Operational Devices for Compaction Optimization and Quality Control

Operational devices for compaction optimization and quality control (Continuous Compaction Control & Light Falling Weight Device)

D. Adam & F. Kopf Department of Civil Engineering, Vienna University of Technology, Austria

Keywords: compaction, roller, quality control, continuous compaction control, dynamic load plate test, earth works, soil dynamics

ABSTRACT: Compaction technologies comprising rollers equipped with different kinds of exciters are presented. Compaction of soil, fill material, etc. normally takes place by vibratory rollers, whereby the vibration of the drum is caused by a rotating mass eccentric. Recently, dynamic rollers with different types of excitation where developed. The paper is focused on the oscillatory roller,the VARIO roller, and the automatically controlled VARIO CONTROL roller. Furthermore, roller-integrated compaction control methods in order to check continuously the compaction success dur-ing compaction are discussed. Investigations have revealed that different operating conditions ofdynamic rollers influence the compaction success and values of the roller-integrated compaction controls. In narrow work spaces and trenches an innovative dynamic load plate test can be appliedinstead of common compaction checking. Moreover, this dynamic load plate can be perfectly usedto calibrate relative compaction values of continuous compaction control.

1 INTRODUCTION

The quality of roads, highways, motorways, rail tracks, airfields, earth dams, waste disposal facili-ties, foundations of structures and buildings, etc. depends highly on the degree of compaction of filled layers consisting of different kinds of materials, e.g. soil, granular material, artificial pow-ders, fly ashes and grain mixtures, unbound and bound material. Thus, both compaction method and compaction equipment have to be selected carefully taking into consideration the used material suitable for the prevailing purpose. Compaction process should be optimized in order to achieve sufficient compaction and uniform bearing and settlement conditions.

If compaction control can be included in the compaction process, time can be saved and cost re-duced. Furthermore, a high-leveled quality management requires continuous control all over the compacted area, which can only be achieved economically by roller-integrated methods. These so called continuous compaction control (CCC) provide relative values representing the developing of the material stiffness all over the compacted area. These values have to be calibrated in order to re-late them to conventional values (deformation modulus of static load plate test) given in contractual provisions and standards.

The dynamic load plate test (Light Falling Weight Device) is the only link to carry out a reliable and economic correlation between CCC-values and the deformation modulus of the static load plate test. Numerous tests on a wide range of fill materials have disclosed clear correlations be-tween the modulus of the static (Ev1) and the dynamic (Evd) load plate test.

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Proceedings of the International Seminar on Geotechnics in Pavement and Railway Design and Construction, Gomes Correia & Loizos (eds) 2004 Millpress, Rotterdam, ISBN 90 5966 038 2

2 OPTIMIZATION OF COMPACTION

Rollers are widely used for compacting soil, layers of filled material, etc. Depending on Layer thickness to be compacted, and material properties like Grain size distribution, maximum grain size, grain shape and degree of non-uniformity, Water content, Water and air permeability rollers are selected, whereby following machine parameters mainly contribute: Total roller weight and static drum load Static compaction (without vibration) or Dynamic compaction caused by different kinds of excitation with parameters:

o Direction of resulting dynamic contact force o Excitation frequency o Theoretical drum amplitude

Surface shape and diameter of drum

2.1 Soil and fill material

The compactibility of a material is primarily depending on the grain size distribution and the de-gree of non-uniformity. Coarse, widely grained material (gravel, sandy gravel) is best suited for compaction (Fig. 1, A). Single size fraction sands (Loess) tend to surface near re-loosening due to dynamic loads and achieve only a low maximum density compared to other soil types. In widely grained soil (clayey, silty and sandy gravel) containing a high amount of fines (; ca. 30 50%) the water content influences the compaction behavior noticeably. The higher the moisture content the more water is trapped in the voids of the low permeable material. Consequently, pore water pres-sures reduce the compactibility more and more (Fig. 1, B). Fine grained soil and artificial material (e.g. fly ash) can hardly be compacted due to the low water and air permeability. Pore water and air produce excessive pore pressure during the compaction process. Sufficient compaction can only be gained by kneading the material to relieve the pore pressures (Fig. 1, C).

Figure 1. Grain size distribution of materials with different compactibility: A - high, B - medium, C - low.

2.2 Static compaction

Rollers with static drums use the effective dead weight of the machine to apply pressure to a par-ticular surface. Thus, soil particles are pressed together and the void content is reduced. Adequate compaction of static rollers is normally achieved only in the upper layers of the material, i.e. effect of static compaction is limited to low depths.

98 2004 Millpress, Rotterdam, ISBN 90 5966 038 2

Cohesive fine grained soil can be compacted sufficiently with static rollers in connection with pad-foot or polygonal drums (Fig. 2). By means of the low permeability of fine grained soil pore water pressures are created by applying (dynamic) compressive stresses. Pore pressures reduce the com-paction effect significantly or prevent compaction at all. However, a statically passing specially shaped drum remolds and kneads the soil near the surface resulting in a reduction of pore pres-sures and void ratio respectively. Nevertheless, only thin layers can be compacted sufficiently.

Fig. 2. Polygonal drum. Optimum compaction of fine grained soil to relieve pore water pressures by varying soil contact pressure (kneading).

2.3 Dynamic compaction

Dynamic rollers make use of a vibrating or oscillating mechanism, which consists of one or more rotating eccentric weights. During dynamic compaction a combination of dynamic and static loads occurs. The dynamically excited drum delivers a rapid succession of impacts to the underlying sur-face from where the compressive and shear waves are transmitted through the material to set the particles in motion. This eliminates periodically the internal friction and facilitates the rearrange-ment of the particles into positions in combination with the static load that result in a low void ratio and a high density. Furthermore, the increase in the number of contact points and planes between the grains leads to higher stability, stiffness, and lower long-term settlement behavior.

Figure 3: Dynamic roller and different kinds of excitation (drum types).

1 2 3 4 5

1

23

4

5

PRES

SURE

[b

ar]

DISTANCE [m]

99Proceedings of the International Seminar on Geotechnics in Pavement and Railway Design and Construction

2.3.1 Vibratory roller The drum of a vibratory roller is excited by a rotating eccentric mass which is shafted on the drum axis (Fig. 3). The rotating mass sets the drum in a circular translatory motion, i.e. the direction of the resulting force is corresponding with the position of the eccentric. Compaction is achieved mainly by transmitted compression waves in combination with the effective static drum load. Con-sequently, the maximum resulting compaction force is supposed to be almost vertical and in fact it is only a little inclined.

The vibration of the roller drum changes in dependence of the soil response. Numerous investi-gations have revealed that the drum of a vibratory roller operates in different conditions depending on roller and soil parameters. Five operating conditions specified in Table 1 can occur; definition criteria are the contact condition between drum and soil and the drum motion cycle as a multiple of the excitation cycle (Adam 1996).

Continuous contact only occurs when the soil stiffness is very low, i.e. in case of low compacted or soft layers, or the drum amplitude is very small. Partial uplift and double jump are the most fre-quent operating conditions. The difference between these two operating conditions consists of the number of excitation cycles; consequently, the motion behavior of the drum repeats itself. When the soil stiffness increases further the motion of the drum axis is no more vertical, the drum starts rocking. Very high soil stiffness in combination with disadvantageous roller parameters can cause chaotic motion of the drum. During rocking and chaotic drum motion the roller is not manoeuvra-ble any more. No useful compaction is possible then. Vibratory rollers are the mainly used rollers world-wide. They can be employed universally for a wide range of soil types and granular material.

Table 1. Operating conditions of a vibratory roller drum.

2.3.2 Oscillatory roller The drum of an oscillatory roller oscillates torsionally. The torsional motion is caused by two op-posite rotating eccentric masses, which shafts are arranged eccentrically to the axis of the drum. Thus, soil is loaded horizontally in addition to the vertical static dead load of the drum and the con-tributing roller frame. These cyclic and dynamic horizontal forces result in additional soil shear de-formation; dynamic compaction is achieved mainly by transmitted shear waves (Fig. 3).

Investigations have revealed that oscillatory rollers operate in two conditions depending on roller and soil parameters. If the force exceeds the friction force (incl. the adhesion) between drum and soil the drum starts slipping. During slipping the compaction effect is reduced, however, the surface is sealed by the slip motion. Consequently, oscillatory rollers are mainly employed for asphalt compaction and cohesive material. Furthermore, oscillatory rollers are used in the vicinity of sensitive structures, because the emitted vibrations are lower than those of vibratory rollers.

drum motion Interaction drum-soiloperating condition soil contact force

application of

CCCsoil

stiffnessroller speed

drum ampli-tude

continuous contact

CONT. CONTACT yes low fast small

PARTIAL UPLIFT yes

DOUBLE JUMP yes

ROCKING MOTION no

chaotic non-periodic loss of contactCHAOTIC MOTION no high slow large

perio

dic

perio

dic

loss

o

f co

nta

ct

left right


2.3.3 VARIO-rollerThe VARIO roller is a development of the company BOMAG. In a VARIO roller two counter-rotating exciter masses, which are concentrically shafted on the axis of the drum, produce a di-rected vibration. The direction of excitation can be adjusted by turning the complete exciter unit (Fig. 3)in order to optimize the compaction effect for the respective soil type. If the exciter direc-tion is (almost) vertical or inclined, the compaction effect of a VARIO roller can be compared with that of a vibratory roller. However, if the exciter direction is horizontal, VARIO rollers compact soil like an oscillatory roller, although the motion behavior of the drum is different. The shear de-formation of soil is caused by a horizontally translatory motion, whereas the drum of an oscillatory roller is working torsionally. Thus, a VARIO roller can be used both for dynamic compression compaction (like a vibratory roller), for dynamic shear compaction (like an oscillatory roller), and a combination of these two possibilities, depending only on the adjustable force direction. Conse-quently, VARIO rollers can be employed universally for each soil type, the respective optimum di-rection can be found by basic investigations on a test field on site (Kopf 1999).

2.3.4 VARIO CONTROL-roller and ACE-roller Based on the findings relating to the ways of operating of different dynamic rollers (Table 1), the company BOMAG developed the first automatically controlled so called VARIO CONTROL roller. The Swiss company AMMANN developed the auto-controlled roller ACE in connection with a roller-integrated control system providing dynamic compaction values independent from roller parameters. Exemplary, in VARIO CONTROL rollers the direction of excitation (vibrations can be directed infinitely from the vertical to the horizontal direction) is controlled automatically by using defined control criteria, which allow an optimized compaction process and, consequently, a highly uniform compaction. The control criteria are explained in the following: Operating criterion: If the drum passes to the operating condition double jump, the excitation

direction is immediately changed, so that the operating condition partial uplift is kept. Force criterion: If the specified maximum compaction force is met, the excitation direction is

automatically changed, so that the applied force does not exceed the maximum force. Two accelerometers, which are mounted on the bearing of the drum, record the dynamic motion

behavior continuously. Soil contact force, energy transmitted to soil, displacements etc. are calcu-lated in a processor unit taking into account the roller parameters like masses, exciter force and fre-quency. The data are transmitted to an integrated control system, where the automatic setting of the parameters is managed. Following benefits of automatically controlled rollers were revealed: Uniform and improved compaction by continuous adjustment of force direction Optimized compaction combined with less roller passes Prevention of over-compaction and re-loosening Reduction of lateral vibrations in the vicinity of sensitive structures

Figure 4. VARIO CONTROL automatic control of excitation direction depending on soil properties.


3 INNOVATIVE COMPACTION CONTROL

Hitherto, compaction control has been carried out mainly by means of punctual test methods with the purpose to check the density or stiffness of the compacted layer. Common testing methods, like the static load plate test for determining the soil stiffness, and the sand equivalent, the water bal-loon method or the TROXLER-nuclear gauge test to determine the density are punctual methods. The uniformity cannot be approved, and faults can hardly be detected by such spot tests. Moreover, the measuring depth range is about 20 to 40 cm only. Last but not least all spot test methods are relatively expensive. This testing delays construction work as well because construction activities have to be stopped in the vicinity of spot test since ground vibrations could affect the test results.

3.1 Continuous compaction control (CCC)

The roller-integrated continuous compaction control (CCC) represents a significant improvement and is based on the measurement of the dynamic interaction between dynamic rollers and soil (Adam 1996). The motion behavior of different dynamically excited roller drums changes in de-pendence of the soil response. This fact is used to determine the stiffness of the ground. Accord-ingly, the drum of the dynamic roller is used as a measuring tool; its motion behavior is recorded, analyzed in a processor unit where a dynamic compaction value is calculated, and visualized on a dial or on a display unit where data can also be stored. Furthermore, an auxiliary sensor is neces-sary to determine the location of the roller or the localization is GPS-based. Control data are al-ready available during the compaction process and all over the compacted area (Fig. 5).

Figure 5. CCC-principle: CCC-components; from measured drum acceleration CCC-values are derived, and displayed area plot comprises the distribution of CCC-values.

3.1.1 CCC-systemsFour recording systems are available for vibratory rollers and VARIO rollers with vertical or any inclined excitation direction (except horizontal direction). All systems consist of a sensor contain-ing one or two accelerometers attached to the bearing of the roller drum, a processor unit and a dis-play to visualize the measured values. The sensor continuously records the acceleration of the drum. The time history of the acceleration signal is analyzed in the processor unit in order to de-termine dynamic compaction values with regard to specified roller parameters.


Table 2. Established CCC-systems, CCC-values and their definitions; producers.

CCC-system CCC-value definition of CCC-value manufacturer

Compactometer CMV[ ] acceleration amplitude ratio (first harmonic div. by excitation frequency amplitude) frequency domain Geodynamik, Sweden

Terrameter OMEGA

[Nm]energy transferred to soil (considering soil contact force dis-placement relationship of 2 excitation cycles) time domain Bomag, Germany

Terrameter Evib

[MN/m]dynamic elasticity modulus of soil beneath drum (inclination of soil contact force displacement relationship during load-ing) time domain

Bomag, Germany

ACE kB[N/m]spring stiffness of soil beneath drum (derived from soil con-tact force displacement relationship at maximum drum de-flection) time domain

Ammann, Switzerland

Table 2 gives a review of the recording systems of CCC. All defined CCC-values have proven suit-able for roller-integrated checking of the actual compaction state. Nevertheless, it is essential to take into consideration the operating condition of the roller drum (see Table 1). Fig. 6 and 7 show the progress of CCC-values depending on soil stiffness and relative vertical drum amplitude.

CMV OMEGA

Evib kB

Figure 6. Influence of operating conditions on CCC-values depending on E-modulus of soil and relative ver-tical drum amplitude, excitation frequency 28 Hz (numeric computer simulations).

cont. contact

partial uplift

double jump

chaoschaos

double jump

partial uplift

cont. contact

chaos

double jumppartial uplift

cont. contact

chaos

double jumppartial uplift

cont. contact

REL

. DR

UM A

MPL

ITUD

E [ ]

R

EL. D

RUM

AM

PLIT

UDE

[ ]

E-MODULUS SOIL [MN/m] E-MODULUS SOIL [MN/m]


REPRODUCIBILITY UNIFORMITY

INCREASE < 5%

MAX MAXIMUM VALUEMV MEAN VALUEMIN MINIMUM VALUE CALIBRATION 0,8MIN... 80% MINIMUM VALUE SD STANDARD DEVIATION < 20%

ROLLER LANE [m]

CCC-

VALU

E [C

MV,

OM

EGA,

Evib

, k B

]

Figure 7. Relative CCC-values depending on soil stiffness at relative vertical drum amplitude of 0.8 (Fig. 6).

The dynamic compaction values are relative values having a clear physical background (see Table 2). If the data shall be compared with common conventional values like the deformation modulus of the static load plate test calibrations have to be performed. There are several possibilities to se-lect the spots where conventional tests can be carried out. Best correlation can be achieved if the spots are selected by means of CCC-results. Spots with high, mean and low dynamic compaction values indicate a wide range of soil properties (Fig. 9).

Furthermore, the different depth range of CCC and the conventional test methods must be con-sidered. In general the measuring depth of CCC (depending on the total roller weight) is larger than the compaction depth and the measuring depth of common spot tests. Thus, soft soils in deeper lay-ers can be detected with CCC which is not possible with conventional tests. The information pro-vided is used to set up control criteria (Fig. 8): Minimum CCC-value in order to locate weak spots and areas Maximum CCC-values in order to locate areas with highest soil stiffness Mean CCC-value in order to assess the general condition of the checked area Standard deviation in order to assess the uniformity of the checked area Increase of CCC-value in order to point out further compactibility Decrease of CCC-values as indicator for loosening, grain crushing or pore water pressure

Figure 8. Progress of CCC-data and control criteria for standardized application; defined limits (right side).

0 %

10 %

20 %

30 %

40 %

50 %

60 %

70 %

80 %

90 %

100 %

0 20 40 60 80 100 120

E - MODULUS SOIL [MN/m]

CCC-

VALU

ES [%

OF

MAX

. VA

LUE]

CMVOMEGAEvibkB

DOUBLE JUMP

CONT

. CO

NTAC

T

PARTIAL UPLIFT


3.1.2 Standardized application of CCC The use of CCC for acceptance testing requires contractual limits for minimum, maximum and mean values. These can be determined by carrying out correlation tests (Fig. 9) taking into account the operating condition of the drum. It is strongly recommended to carry out such a calibration pro-cedure using the dynamic deformation modulus Evd of the dynamic load plate test in form of the Light Falling Weight Device (LFWD). This innovative test provides much more reliable results than the static load plate test (Ev1). Information referring to this is given in chapter 3.2.

Extensive investigations and practical site experience have revealed that the standard deviation of CCC-data shall not be > 20% (related to the mean value), the increase of dynamic compaction values between two passes < 5%.

In Austria a new standard for road and motorway construction concerning CCC was established in 1998 and declared obligatory in 1999. This RVS 8S.02.6 (Kontinuierlicher walzenintegrierter Verdichtungsnachweis) contains both fields of application, requirements to roller, soil, CCC-systems, and contractual limits for acceptance testing including the setting up of these limits by calibration, and moreover, distribution of responsibility and cost.

Figure 9: Calibration of CCC-values using Evd according to technical provision RVS 8S.02.6; example.

3.2 Dynamic load plate test (Light Falling Weight Device LFWD)

The dynamic load plate test in the form of the Light Falling Weight Device (LFWD) (Fig. 10) was developed as a testing device to determine the dynamic deformation modulus (Evd) of soils and other granular materials. There exist good correlations between the dynamic Evd and the CCC val-ues because both control methods are based on dynamic soil response. Moreover, the dynamic load plate test is very suitable for compaction control under confined conditions, e.g. in narrow trenches.

The evaluation procedure is based on a very simple principle. After the test execution the meas-ured accelerations or velocities are integrated twice or once, respectively, thus providing the maxi-mum plate deflection zmax. All other parameters required for the determination of the dynamic de-formation modulus are assumed to be constant. This approximation has shown to be reasonable (Brandl and Adam 2004). The dynamic load plate test is evaluated on the basis of the equation for the static load plate test, which is based on the theory of the elastic halfspace under static load. Ve-locity-dependent terms and forces of inertia are not taken into account due to this simplified con-sideration. Moreover, the maximum average soil stress occurring during the test execution is hy-pothetically assumed to be constant (0.1 MN/m). Thus, following simple equation is used to determine the dynamic deformation modulus for a load plate with a diameter of 300 mm:

][

5.22]/[

max mmzmMNEvd =

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60

Evd [MN/m]

CCC-

VALU

E [ ]

highvalues

lowvalues

meanvalues

MAX = 103,74MV = 80,44

MIN = 69,160,8 MIN = 55,33

r = 0,87limit EV1 limit EVd = 35 MN/m

+ 5%- 5%

MV

MAX

MIN

0,8 MIN

(MV double jump)


Figure 10. Components of LFWD. Figure 11. General correlation between Evd and Ev1.

Uniform and properly working measuring equipment is essential for a successful application of the LFWD. Attention has to be focused on exact specifications of the deflection measuring device (fre-quency range, filter characteristics, temperature, etc.) which are listed in the German technical pro-vision TP BF-StB part B 8.3. Because of the simplified assumption the spring-damper-elements of the loading device have to have constant characteristics within a temperature range from 0C to 40C. Disc springs made of steel meet this requirement, while the characteristics of synthetic ele-ments (rubber springs) change for even slight temperature differences. The measuring range of the dynamic deformation modulus is limited to Evd = 10 90 MN/m. The measuring depth of the LFWD is approximately 60 cm, i.e. twice the plate diameter (Brandl and Adam 2004).

Numerous test series and comparative tests between the dynamic and the static load plate test have disclosed general correlations between Evd and Ev1. Because of the different behavior of soft cohesive soil compared to coarse-grained soil due to the impact-like load (effect of pore water pressures) two different regression lines for low soil stiffness Evd < 30 MN/m exist. While for co-hesive soil a linear relationship is valid for the entire measuring range, for non-cohesive soil either a bi-linear relationship or a logarithmic function can be used (Fig. 11).

4 CONCLUSIONS

Various roller types containing different drum excitations are available for optimum compaction of different soils. Roller-integrated continuous compaction control (CCC) represents a significant im-provement for high-levelled quality management systems. CCC-values can be reliably correlated to common static deformation modulus by application of the Light Falling Weight Device (LFWD).

REFERENCES

Adam, D.: Roller-integrated continuous compaction control of soils by means of vibratory rollers. (in Ger-man). Doctors Thesis. Technical University of Vienna, 1996.

Brandl, H. and Adam, D.: Basics and Application of the Dynamic Load Plate Test in Form of the Light Fal-ling Weight Device. A.W. Skempton Memorial Conference. Proc. of Imperial College, London, 2004.

Kopf, F.: Roller-integrated continuous compaction control of soils by means of different dynamic rollers. (in German). Doctors Thesis. Technical University of Vienna, 1999.

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Evd [MN/m]Ev

1 [M

N/m

]

measuring range LFWD

5,1245

1 = vdv EE

vdv E

E

=

180180ln1501

cohesive

non-cohesive

vdv EE 65

1 =


Operational Devices for Compaction Optimization and Quality Control

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