FIELD TESTING TO DETERMINE THE SUITABILITY OF BIOSOLIDS FOR EMBANKMENT FILL. and Phone: ABSTRACT

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1 FIELD TESTING TO DETERMINE THE SUITABILITY OF BIOSOLIDS FOR EMBANKMENT FILL V. Suthagaran 1, A. Arulrajah 1, M.W. Bo 2 and J. Wilson 1 1 Swinburne University of Technology, Melbourne, Australia, 2 DST Consulting Engineers, Ontario, Canada. aarulrajah@swin.edu.au and Phone: ABSTRACT Biosolids are defined as appropriately treated sewage sludge which consists of untreated organic slurry residue derived from wastewater treatment processes. The use of biosolids and other waste materials in a sustainable manner is currently being investigated in several countries around the world. This paper reports on results obtained from field investigations to assess the viability of using biosolids as stabilised fill for road embankments. Field tests were recently carried out on biosolids stockpiles at the Western Treatment Plant in Victoria, Australia. The field tests included field vane shear tests, standard penetration tests (SPT) and dynamic cone penetrometer (DCP) tests. Undisturbed tube samples and disturbed bulk samples were also collected for future laboratory testing. Geotechnical parameters from the field test were obtained and compared with the existing local road authority specification for fill material. KEY WORDS bearing capacity, biosolids, field testing, shear strength. INTRODUCTION With the advent of global industrialization, extensive amounts of waste are generated daily by various industries and human activities. The disposal of solids waste such as biosolids is a major problem throughout the world. The sustainable usage of waste materials in engineering applications is of social and economic benefit to industrialized nations. Due to the shortages of natural mineral resources and increasing waste disposal costs, recycling solids wastes has become significant in recent years. Biosolids refers to dried sludge having the characteristics of a solid typically containing 50% to 70% by weight of oven dried solids. Sludge refers to solids-water mixture pumped from wastewater treatment lagoons having the characteristics of a liquid or slurry typically containing between 2% to 15% of oven dried solids. The quantity of the municipal biosolids produced annually in the world has increased dramatically over the decades. The engineering characteristics of biosolids is required to be investigated to assess the suitability of biosolids in engineering projects such as roads, embankments and other stabilised fill applications. The characteristics of the biosolids around the world vary as the properties of the biosolids depend on factors such as the type of waste, type of treatment process and age of the biosolids. This paper presents a literature review and results from field tests undertaken on biosolids at a waste water treatment plant in Victoria, Australia. The current research investigates the potential usage of biosolids as engineering fill in road embankments. Depending on the type of construction project, the engineering parameters of biosolids must be investigated to determine the viability of biosolids as a construction material. California Bearing Ratio (CBR), bearing capacity and the shear strength of biosolids are essential parameters in the investigation into biosolids use as a fill material for stabilised embankments. Field tests were recently carried out on biosolids stockpiles at a Western Treatment Plant in Victoria, Australia. The field tests included field vane shear tests, standard penetration tests (SPT) and dynamic cone penetrometer (DCP) tests. Geotechnical parameters for the biosolids were obtained from the field test and compared with the existing local road authority specification for fill material.

2 PAST STUDIES ON BIOSOLIDS The important parameters for designing and analysis of stabilised biosolid embankments include the compaction and shear strength parameters. A literature review indicates that very few studies on geotechnical properties of untreated and stabilised biosolids are available. The geotechnical aspects of sewage sludge that have been studied in recent years in various countries such as UK, Hong Kong, USA, Turkey and Singapore are summarised in this section for background information. In one of the few available studies on biosolids, Hundal et al. (2005) have presented a study to evaluate the geotechnical parameters such as basic physical properties, compressibility, consolidation characteristics and shear strength parameters of the untreated municipal wastewater treatment plant in Chicago, USA. The tri-axial and unconfined compressive strength tests were carried out to determine the shear strength parameters of the untreated biosolids. The Illinois bearing ratio (IBR) tests were conducted to derive the bearing capacity of the biosolids, it ranged between 1.6 to 4.8. Hundal et al. (2005) concluded that biosolids are suitable fill material for embankment construction and bearing capacity can be increased by blending the biosolids with topsoil or other residuals. Reinhart et al. (2003) have reported on the compressibility and shear strength properties of the municipal solid waste mixed with the biosolids and the lime sludge. The modified compaction test and the large scale direct shear test were conducted to establish strength and compressibility properties of both solid waste and the mixture of the solid waste with biosolids and the lime sludge. Laboratory tests showed that, lime sludge had more inherent shear strength than biosolids. The mixing of biosolids and lime sludge reduces the strength of the waste in the landfill (Reinhart et al. 2003). Vajirkar (2004) has reported on the strength characteristics of biosolids when mixed with municipal solid waste based on cone penetration tests carried out in Florida, USA whilst Kocar et al. (2003) has reported on the use of fly ash as an additive in the stabilisation of biosolids and sludge in Turkey. Koenig at al. (1996) have reported the shear strength of the dewatered sewage sludge in Hong Kong by using the vane shear test. Koenig at al. (1996) further reported that the consolidation behaviour of the sewage sludge followed that of the conventional consolidation theory. Chu et al. (2005) has reported on the consolidation properties of cement-treated sewage sludge in the Republic of Singapore with the use of prefabricated vertical drains. Chu et al. (2005) and Goi (2004) have reported on the geotechnical properties of sewage sludge in Singapore and proposed the option of using cement-treated sewage sludge as a fill material for land reclamation activities in Singapore. Pore pressure dissipation of the sewage sludge was measured during the consolidation process in a large-consolidometer to enable the consolidation around prefabricated vertical drain. Ordinary Portland cement and hydrated lime were used as binder materials for the consolidation test that lasted 550 hours. Lo et al. (2002) has reported on the geotechnical characterisation of dewatered sewage sludge generated from the Stonecutters Island treatment plant in Hong Kong. Compaction tests carried out indicated that the dewatered sewage sludge exhibits compaction characteristics similar to that of clayey soils. Lo et al. (2002) confirmed on the findings of Klein and Sarsby (2000) that sludge once placed in landfills can be considered as geotechnical material similar to normally consolidated cohesive material with high organic contents. In addition to consolidation and compaction tests, direct shear tests were also carried out on the sludge mixtures. Kelly (2004, 2005, and 2006) reported on the various geotechnical characteristics of sludge at the Tullamore wastewater treatment plant in the United Kingdom in terms of their strength, compaction, compressibility and other geotechnical properties. Kelly (2004) reported that in the United Kingdom, the sewage sludge is eventually disposed in landfills (sludge-to-landfill) which is different from the typical requirement of 3 year air-drying and subsequent stockpiling of biosolids in Australia. Kelly (2004) stated that sludge material in various treatment plants can have different engineering properties due to different input levels of domestic and industrial wastewater.

3 GEOTECHNICAL FIELD TESTING OF BIOSOLIDS Site descriptions Field sampling and tests were carried out on a recently constructed Biosolid Stockpile Area located in the Western Treatment Plant in Victoria, Australia. The treatment plant is located approximately 50 km to the west of Melbourne. Following the construction of the 18 ha Biosolid Stockpile Area, approximately 150,000 m 3 of biosolids were harvested from sixteen existing Sludge Drying Pans and stockpiled. The Biosolid Stockpile Area was constructed with provision for the stockpiling of seven rows of biosolids stockpiles each up to 5 meters high and separated by access roads. Geotechnical sampling and field testing works were carried out from the top of the three existing biosolids stockpiles within the Biosolid Stockpile Area (Figure 1). Figure 1: Biosolids stockpiles at the Western Treatment Plant, Victoria (Australia) Sampling and testing In total, twelve boreholes were drilled from the top of the biosolids stockpiles for the full 4-5 m depth of the biosolids with a geotechnical drilling rig. Four undisturbed samples were obtained with 100 mm diameter sample tubes in each of the borehole. A total of 48 undisturbed samples were obtained from the field and transferred to the laboratory. The standard penetration test results of biosolids at the Western Treatment Plant in Victoria are tabulated in Table 1. Four standard penetration tests (SPT) were carried out in some of the boreholes as shown in Figure 2. Table 1: Summary of SPT test results at the Western Treatment Plant, Victoria Borehole Stockpile Depth (m) SPT N (Blows) Allowable bearing Capacity (kpa) Consistency BH Firm BH Firm BH Very Stiff BH Firm

4 Figure 2: Standard Penetration Test (SPT) at a borehole location Twenty numbers of field vane shear tests were carried out within the boreholes at one meter depth intervals to determine the in-situ vane shear strength of the biosolids. Peak and residual field vane shear strength of each stockpile is summarised in Table 2. Twelve dynamic cone penetrometer (DCP) tests were carried out adjacent to each borehole (Figure 3). Table 3 shows the bearing capacity and the California Bearing Ratio (CBR) of each stockpile obtained from dynamic cone penetration tests. Approximately 2500 kg of bulk biosolid samples were also obtained from the biosolids stockpile area for laboratory testing purposes. Bulk samples were collected in large bags which were sealed to retain the natural moisture content of the biosolids.

5 Table 2: Field vane shear strength of biosolids at the Western Treatment Plant, Victoria Borehole Stockpile BH2 1 BH4 1 BH6 2 BH8 2 BH10 3 BH12 3 Depth Peak Shear strength (kn/m 2 ) Residual Shear strength (kn/m 2 ) Sensitivity Consistency 1.0m Very Stiff 2.0m Very Stiff 3.0m Hard 4.0m Hard 1.1m Very Stiff 2.1m Hard 3.1m Very Stiff 4.1m Hard 1.0m Firm 2.0m Very Stiff 3.0m Hard 4.0m Stiff 1.0m Very Stiff 2.0m Very Stiff 3.0m Very Stiff 4.0m Very Stiff 1.0m Stiff 2.0m Hard 3.0m Hard 4.0m Very Stiff 1.0m Firm 2.0m Firm 3.0m Very Stiff 4.0m Hard Figure 3: Dynamic Cone Penetrometer (DCP) tests at the Western Treatment Plant

6 Table 3: Stockpile Summary of DCP test results at the Western Treatment Plant, Victoria DCP Depth (m) CBR Allowable Bearing Capacity (kn/m 2 ) Consistency DCP Firm Very Stiff Very Stiff Hard Hard DCP Firm Very Stiff Very Stiff Hard Very Stiff Hard DCP Firm Stiff Stiff Very Stiff Very Stiff Hard DCP Firm Very Stiff Stiff Hard Very Stiff Hard DCP Firm Very Stiff Stiff Very Stiff Very Stiff Hard DCP Firm Very Stiff Very Stiff Very Stiff Hard DCP Firm Very Stiff Stiff Very Stiff Stiff Very Stiff DCP Firm Very Stiff Stiff Very Stiff Very Stiff Hard DCP Firm Stiff Stiff Stiff Firm Very Stiff DCP Firm Stiff Stiff Stiff Very Stiff DCP Firm Very Stiff Stiff Very Stiff Stiff Very Stiff DCP Firm Stiff Stiff Very Stiff Very Stiff Hard EVALUATION OF FIELD TESTING RESULTS A firm layer of biosolids (4< SPT <8) was encountered in the three stockpiles at depths ranging from 1.5 m to 3.0 m whilst a very stiff layer of biosolids (16< SPT <30) was encountered in Stockpile 2 at a depth of 4.0 m. The allowable bearing capacity of the biosolids was found to vary between 70 to 80 kpa at a depth of 1.5 m to 3.0 m in the three stockpiles whilst the allowable bearing capacity of the biosolids in Stockpile 2 was found to be 230 kpa at depth of 4.0 m. The field vane shear tests results indicated that the undrained shear strength of biosolids generally increased with the depth of the stockpile. Consistency of the biosolids based on the field vane shear test can be classified as very stiff to hard and the sensitivity of the biosolids was found to vary between 2.3 to 6.8. The DCP test results obtained from Stockpiles 1 and 2 indicate that the biosolids are firm to very stiff at depths from 0 to 0.5 m whilst below the depth of 0.5 m, the biosolids are found to be stiff to hard. The DCP test results obtained from Stockpile 3 indicate that the biosolids is firm to stiff at depths from 0 to 0.5 m and stiff to very stiff below a depth of 0.5 m. The average CBR and allowable bearing capacity values of the stockpiles are summarised in Table 4.

7 Table 4: Stockpile Average CBR and allowable bearing capacity with depth Depth (m) Average In-situ CBR Average allowable bearing Capacity (kpa) It is noted that the various field testing methods consistently indicate that the biosolids at the stockpiles are firm to hard. The slight variability between the various field testing methods is expected due to the various assumptions and empirical equations used in each test methods. COMPARISON WITH LOCAL ROADWORK SPECIFICATIONS VicRoads is the local road governing authority in Victoria, Australia. VicRoads classifies fill material for the earthworks into three types which are Type A, B and C. VicRoads requirement for Type B fill is a California bearing ratio (CBR) value of 2% to 5% (VicRoads, 2006). VicRoads also classifies fill material based on the type, particle size and physical and mechanical properties of the material. Type B fill material is also defined by VicRoads (2006) to be free of top soil, deleterious and/or perishable matter and after compaction shall have a maximum particle dimension of not more than (i) 150 mm within 400 mm of subgrade level; (ii) 400 mm at depths greater than 400 mm below subgrade. In-situ CBR values obtained from the DCP test results ranged between 2% and 25% for the uncompacted biosolids which satisfy the VicRoads specification for Type B fill material without further compaction. This preliminary finding indicates the potential for biosolids use as fill material in embankments though this will have to be confirmed in the next phase of laboratory testing. CONCLUSIONS The consistency of biosolids in the stockpiles was found vary from very stiff to hard based on the field vane shear tests, firm to very stiff based on the standard penetration tests and firm to hard based on the dynamic cone penetrometer tests. In-situ CBR values obtained from the dynamic cone penetrometer test results ranged between 2% and 25% for the uncompacted biosolids which satisfy the VicRoads specification for Type B fill material without further compaction. The results presented here indicate the potential for reuse of biosolids as a construction material for embankment fills. The next phase of this research will be to undertake laboratory tests to confirm the CBR values of untreated as well as stabilised biosolids and compare the laboratory results with the field results. ACKNOWLEDGEMENT The authors would like to acknowledge the Smart Water Fund for funding this research project (Project No: 42M-2059). The Smart Water Fund is an initiative of the Victorian Government and the Victorian water industry in Australia aimed at encouraging innovative solutions to water conservation, water management and biosolids management.

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