Relative performance of compacted and uncompacted rockfill

Matheson, G.M. (Schnabel Engineering Associates)


ABSTRACT Introduction Rockfill is frequently used as a construction material for roadways, embankments dams, and valley fills for commercial, residential, or industrial developments. The construction practices used to place rockfill vary widely depending on the fills ultimate purpose. The behavior of rockfill, in terms of long-term settlement potential, has been a significant concern of the engineering community for many years. Dascal (1987), Clements (1984), and Penman (1971) have provided documentation of the behavior of rockfill dam embankments. Little literature is available, however, on the behavior of embankments constructed as valley fills for roadways or structural support. This paper presents the results of monitoring of several large valley rockfill embankments that have been monitored for settlements for periods of up to 95 months. An unique aspect of the data is a comparison of long-term settlements of a fill that was completed in two parts; one compacted during construction and the other not compacted. This data in combination with other rockfill settlement data, clearly demonstrates the benefits of compaction and the behavior of rockfill and the relative magnitude and time rate of settlement that can be expected from the fills. In addition, data is available that has allowed an estimate of the deformation moduli of rockfill. Description of Rockfills The rockfills were constructed as part of the Colony Shale Oil Project in Western Colorado. Three major fills were constructed as part of this project that consisted of an aggregate total of greater than 15 million cubic yards of rockfill. The fills under consideration are termed the North Valley fill, South Valley fill, and the Mine Bench fill. The North and South Valley fills were constructed as part of a plant site development. For this development, adjacent hillsides were excavated by ripping and blasting and then placed into adjacent valleys to create a level site area. The fills are to be used for support of plant site structures and, as a result, it was desired to minimize the long-term settlement potential of the fills. The rock used in the plant site development consisted of friable sandstones and siltstones of the Unita Formation. These rock particles typically had an average unconfined compressive strength of about 2600 psi. The fill was generally poorly graded with the majority .of the fill particles less than 12 inches in minimum dimension, 25 to 40 percent of the material passing the No. 4 Sieve, and less than 20 percent of the material passing the No. 200 Sieve. The fine fraction of the material was non-plastic as defined by the Atterberg Limits test. For further detail on these fills and their characteristics see Matheson (1986) and Matheson and Parent (1989). At the end of fills construction in 1982, settlement monuments were placed on each fill to document their long-term settlements. Figure 1 and 2 present the configuration of the fills and the location of the settlement monuments. As shown on these figures, the depth of fill below each monitoring point varied from about 120 to 155 feet for the South Valley fill and 30 to 95 feet for the North Valley

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