EFFECT OF HYDRAULIC PLATE COMPACTOR AND LIFT THICKNESS ON UTILITY TRENCH BACKFILL COMPACTION

ABSTRACT

 

For utility trench backfill compaction, compaction-induced earth pressure in the backfill zone and deformation along the pipe are two important issues to be addressed.  Backfill materials should be adequately compacted to lock the pipe in place and reduce potential settlement from external loading (e.g., traffic loading).  On the other hand, excessive backfill compaction from compaction equipment may damage the pipe.  Backhoemounted hydraulic plate compactors (hoe-packs) have been increasingly used for soil compaction in trenching, street repairing, or site preparations.  Comparing to traditional compaction tools such as roller compactors, hoe-packs are advantageous for utility trench backfill compaction as they can be operated by the backhoe operator and requires no lifting of compaction machine during compaction process.  However, uncertainty remains with regard to the maximum lift thickness to consistently achieve desired compacted dry mass density by hydraulic plate compactors.  The large impulse energy and down pressure exerted by hydraulic plate compactors also raise concerns on potential damages to utility pipes.

The objective of this study is to assess the capability of a hoe-pack for utility trench backfill compaction.  Field tests were conducted to investigate the effect of hoepack on compacted dry mass density, compaction-induced earth pressures, and compaction-induced strains in pipe.  Different values of lift thickness and pipe materials were used.  Compaction tests using hand-held vibratory roller compactors which followed the specifications of the Pennsylvania Department of Transportation (PennDOT) were conducted as a control set.  It is concluded that the hoe-pack used in this study can consistently achieve relative densities above 100% of the Standard Proctor Density (SPD) with a lift thickness of 8 inches or 12 inches.  However, the hoe-pack is not able to consistently achieve relative densities above 100% of SPD 12 inches below the compacted surface when the lift thickness is 18 inches or greater.  Comparing the performances of the hoe-pack and vibratory roller compactor used in this study for a lift thickness of 8 inches, the hoe-pack is more efficient in performing compaction.  The hoepack used in this study is likely to induce higher dynamic earth pressures in the backfill

 

zone, but may not induce higher static earth pressures depending on the pipe material and diameter.  The hoe-pack used in this study generally induces similar strains along pipe as the vibratory roller compactor does.  For the hoe-pack used in this study, the static earth pressures in the backfill zone are relatively insensitive to lift thickness, whereas the dynamic earth pressures may decrease as the lift thickness increases, depending on the pipe.  The effect of lift thickness on strains developed along pipe also depends on the pipe.

 

 

TABLE OF CONTENTS

List of Figures …………………………………………………………………………………………………………..  vii

List of Tables …………………………………………………………………………………………………………… xiii

Acknowledgements …………………………………………………………………………………………………… xiv

Chapter 1 Introduction and Literature Review ………………………………………………………………    1

1.1   Introduction ……………………………………………………………………………………………………..    1 1.2   Working Theory of Hydraulic Plate Compactor ……………………………………………………    2

1.3   Specifications of Common Models of Hydraulic Plate Compactors ………………………..    6

1.4   Selection of Hydraulic Plate Compactors …………………………………………………………….    8 1.5   Past Research on Backfill Compaction ………………………………………………………………..    9

1.6   Objective and Scope of Work …………………………………………………………………………….  11

1.7   Thesis Outline ………………………………………………………………………………………………….  11 Chapter 2 Instrumentation Plan and Data Acquisition System …………………………………………  12

2.1   Strain Gage Instrumentation ………………………………………………………………………………  13

2.2   Pressure Cell Instrumentation …………………………………………………………………………….  16

2.3   Connector Block and Data Processor …………………………………………………………………..  16

2.4   Data Acquisition Program ………………………………………………………………………………….  19 Chapter 3 Field Experiments and Results ……………………………………………………………………..  22

3.1   Test Plan ………………………………………………………………………………………………………….  22

3.2   Trench Backfill Compaction Tests at State College ………………………………………………  25

3.2.1  Test SC-1: 6 inch Diameter SDR-35 Pipe in 2B Stone …………………………………………..  25

3.2.2  Test SC-2: 6 inch Diameter SDR-35 Pipe in 2A Aggregates ………………………………….  33

3.2.3  Test SC-3: 18 inch Diameter Reinforced Concrete Pipe ………………………………………..  39

3.2.4  Test SC-4: 18 inch Diameter HDPE Drainage Pipe……………………………………………….  45

3.3   Trench Backfill Compaction Tests at Harrisburg ………………………………………………….  51

3.3.1  Tests HB-1 to HB-4: 6 inch Diameter SDR-35 Pipe in 2B Stone ……………………………  52

3.3.2  Tests HB-5 to HB-8: 6 inch Diameter SDR-35 Pipe in 2A Aggregates ……………………  65 3.3.3  Tests HB-9 to HB-12: 18 inch Diameter Reinforced Concrete Pipe ………………………..  78 3.3.4  Tests HB-13 to HB-16: 18 inch Diameter HDPE Drainage Pipe …………………………….  91

Chapter 4 Conclusions and Recommendations ……………………………………………………………… 104

4.1   Effect of Hydraulic Plate Compactor on Compacted Dry Mass Density ………………….. 104

4.2   Effect of Hydraulic Plate Compactor on Compaction-Induced Downward Earth

Pressure ………………………………………………………………………………………………………….. 105

4.3   Effect of Hydraulic Plate Compactor on Compaction-Induced Strains in Pipe …………. 105 4.4   Effect of Lift Thickness for Hydraulic Plate Compactor ……………………………………….. 110 4.5   Conclusions and Recommendations …………………………………………………………………… 111

4.6   Limitations of Current Study …………………………………………………………………………….. 112 REFERENCES ………………………………………………………………………………………………………… 122

Chapter 1 

 

Introduction and Literature Review

1.1  Introduction

Pipelines are an important infrastructure system.  As most pipes are buried underground, backfill compaction plays an important role in the performance of buried pipelines.  Backfill materials should be adequately compacted to lock the pipe in place and reduce potential settlement from external loading (e.g., traffic loading).  On the other hand, excessive backfill compaction from heavy compactors (e.g., trench roller, hydraulic plate compactors) may damage the pipe.  For example, Hansen et al. (1997) studied the relationship between compaction loading and cracks of small-diameter concrete pipes and concluded that pipes can be damaged by compaction loading.

The excavator mounted hydraulic compactor was first introduced in 1980’s (St.  Louis 1980) which is also called ‘Hoe-Pack’ or ‘Ho-Pac’.  A hoe-pack consists of a hydraulic plate compactor mounted on the machine arm of a backhoe or an excavator.  Compaction tasks with hoe-pack are usually done by pressing backfill material using the machine arm together with operating the hydraulic plate compactor on the end of machine arm.  Using this equipment, there is no need for construction crew to get compaction equipment (e.g., roller compactor) into and out of a trench, thus the efficiency is greatly increased.  However, the relatively high compaction effort also increases the risk of damaging the pipe.  To reduce pipe damage and ensure adequate backfill compaction, standard specifications for lift thicknesses are provided (Pacific Northwest Concrete Pipe Association 1993).

1.2  Working Theory of Hydraulic Plate Compactor

Compacting equipment compacts soil by applying one or a combination of the following types of compaction effort: static pressure, impact, vibration, and kneading.  These different types of effort are found in the following two principal types of compaction forces: static and vibratory (Multiquip 2011).

Static force is often the deadweight of the machine, applying downward force on the soil surface, compressing the soil.  The effective compaction force can be changed by adding or reducing the weight of the machine.  Static compaction is typically limited to soils/materials near the surface and is most effective for thin layers of non-granular materials and asphalt (Allied 2004).  Kneading and static pressure are two examples of static compaction.

Vibratory force uses impact or vibration, usually engine-driven, to create a downward dynamic force in addition to the machine’s static weight.  The mechanism is usually a rotating eccentric weight or piston/spring combination (in rammers).  Impact compaction equipment (e.g., rammers, tampers) generates a low-frequency, long-stroke motion, which can break soil “clumps” into smaller pieces and rearrange the pieces together.  Impact compaction is effective for soils with less than 50% granular content (i.e., fine-grained soils).  On the other hand, vibration compaction equipment generates a higher-frequency, shorter-stroke motion, resulting in stress waves propagating through the soil, setting particles in motion and rearranging them into a higher density.  Vibration compaction is effective for soils with 50% or more granular content (i.e., coarse-grained soils) (Allied 2004).

Factors such as soil type, degree of compaction required, moisture content, etc., must be taken into consideration when choosing among various compactors.  A comparison of the relative performance of typical compaction equipment for different soil types is presented in Table 1.1.  Table 1.1 indicates that vibration is effective in densifying granular soils (i.e., sand and gravel).  In addition, vibratory compaction can work in materials with some cohesion (Selig and Yoo 1977).  When vibration is added to a static component, compaction is significantly increased, as shown in Figure 1.1.  For soils compacted on the dry side of optimum, adding the dynamic component results in increased density (Holtz and Kovacs 1981).

Table 1.1  Relative Performance of Typical Compaction Equipment for Different Soil Types (after Multiquip 2011)

Soil Types	Vibrating Sheepsfoot Rammer	Static Sheepsfoot Grid Roller Scraper	Vibrating Plate              Compactor Vibrating Roller Vibrating Sheepsfoot	Scraper Rubber-tired Roller Loader Grid Roller
	Impact	Static Pressure with kneading	Vibration	Kneading with static pressure
Gravel	Poor	Not used	Good	Very good
Sand	Poor	Not used	Excellent	Good
Silt	Good	Good	Poor	Excellent
Clay	Excellent	Very Good	Not used	Good

 

 

Figure 1.1  Compaction results on 30 cm (12 in.) layers of silty sand, with and without vibration, using a 7700 kg (17000 lb) towed vibratory roller (after Parsons et al.  1962, as cited by Selig and Yoo 1977)

Hydraulic plate compactors utilize a combination of static pressure and vibration to compact granular soils.  The static pressure is applied by the hydraulic system and extended arm of the carrier (e.g., excavator); the vibration and impulse energy are applied using eccentric weight rotating at a high rpm (revolutions per minute).  The rate and density of compaction depend on factors such as moisture content of the soil, condition of the compactor and carrier, and the skill of the operator.  The following factors are discussed.

Vibration Frequency: The effect of vibration frequency on compaction by smooth-drum vibratory rollers for different soils is shown in Figure 1.2.  Figure 1.2 indicates that a peak in the density-frequency curve develops for most soils, including clays.  The frequency at which a maximum density is achieved is called the optimum frequency (or resonant frequency), which is a function of the compactor-soil system, and it changes as the density increases during the process of compaction (Converse 1952; Holtz and Kovacs 1981).  Granular soil particles (e.g., sand and gravel) respond to different vibration frequencies differently depending on particle size.  The smaller the particle, the higher the frequency necessary to move/excite it.  For compacting soils consisting of large particles, moving up to a larger compactor with a lower frequency and higher vibration force may be advantageous.  Therefore, it is desirable for a compactor to have the capability to vary its operating frequency and have the range required to obtain maximum density.  However, the peaks are gentle as shown in Figure 1.2 and a wide frequency range may not be important (Holtz and Kovacs 1981).  For utility trench backfill, 2A aggregates (sandy gravel) are used according to PennDOT (Pennsylvania Department of Transportation) Publication 408/2011.  Figure 1.2 suggests that vibration frequencies at about 2000 cpm (cycles per minute) are effective for compacting this material.

For hydraulic plate compactors, the vibration frequency is controlled by the hydraulic flow input to the compactor.  A higher flow rate results in a higher vibration frequency but does not necessarily improve performance; on the contrary, it may result in fluid overheating, and contributes to early bearing failure.

Impulse Force and Downward Pressure: The impulse force is proportional to the product of eccentric mass and eccentric distance for a given vibration frequency.  An increase in impulse force and downward pressure results in higher compaction energy delivered to the soil and a greater effective compaction depth; however, fines content, location of ground water, the presence of a hard underlying layer that reflects vibrations, and other factors can all have a significant impact on the maximum depth and effectiveness of densification (Whetten and Weaver 1991).

 

 

Figure 1.2  Effect of vibration frequency on compaction by smooth-drum vibratory rollers (after several sources as cited by Selig and Yoo 1977)

 

Baseplate Dimensions: An increase in baseplate dimensions results in a greater effective compaction depth.  Some field compaction tests have indicated that excellent compacted dry densities were obtained to depths of one and a half time of the width of surface plate (e.g., Converse 1952).  For utility trench backfill, the closer the width of the baseplate is to the width of the trench, the higher the delivered compactive effort due to the effect of confinement from the trench.  Adjacent compacted sections should overlap slightly; excessive overlap may loosen the soil previously compacted.

1.3  Specifications of Common Models of Hydraulic Plate Compactors

Table 1.2 presents specifications of common models of hydraulic plate compactors from major manufacturers.  Table 1.2 indicates that most models come with a fixed vibration frequency at about 2,000 cpm, which is effective for compacting granular materials including 2A aggregates (see Figure 1.2).  Several manufacturers offer adjustable vibration frequencies in their models (e.g., Tramac by Montabert, Astec – Breaker Technology, Inc.).  As previously discussed, the advantage of adjustable vibration frequency may not be important for compacting 2A aggregate materials.

Table 1.2  Specifications of Common Models of Hydraulic Plate Compactors

Specifications	Allied Construction Products, LL				C
	300B	500B	1000B	1600B	2300B
Impulse Force (lbs.)	3,000	5,000	8,000	16,000	24,000
CPM	2,000	2,000	2,000	2,100	2,100
Baseplate Dimensions (in.)	12×22	13×27	24×28	29×32	34×36
Weight (lbs.)	162	162	200	320	N/A

Source: Allied (2014)

Specifications	Astec – Breaker Technology, Inc.
	TC51	TC71	TC92	TC152	TC301
Impulse Force (lbs.)	1,930 – 3,000	2,600 – 5,000	5,500 – 8,200	11,130 – 16,630	16,330 – 24,400
CPM	1,800 – 2,240	1,800 – 2,500	1,800 – 2,200	1,800 – 2,200	1,800 – 2,200
Baseplate Dimensions (in.)	12.5×28	15×31.5	23×34.9	28×45.7	34×48.4
Weight (lbs.)	295	610	1130	1820	2150

Source: Rock Breaker (2014)

 

 

 

Specifications		Caterpillar
	CVP16	CVP40	CVP75	CVP110
Impulse Force (lbs.)	3,650	8,928	16,508	24,669
CPM	2,200	2,200	2,200	2,200
Baseplate Dimensions (in.)	12×24	23×38	29×43	34×47
Weight (lbs.)	474	884	1,765	2,319

Source: CAT (2014)

Specifications			Furukawa Rock Drill
	HP35ME	HP65II	HP75	HP135II	HP210II
Impulse Force (lbs.)	3,000	6,500	7,800	13,500	21,000
CPM	2,000	2,000	2,000	2,000	2,000
Baseplate Dimensions (in.)	12×26	24×34	24×34	28×40	34×46
Weight (lbs.)	350	850	850	1,335 – 1,770	2,150 – 2,730

Source: FRD (2014)

Specifications			Hudco Manufacturing, Inc.
	HC-10	HC-12	HC-15	HC-20	HC-30	HC-40	HC-50
Impulse Force (lbs.)	3,000	3,500	5,000	6,500	13,500	20,000	22,000
CPM	2,100	2,100	2,100	2,000	2,000	2,000	2,100
Baseplate Dimensions (in.)	12×30	12×31	16×36	24×34	29×40	34×47	34×47
Weight (lbs.)	280	350	700	900	1,600	2,600	2,600

Source: Hudco (2014)

Specifications			Kenco
	KC-12	KC-15	KC-20	KC-30	KC-40	KC-50
Impulse Force (lbs.)	3,500	5,000	6,500	13,500	20,000	22,000
CPM	2,100	2,100	2,000	2,000	2,000	2,100
Baseplate Dimensions (in.)	12×31	16×36	24×34	29×40	34×47	34×47
Weight (lbs.)	350	700	900	1,600	2,600	2,600

Source: Kenco (2014)

Specifications	Stanley Black & Decker, Inc.
	HSX3	HSX6	HSX11	HSX22
Impulse Force (lbs.)	3,400	6,400	11,350	22,000
CPM	2,100	2,000	2,000	2,100
Baseplate Dimensions (in.)	19×20	24×26	27×30	32×42
Weight (lbs.)	370	850	1,425	2,200

Source: Stanley (2014)

Specifications		Tramac by Montabert
	TR-6	TR-9	TR-14	TR-21	TR-40TM
Impulse Force (lbs.)	1,800 – 3,200	3,800 – 7000	7,000 – 14,500	15,000 – 21,500	21,000 – 40,000
CPM	2,600 – 3,740	1,800 – 2,600	1,800 – 2,600	2,000 – 2,300	1,500 – 2,100
Baseplate Dimensions (in.)	12×18	23×31	23×35	23.5×41	35×41
Weight (lbs.)	250	715	850	1,500	2,500

Source: Tramac (2014)

1.4  Selection of Hydraulic Plate Compactors

The hydraulic plate compactor should be selected properly for the carrier on which it is mounted and the compaction work it needs to perform.  For utility trench backfill with 2A aggregate materials, most models of compactors can provide good compaction if proper lift thickness is used.  If the lift is too thick, it will either take long compaction time to reach the desired level of compaction, or the desired level will be unattainable.  If the lift is too thin, soil may also be over compacted, thereby wasting time, causing “cracking” of the compacted layers and creating unnecessary wear on the compaction equipment as excessive impact force is transferred back into the compactor (Allied 2004).  Generally speaking, compactors with higher impulse energy can use a higher lift of materials to achieve the same compacted dry density, which increases productivity.  However, higher impulse energy and downward pressure may potentially damage utility pipes beneath the soil being compacted, particularly during the compaction of the first lift above the pipe at crown (Demartini et al. 1997; Kararam 2009).  It may be necessary to try different lift heights to determine the most effective lift to achieve the desire level of compaction and productivity.  An instrumented field study is hence warranted to investigate the effect of lift thickness on the level of compaction attained and compaction-induced downward pressure in compacted fill and strains developed in utility pipes.

Generally speaking, the amount of impulse force necessary to achieve desired compaction depends on soil particle size.  For compacting soils consisting of large particles, moving up to a larger compactor with a higher impulse force may be advantageous.  For 2A aggregates (sandy gravel) used in utility trench backfill, a large percentage of the mixture has particle sizes between 50 mm and 4.75 mm (#4 sieve).  For this range of particle sizes, a moderate level of impulse force is necessary for effective compaction.  Therefore, hydraulic plate compactors with mid-range impulse forces may be desirable.  The top three hydraulic plate compactors that may best fit the needs of PennDOT are considered to be: Model 1000B from Allied Construction Products LLC, Model CVP40 from Caterpillar, and Model KC-20 from Kenco (see Table 1.2).

1.5  Past Research on Backfill Compaction

The theory of earth loads on buried pipes was developed by Marston (1930).

Based on Marston’s theory, Spangler (1933) determined the supporting strength of buried rigid pipes when subjected to earth load.  The theory developed by Marston and Spangler formed the basis of current “Indirect Design” method.  The design of pipes (e.g., pipe material and geometry) has been greatly improved since our knowledge about the behavior of soil-pipe system was developed.  New materials such as PVC (polyvinyl chloride) and HDPE (high density polyethylene) were developed for making pipes at low cost and resistant to rust and corrosion.

Aside from the pipe itself, the performance of a buried pipe is also affected by the bedding materials (Marston et al. 1917; Schlick 1920).  Parmelee (1973) studied the behavior of buried pipe under imposed loading, including field test and FEM (finite element method) simulation.  Another field-scale test evaluating soil-pipe interaction was conducted by Webb et al. (1996), the results showed that rammer compactors were better than vibratory plate compactors in producing greater compaction density, however, it could also produce higher impact load onto the pipe.  Zoladz et al. (1996) conducted laboratory tests of soil-pipe interaction under various backfill compaction conditions.  Test variables such as pipe types, trench conditions, backfill materials, compaction methods, bedding conditions and haunch effort were considered.  It was concluded that wider trenches can result in higher upward deflections during sidefill compaction; rammer compactor was most efficient by pushing backfill materials into the haunch zone and thus developing lateral earth pressure at the side of the pipe; coarse materials need less compaction energy to achieve the same compaction density than fine materials.  As a following up to the previous research, Webb et al. (1998) studied the supporting capability of low strength materials and proved that they can provide excellent support for pipe in hard-to-reach areas.

Instrumentation plan of soil-pipe system tests is crucial in yielding accurate and reliable experiment data.  McGrath et al. (1999) suggested measurements including pipe dimensions, pipe deformation, soil-pipe interface pressure, soil density, soil pressure, and soil deformation data to be collected.  McGrath et al. (2000) conducted full-scale field test based on his proposed instrumentation plan to evaluate soil-pipe interactions while backfilling and compacting, the test protocol worked very well in obtaining comprehensive soil-pipe interaction behavior.  When it came to the aspect of sensors used in soil-pipe systems, Talesnick (2005) studied the design of soil contact pressure sensors and observed that, these sensors, if well calibrated, showed good response in measuring soil-pipe interaction behaviors.

Kararam (2009) conducted a comprehensive research on the effect of hoe-pack on backfill compaction of underground conduit.  He investigated the behavior of a reinforced concrete pipeline under a compaction process using both experimental and numerical simulation methods.  Strain gages and pressure cells were installed on the concrete pipes to measure the deformation and compaction-induced pressure respectively.  During the compaction tests, cracks and fractures were developed on the sidewall of concrete pipes.  The test results showed that the pipes were most likely to be damaged due to the compaction force when the first layer of backfill above the pipe’s crown was being compacted.  Furthermore, the most critical compaction location was at the joints of the pipe system.  Kararam (2009) also developed a FEM model simulating “D-load test” (three-edge bearing test) of concrete pipes.  Full-scale experiments were also conducted

to verify the numerical simulation results.  It was concluded that the FEM model coupled with a discrete crack model yielded matching results with the experiments.

1.6  Objective and Scope of Work

Backhoe-mounted hydraulic plate compactors (hoe-packs) have been increasingly used for soil or material compaction in trenching, street repairing, or site preparations.  Comparing to traditional compaction tools such as roller compactors, hoe-packs are advantageous for utility trench backfill compaction as they can be operated by the backhoe operator and requires no lifting of compaction machine during compaction process.  However, uncertainty remains with regard to the maximum lift thickness to consistently achieve desired compacted dry mass density by hydraulic plate compactors.  The large impulse energy and down pressure exerted by hydraulic plate compactors also raise concerns on potential damages to utility pipes.

The objective of this research project is to assess the capability of vibratory hydraulic plate compactors for compaction in utility trench backfill.  Specifically, the project aims to investigate: (1) the maximum lift thickness to consistently achieve the minimum specified Standard Proctor Density (SPD) of trench backfill in the cover zone as per PennDOT Pub 408 (2011.5), Section 206.3(b)1, (2) the effect of lift thickness on compaction-induced downward earth pressure in the backfill zone, and (3) the effect of lift thickness on compaction-induced longitudinal and hoop strains in typical pipes.

1.7  Thesis Outline

This thesis will be presented in four chapters.  Chapter 1 includes background information and research objectives.  Chapter 2 will present methodology and instrumentation plan for the field tests conducted.  Chapter 3 will present the results from the field tests conducted.  Chapter 4 will present the conclusions and recommendations for future studies.

EFFECT OF HYDRAULIC PLATE COMPACTOR AND LIFT THICKNESS ON UTILITY TRENCH BACKFILL COMPACTION

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