Date of Award

12-2013

Degree Name

MS in Civil and Environmental Engineering

Department

Civil and Environmental Engineering

Advisor

James Hanson

Abstract

An extensive laboratory and field investigation was conducted at Santa Maria Regional Landfill (SMRL) in Santa Maria, CA to determine the effects of waste placement practices on the engineering response of municipal solid waste (MSW). Laboratory and field testing was used to determine the engineering properties and monitor field response of MSW.

The specific gravity (Gs) of manufactured MSW (MMSW), fresh MSW (FMSW), and old MSW (OMSW) was determined experimentally using a modified version of standard soil testing procedures. Effects of particle size, compactive effort, and degradation on the specific gravity of waste were evaluated. Specific gravity of manufactured waste samples increased with decreasing particle size, with compaction, and with increased degradation. The average specific gravity of uncompacted MMSW samples was 1.333, 1.374, and 1.424 for coarse, medium, and fine particle sizes, respectively. Specific gravity of coarse, medium, and fine MMSW samples compacted at dry of optimum (= 30%) was determined to be 1.497, 1.521, and 1.552, respectively and at wet of optimum ( = 90%) to be 1.500, 1.542, and 1.570, respectively. The compacted and uncompacted specific gravity of fresh MSW was lower than manufactured and old MSW. The average Gs of uncompacted and compacted fresh MSW was 1.072 and 1.208, respectively whereas old MSW had Gs of 2.201.

Additional physical and engineering properties of MSW were determined for fresh and old wastes. A total of 8 magnetic extensometer settlement arrays and 4 thermocouple arrays were installed in old wastes. The settlement and temperature data were collected for an approximate duration of 1 year. In addition, laboratory experiments were conducted to determine the particle size distribution, organic content, and moisture content of fresh waste sampled from the active face of the landfill and from old waste sampled from different depths. The particle size distribution of OMSW was comparable to a well-graded coarse-grained soil. The average baseline moisture content of incoming MSW at SMRL was 42.7% (dry-weight basis). The average moisture content of residential MSW, commercial MSW, and self-delivered MSW were determined to be 57.7, 46.3, and 12.0%, respectively. The organic content of fresh and old MSW was determined to be 77.2 and 23.5%, respectively. Temperature increased over time due to heat generation of the waste mass. The temperature increased on average 3 to 6°C between the initial and final day of measurements for wastes that were 0.3 to 9 years old.

Fresh and old wastes at SMRL exhibited unique compression behavior. A majority of the waste was undergoing secondary compression characterized using a secondary compression ratio () ranging from 0.013 to 0.067 with an average of 0.030. In addition, the fresh and old wastes exhibited recompression behavior. Fresh waste lifts were determined to be slightly overconsolidated such that the self-weight of the fresh waste was less than the preconsolidation stress. The old waste exhibited recompression behavior during loading and unloading of an earthen embankment. The modified recompression indices () for fresh and old wastes were determined to be 0.076 and 0.012, respectively. The initial compression ratio for old wastes () was quantified for the old waste lifts to be between 0.069 and 0.332.

Finally, meso- and full-scale field compaction experiments were conducted to determine the effects of systematic moisture addition prior to compaction on placement efficiency and compaction characteristics of MSW. Two 16 x 46 m test plots were constructed for the meso-scale compaction tests. Approximately 890 kN (100 tons) of residential MSW (RMSW) was placed into a test plot and compacted at target moisture contents of 55 (baseline as-received), 65, 80, 95, and 110%. Compaction curves generated for RMSW were bell shaped and similar to soil compaction curves. The maximum dry unit weight () and operational unit weight () for the meso-scale compaction study were 8.5 and 13.3 kN/m3 with corresponding optimum moisture contents of and , 78.5 and 79.5%, respectively. Moisture addition prior to compaction yielded beneficial waste placement results. An operational waste placement factor (OWPF) was defined as additional amount of waste that could be placed in one unit of volume. OWPF values were determined to be 1, 1.33, 1.66, 1.37, and 0.83 for RMSW compacted at target moisture contents of 55, 65, 85, 90, and 110%, respectively.

The full scale compaction investigation was conducted in a similar manner to the meso-scale investigation. However, the compaction tests were conducted on the active face of the landfill and representative of the entire incoming daily waste stream. A daily average of 2940 kN (330 tons) of MSW was placed and compacted at target moisture contents of 45 (baseline as-received), 65, 85, and 105%. Compaction curves for the delivered MSW were bell shaped and similar to soil compaction curves. The maximum dry and operational unit weights for the full-scale test were 7.0 and 9.8 kN/m3, respectively corresponding to optimum moisture contents of and , 76 and 75.5%, respectively. OWPFs were calculated to be 1, 1.28, 1.55, and 0.80 for target moisture contents of 45, 65, 85, and 105%, respectively.

The spatial variability associated with moisture addition also was determined for the meso- and full-scale compaction tests and verified using test pits and spatial sampling of the surface of the active face. Particularly, the variations in degree of saturation (S) and volumetric moisture content () due to moisture addition were estimated. For waste compacted at target moisture contents of 55, 65, 80, and 110% during the meso-scale tests, S increased by 19, 4.5, 4.4, and 4.3%, respectively while increased by 28, 7.7, 8.1, and 5.7%, respectively. For the full scale compaction tests, S increased by an average of 43% and increased by an average of 78%. The average moisture content of waste at the surface after compaction at 45% moisture content (i.e., as-received) and at 80% moisture content (i.e., near optimum) were 34 and 133%, respectively. The results of the investigation have environmental, operational, and financial implications such as extend the life of a landfill, ability to place more wastes in a unit landfill volume, and increasing to values above field capacity with moisture addition during compaction.