIM ]{AM { .......................................................................................................................
! DO18 213/200 910815 0837 lOm Om ! DO19 214/200 910815 0840 9m Om ! D020 215/200 910815 0936 llm Om ' D021 216/200 910815 0938 lOm Om ' D022 2/7/200 910815 0942 13m Om ' I)023 2/8/200 910815 0945 12m Om ! D024 2/9/200 910815 1119 lOm Om ! D025 2110/200 910815 1122 12m Om ! D026 2111/200 910815 1130 12m Om ! D027 2/12/200 910815 1132 13m Om ! D028 2113/200 910815 1134 13m Om ] D029 2/14/200 910815 1136 16m Om ! D030 2/15/200 910815 1139 15m Om ! D031 2/16/200 910815 1141 16m Om !
, DO21 : II_IIIXlII ' D022 : £XlXXXXlIIIII ' D023 : £XXXXXXfffXX ! D024 : I l l l l / l l l / I ~25 : I_ilIXf£XX£ ! D 0 2 6 : £///X/X/III! , D027 : XI_IIIIXXXXlX ] D028 : fXXXXfXXXXX//
Fig.13 Pr in tout of resul ts f r o m the D y n a p a c C C S - R A
13
1.60
1.50 -
co E
1.40 -
0
1.30 -
1.20
0
1.90
H E A V Y CLAY
1.80 -
E
~ 1 . 7 0 - c
"o
1.60 -
1.50 0
F ig .14
= <10% air voids I
Moisture content 1 23 to 28 per cent
I I I I
1 2 3 4 CMV
SANDY CLAY
1
0
~ 16 passes l 1
~ \ 8 passes ~0 ~
% %
~,, (~ 4 passes ,, %
O ~ %
'b ~ 2 passes
I I I
5 6 7
• = <10% air voids I
2 passes
%
16 passes %
8 passes
4 passes \
\
d~
\
Moisture content \ \ \ \ \
15.5 to 18.5 per cent \ 0 \
i t I I I I I
1 2 3 4 5 6 7
CMV
Re la t i ons b e t w e e n C M V and d r y d e n s i t y
fo r t he cohes ive soi ls
2.10 WELL--GRADED SAND
.E
E o~
E3
2.05
2.00
1.95
1.90
1.85
1.80
I • • = <10% air voids ]
0
~ . A A • •
. A • •
• 0 0
0 0
0 • 0
0
%
0
0 I oisture content
0 = 7.4 percent A = 9.1 per cent
I 10
I I 20 30
CMV 40
2.10 G R A V E L - S A N D - C L A Y
&--
-(3
£3
2.05
2.00
1.95
1.90 --
1.85 --
1.80 0
IO • = < l O % a i r v o i d s I
, : Io @
O
A 0 0
O
O z~
O
Moisture content
0 = 6.7 per cent
= 7.3 per cent
t I q 1 i 10 20 30 40 50
CMV
F ig .15 Re la t ions be tween C M V and d r y d e n s i t y fo r the we l l -g raded granu la r soils
60
14
21 for a moisture content of 7.4 per cent. At a moisture content of 9.1 per cent however, a CMV of about 11 was required. Similarly, with the gravel-sand-clay, CMVs of about 30 and 19 were required at moisture contents of 6.7 and 7.3 per cent respectively, before a satisfactory state of compaction was achieved. Results obtained on the 200mm thick layer of gravel-sand-clay are presented in Table 3.
Therefore, with these well-graded granular soils, it was only possible to determine a unique CMV, corresponding to a specified state of compaction, when the soil was in a homogeneous condition. Such a condition rarely exists within a typical fill material used for bulk earthworks construction in the UK, and must therefore place a serious limitation on the number of sites where the compaction monitoring system could be used.
The conclusions reached reinforced those found with the Bomag Terrameter BTM with one exception. As the Dynapac system did not compare values recorded on consecutive passes of the roller, and relied upon an absolute value for acceptance of the state of compaction, its performance was unacceptable on both cohesive and granular fills. Therefore, the Dynapac system as tested could not be considered as a suitable method for control- ling the compaction of general earthwork fills in the UK.
3.4 T A C H O G R A P H S
In France, tachographs are used to record the speed of travel, frequency of vibration and working times of compaction plant (Leflaive et al, 1980; Machet, 1980). These daily charts allow simple calculations of the surface area covered by the compaction plant, required
TABLE 3
CMV, dry density and air void values measured on a 200mm layer of gravel-sand-clay
as an element in the calculation of compliance with their complex method specification and also provide a perma- nent record (Ministere de I'Equipment, 1976).
The use of tachographs on plant working to a UK method specification on small sites, such as in a trench reinstate- ment where continuous supervision is not always possi- ble, may be worth consideration as an aid to controlling compliance. A simple comparison of accumulated machine hours and volume of compacted fill would provide a good indication of whether or not compliance with a method specification had been achieved (Parsons, 1992).
4. CONCLUSIONS
Achievement of a satisfactory state of compaction at a minimum specified dry density value, or maximum air voids, is the primary function of the compaction process. Therefore, it is essential that the result obtained from any method, or device, used to monitor the state of compac- tion of earthwork fill and trench reinstatement materials is dependent on density and not strength.
End product specifications, as detailed in Clause 612 SHW (1986), require dry density measurements using methods given in BS1377:1990. These now include the use of nuclear methods which are significantly faster to carry out on site than sand and water replacement methods. It should be emphasised, however, that the nuclear gauge must be calibrated for compliance testing in the compacted fill material, and operated by authorised personnel. In addition, the direct transmission mode of measuring bulk density through the full depth of the Compacted layer should be used in preference to the backscatter mode operating on the compacted surface.
Apart from a possible application of the Bomag Terrameter mounted on a vibratory roller for the compac- tion of granular fills complying with the relevant material specification in the SHW (1986), none of the devices designed to measure the strength or bearing capacity of a compacted layer should be considered for monitoring or controlling the compaction of earthworks.
Hard copy from tachographs may provide evidence on which to assess compliance when used in combination with existing method specifications, particularly on small sites. The output from vibratory roller mounted compac- tion meters, if considered suitable after site trials, may also assist in assessments for compliance with method specifications and Quality Assurance schemes.
5. A C K N O W L E D G E M E N T S
The work described in this Report forms part of the research programme of the Ground Engineering Division (Head of Division Dr J Temporal) of the Structures Group of TRL. Thanks are due to the manufacturers of devices for their co-operation and technical advice.
6. REFERENCES
BRITISH STANDARDS INSTITUTION (1990). Methods of test for soils for civil engineering purposes. British Standard BS1377: 1990. British Standards Institution, London.
CLEGG B (1976). An impact testing device for in situ base course evaluation. Proceedings, 8th ARRB Confer- ence, Vol 8, Perth.
DEPARTMENT OF TRANSPORT (1986a). Specification for Highway Works. Part 2, Series 600 Earthworks. HMSO, London.
DEPARTMENT OF TRANSPORT (1986b). Notes for Guidance on the Specification for Highway Works. Part 2, Series 600 Earthworks. HMSO, London.
FLOSS R, GRUBER N and OBERMAYER J (1983). A dynamical test method for continuous compaction control. Proc Eighth European Conference on Soil Mechanics and Foundation Engineering, Voll, pp25-30, FGS, May 1983, Helsinki.
FORBES A J (1986). Clegg Impact Tester- field evalua- tion results. Report No ABTR/RD/RR-86/08, Alberta Transportation Research and Development, Edmonton, Alberta.
FORSSBLAD L (1980). Compaction meter on vibrating rollers for improved compaction control. International Conference on Compaction, Vol 2, pp 541-546, ENPC, April 1980, Paris.
FREER-HEWISH R J (1982). Developments in field control of earthworks compaction. The Eleventh ARRB Conference, Vol II, Part 3, Melbourne, August 1982.
KLEYN E G (1975). The use of the Dynamic Cone Penetrometer (DCP). Materials Branch, Transvaal Roads Department, South Africa.
LEFLAIVE B, SCHAEFFNER M, LENY G and PUIG G (1980). Compaction control of earthworks (in French). International Conference on Compaction, Vol 2, pp 571- 576, ENPC, April 1980, Paris.
LINDH E (1989). Production-integrated compaction testing. Unbound Aggregates in Roads (UNBAR 3), pp 38-45, editors R H Jones and A R Dawson, Butterworths, London.
MACHET J M (1980). Compactor-mounted control devices (in French). International Conference on Com- paction, Vol 2, pp 577-581, ENPC, April 1980, Paris.
MINISTERE DE L'EQUIPMENT (1976). Recommenda- tions for the construction of road earthworks (in French). SETRA, Laboratoire Central des Ponts et Chaussees, Paris.
16
O'REILLY M P, RUSSAM K and WILLIAMS F H P (1968). Pavement design in the tropics: investigations of subgrade conditions under roads in East Africa. Road Research Technical Paper No 80. Ministry of Transport, HMSO, London.
PARSONS A W (1992). Compaction of soils and granular materials: a review of research performed at the Trans- port Research Laboratory. HMSO, London.
PUNCH J J G (1981). An assessment of the Clegg Impact Soil Tester. Main Roads Department, Report No MED 81/1, Perth, Western Australia.
SCHWAB E F, PREGL O and KIERES W (1983). Compaction control with the Compactometer. Proc Eighth European Conference on Soil Mechanics and Foundation Engineering, Vol 1, pp 73-82, FGS, May 1983, Helsinki.
SERVAIS S G C and YORK K I (1990). Comparison of the precision of two methods of determining field density of earthworks. Australian Road Research 20(2), pp 23- 37, June 1990.
SMITH R B and PRATT D N (1983). A field study of in- situ California Bearing Ratio and dynamic cone penetrometer testing for road subgrade investigations. Australian Road Research 13(4), pp 285-294, December 1983. Australian Road Research Board.
THURNER H F (1991). Quality Assurance and Quality Control in compaction. 3rd International Symposium on Field Measurements in Geomechanics, September 1991, Oslo.
THURNER H F and SANDSTROM A (1989). Compaction meter and compaction documentation system. Unbound Aggregates in Roads (UNBAR 3), pp 46-51, editors R H Jones and A R Dawson, Butterworths, London.
TODRES H A (1986). Soil compaction verification. Paper presented at 1986 Distribution Transmission Confer- ence, Operating Section, American Gas Association, Chicago.
WINTER M G and SELBY A R (1991). Clegg meter performance assessment with reference to the reinstate- ment environment. Highways and Transportation, pp 17- 23, June 1991.
Printed in the United Kingdom for HMSO Dd8222834 10/92 C4 G2516 10170
RR316 Analyses of the performance of vibrating and impacting compaction plant, M P O'Reilly, Price Code C
RR300 Analyses of the performance of dead-weight rollers compacting soil, M P O'Reilly, Price Code C
RR208 The compact ion of soil using l ightweight vibrating plate compactors, A F Toombs, Price Code B
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