In this study, the strength of soft cohesive subgrade soil was improved by applying sewage sludge ash as a soil stabilizer. Test results obtained were compared with earlier tests conducted on soil samples treated with fly ash. Five different proportions of sludge ash and fly ash were mixed with soft cohesive soil, and tests such as pH value, compaction, California bearing ratio, unconfined compressive strength (UCS), and triaxial compression were performed to understand soil strength improvement because of the addition of both ashes. Results indicate that pH values increase with extending curing age for soil with sludge ash added. The UCS of sludge ash/soil were 1.4–2 times better than untreated soil. However, compressive strength of sludge ash/soil was 20–30 kPa less than fly ash/soil. The bearing capacities for both fly ash/soil and sludge ash/soil were five to six times and four times, respectively, higher than the original capacity. Moreover, the cohesive parameter of shear strength rose with increased amounts of either ash added. Friction angle, however, decreased with increased amounts of either ash. Consequently, results show that sewage sludge ash can potentially replace fly ash in the improvement of the soft cohesive soil.

Fourteen Texas SPS-3 test sites were studied to determine the effectiveness of preventative maintenance treatments. These sections were built on four highway classifications (IH, US, SH, and FM) in different climates and with different levels of traffic and subgrade support. Almost all 14 SPS-3 sites were given preventative maintenance treatments (thin overlay, slurry seal, crack seal, and chip seal) in Fall 1990. The distress score concept used by the Texas Department of Transportation (TxDOT) was adopted in this study to judge the effectiveness of preventative maintenance treatments. TxDOT has used this concept since the early 1980s, though the utility factors have been revised few times. The distress score quantifies the visible surface wear due to traffic and environmental influences. Only very few sections experienced premature failures on the SPS-3 sites in Texas. In many cases, superior underlying pavement conditions have been found. The chip seal has the most sites in which it is rated the best performer. The chip seals performed well on a wide range of pavement conditions. In fact, chip seals have the highest distress score for both high and low traffic areas. When initial cost is considered, crack seal provides the best alternative for low traffic routes that have a sound underlying pavement structure. For high traffic routes, chip seal is a better choice. However, a thin overlay is the most effective for rut resistance. Since the thin overlay has the highest initial cost, it is best used on high traffic routes where rutting is a major concern. If rutting is not a concern, chip seal is the best choice for a high traffic area. The treatments applied to US84 sections were too late and did not reach seven years of life as normally was expected, which reconfirms that the timing for preventive maintenance treatment is very important.