ORIGIN AND TREATMENT OF HYDROCOMPACTION IN THE SAN JOAQUIN VALLEY, CA, USA Nikola P. Prokopovich United States Bureau of Reclamation Sacramento, California Abstract Hydrocompact ion is a property of some dry sediments which causes them to spontaneously slump, crack and collapse after wetting. In California's San Joaquin Valley, hydrocompaction with vertical displacements up to 5 m and cracks up to 2 m wide has spotty occurrences in Pleistocene mudflow deposits. After completion of the process, the originally hazardous deposits become stable. The probable origin of hydrocompact ion in Cali­ fornia and in some loess deposits is Pleistocene, periglacial, sublimation of near surface wet deposits, expanded by previous freezing. Hydrocompaction in California affected two reaches of the San Luis Canal and alinements of several associated pipelines. In order to prevent postconstruction damage, 32.5 km of canal alinement and 90 km of selected pipeline alinements were flooded prior to construction, at a cost of some 8 million dollars. No failures have occurred in prewetted areas. Addi­ tional wetting of some untreated road crossings and pipeline laterals is under consideration, due to numerous failures in nonprewetted areas. Introduction The paper briefly summarizes data on distribution and treatment of hydrocompaction in a major irrigation project in the semiarid, west- central portion of the San Joaquin Valley, California, USA (Fig. 1). The project, known as the San Luis Unit of the Central Valley Project, consists of several dams, canals, underground distribution pipelines, and associated structures (Anonymous, 1981). Bureau studies of hydrocom­ paction included delineation of areas potentially susceptible to hydrocom­ paction, estimates of ultimate amounts of hydrocompaction, selection of methods to control the destructive effects of hydrocompaction, precon- struction treatment of selected alinements and evaluation of the effi­ ciency of this treatment. The text is based mostly on.data collected during two decades of preconstruction, construction and postconstruction studies by the Federal Bureau of Reclamation. The ideas expressed in this paper are, however, those of the author and may not represent the official view of the Bureau. Hydrocompaction - General Data Hydrocompaction occurs in some dry, unconsolidated, porous semiarid, and arid deposits when they lose their dry strength after wetting (sometimes rapidly), and develop spontaneous settling, slumpage, and cracking. The process can be one of the most rapid and highly destructive forms of subsidence. Typical surface expressions of active hydrocompaction are soil cracks and settling, development of "sinkholes" similar to karst sinks, undulatory fields, piping, and structural damage to houses, industrial and agricultural structures, railroads, highways, and bridges. Particularly severe damage occurs to dams, canals, ditches and wells (Fig. 2). The process is frequently related to human activity, such as irrigation, construction of canals, urbanization, disposal of industrial wastewater, etc. 537 SACRAMENTO I TRACY ) /A PUMPING PLANT^Â.6j? DELTA-MENDOTA CANAL CALIFORNIA AQUEDUCT SAN LUIS DAM, RES. a -^<:\^?|>/ PUMPING-GENERATING PLANT \jA LOS BANOS DETENTION DAM S RESERVOIR LITTLE PANOCHE DETENTION DAM d RES. SAN LUIS CANAL .-, FRIANT 2 \ DAM PLEASANT VALLEY PUMPING PLANT a COALINGÀ CANAL SAN LUIS 5 DRAIN HYDROCOMPACTION (CONSTRUCTED) SAN JOAQUIN VALLEY SERVICE \ 5 AREA w BOUNDARY 0 ,1 FEDERAL PROPOSED /^-CALIFORNIA AQUEDUCT Hit Mil FEDERAL CONSTRUCTED /'•/ • / •FRIANT- STATE CONSTRUCTED .It? / KERN t! / CANAL JOINT CONSTRUCTED t e s* « PUMPING PLANT BAKERSFIELD 0 50 Ml h 80 KM \. / Fig. 1. San Luis Unit and other major Federal and State conveyance systems in the San Joaqin Valley, California. 538 Fig. 2. Typical damage due to hydrocompaction in the San Joaquin Valley: (A) Hydrocompaction cracks. (B) 3-4 m deep "sink holes", partially flooded by irrigation runoff in an originally flat leveled field. Note high-power line for scale. (C) An originally flat field-road became undulatory by hydrocompaction due to irrigation. (D) Cracking of a farm workshop due to irrigation of surrounding fields. (E) Cracking and separation of concrete lining of a small irrigation ditch. (F) Damages to the 10-cm-thick concrete lining in a test canal section (State of California, "Mendota Test Site"). Hydrocompaction is known to exist in several areas of the United States and abroad (Anonymous, 1959, 1963; Bull, 1961, 1964; Drashevska, 1962; Dudley, 1970; Lin and Liang, 1980; Lofgren, 1969) . The phenomenon is described under various terms such as "shallow subsidence", "near-surface 539 subsidence", "hydroconsolidation", "soil settling", "soil settling by wetting", "collapsing soils", etc. Some of these terms are misleading. For example, the term "shallow subsidence", previously used in the San Joaquin Valley (Poland, 1958), is misleading because (1) the process may not be restricted to near-surface deposits and can occur at depths of over 30-40 m and (2) the uppermost near-surface deposits may be stable, while deposits occurring at depth can be susceptible to hydrocompaction. The term, "hydroconsolidation", can be associated with some form of consoli­ dation by cementation, etc. The term "hydrocompaction", used by the Bureau of Reclamation (Anonymous, 1963; Hall and Carlson, 1965; Prokopo- vich, 1963) and by the U.S. Geological Survey (Lofgren, 1969), and others properly reflects the nature of the process and is used in the following text. Rates and total amounts of hydrocompaction vary greatly depending upon the initial susceptibility of the sediment to hydrocompaction, past hydrocompaction, and the amount and type of water application. Gentle water application by sprinkling may cause less damaging but longer lasting hydrocompaction. After the completion of hydrocompaction, sometimes several years after the initial wetting, the originally hazardous deposits become stable. Susceptibility to hydrocompaction is controlled by (1) the development of an excessively porous deposit with a low dry density and a low moisture content, but with a relatively high dry strength which prevents spon­ taneous compaction and (2) aridity of past and present climates which prevents "natural hydrocompaction" of deposits. The origin of excessively porous deposits susceptible to hydrocompaction is not completely estab­ lished and may be polygenetic. Several theories of origin were considered for the San Joaquin Valley. Periglacial, Pleistocene sublimation of ice in frozen, newly deposited, initially wet sediments, expanded by freezing, seems to be a reasonable explanation of the origin of hydrocompaction in California's San Joaquin Valley and in many loess deposits in both North America and Eurasia. Such origin was duplicated in laboratory experiments by the author. In California, the "freeze-drying" was accomplished by westerly winds bringing downward moving and, therefore, relatively dry, air masses into the Valley (Fig. 3). SAN JOAOUIN VALLEY Fig. 3. Air circulation patterns in the San Joaquin Valley, California. 540 Hydrocompaction in California In California, at the present time, hydrocompact ion has "spotty" occur­ rence along the Coast Range foothills on the western margin of the southwestern portion of the arid San Joaquin Valley (Fig. 4), where it affects two major water conveyance systems — the San Luis Canal and the California Aqueduct. Typical deposits here, affected by hydrocompaction, are composed of clayey Pleistocene inter fan piedmont alluvium deposited mostly as mudflows. Such interfans are composed of minor, coalescent poorly defined fans of small arroyos. These isolated patches of alluvium, susceptible to hydrocompaction, are separated by fluvial alluvium of major ephemeral Coast Range streams (Fig. 4). The maximum amount of hydrocom­ paction here is up to 5m. Some hydrocompaction cracks were over 2 m wide and were probed to a depth of about 10 m. The initial total thickness of sediments susceptible to hydrocompaction was in an order of 65 m. At the present time, much of the alluvium initially susceptible to hydrocompaction has become "stable" due to wetting caused by irrigation. Since the 1940's, with advanced farming and widespread irrigation, hydrocompaction caused extensive damages to irrigation wells, ditches, pipelines, roads, bridges, fields and buildings (Fig. 2). Hydrocompaction in the region was probably first noted in 1915, after construction of the now abandoned Chaney Pumping Station in an undeveloped terrain (Sneddon, 1951). Hydrocompaction here was kept well under control for decades by keeping the station perimeter dry through diversion of wastewater and by restriction of irrigation. Since 1955-60, the hydro- compaction has become particularly important due to scheduled con­ struction of the San Luis Canal and its distribution system by the Federal Bureau of Reclamation, and associated developments of the State Water Project (Anonymous, 1974, 1974A). U.S. Bureau of Reclamation Studies of Hydrocompaction U.S.B.R. studies of hydrocompaction, initially carried out as part of an interagency investigation of subsidence in California (Poland, 1958), were accelerated since 1961-62, as engineering-geologic preconstruction, construction and postconstruction studies of hydrocompaction. These studies included (1) evaluation of numerous proposed methods of detection of sediments susceptible to hydrocompaction (Prokopovich, 1963, 1984), (2) estimates of ultimate amounts of future hydrocompaction,