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Construction Problems Experienced with Loess Soils

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Construction P roblems Experienced With Loess Soils W. J. TURNBULL, Chief of Soils Division, U. S. Army Engineer Waterways Experiment Station •AMONG engineers and soil scientists, loess soil has long been one of the most inter­ esting soil types. Many articles from the soil scientists' and geological viewpoints have been written about loess soil and appear in the literature. Probably one of the most interesting and authoritative summaries was written by Scheidig (1). A lesser number of articles have been written on engineering properties (2, 3, 4, 5) or problems (6, 7, 8) concerning loess soil. This paper is based on the author's personal experience and s tudy of loess from two areas, Mississippi and Nebraska. The general behavior and structural makeup of the loess soils of Mississippi and Nebraska are quite similar, ex­ cept Mississippi loess is more uniformly clayey and tends to have a greater in situ density. A close similarity exists between construction problems encountered with loess soil in the two areas. Before discussing construction problems in loess, its phys­ ical properties and structural makeup are described briefly, principally with reference to the manner in which they contribute to the behavior of loess in various types of con­ struction work. Tl1e principal background of the author's experience in construction work with loess soil was gained on the Central Nebraska Public Power and Irrigation District project from 1934 to 1941. Much information was also obtained on the structural makeup of the loess soils by personal observation, study, and close contact with the late George E. Condra (9) of Nebraska, who had great knowledge of loess soil and its behavior. The knowledge ga ined by the author concerning the structural makeup of Mississippi loess soil has been through studies at the Geology Branch, Soils Division, U.S. Army Engi­ neer Waterways Experiment Station, Vicksburg, Miss., during the last 25 years. In addition, considerable knowledge in construction work has been gained by observing the behavior of Mississippi loess soil, principally on state highway projects and in building construction in Vicksburg, Miss. GENERAL On the basis of available information, loess deposits cover approximately 11 percent of the land area of the world. This figure is about 17 percent for the United States. Figure 1 shows the most important loess areas of the United States. Figure 2 shows the age relationship of the soils of the Central Great Plains area and the southern region of the United States. Some difference of opinion exists between geologists of the various regions regarding the relationship of specific loess deposits; however, this disagreement is not partic­ Figure l. General loess areas of the United ularly pertinent. Probably one of the most States. Paper sponsored by Ad Hoc Committee on Loess and presented at the 45th Annual Meeting. 10 11 CENTRAL AGE ILLINOIS MISSISSIPPI GREAT PLAINS 0 ,_ VALDERAN BIGNELL LDESS z TWOCREEKAN BRADY SOIL / UJ LEACHED () :< l'.l LOE SS <( UJ Q'. 0 Q'. PEORIA {"'°"CA•OLOE$S ·n WOODFOROIAN ::J _J 20,000 PEORIA LOE SS Ul LOE SS MORTON (Il w Ul z '<'. 0 CALCAREOUS - 1..0ESS _J u 0 Ul " FARMOALIAN BASAL Q'. Q'. > ) (Il <( LOE SS w TRANSITION FARMDALE SILT n: >- ZONE ANO PEAT CL z ;;; 0 Ul (Il ;;; Q'. z I Ul <( 40, 000 • <( :< u z I S'.' iii 0 z ROXANA I <( ALTONIAN SANO AND LOE SS \,? Q'. 0 I () GRAVEL <1) I ~ ZONES I TO I\' I BASAL I 60 ,000 TRANSITION I ZONE I I I SANGAMONIAN SANGAMON SOIL SANGAMON SOIL I I ' I BUFFALO HART PRE-VICKSBURG j UJ u Ul LOESS z w Ul J w _J LOVELAND w ::J <( z 0 <( LOE SS _J 0 u Ul w 0 0 NOTE STRATIGRAPHIC Ul w z z w :< - <( POSITIONS OF _J JACKSONVILLE CRETE SAND > ,_ _J BASAL TRANSi- _J PAYSON w ;:: - AND GRAVEL <( 0 > TION ZONE ANO z 0 _J PRE-VICKSBURG w _J Q'. PETERSBURG LOESS l\RE LOE SS TENTATIVE T YARMOUTHIAN YARMOUTH SCIL YARMOUTH SOIL Figure 2. Age relations in loess soils of the Central Great Plains, Illinois, and Mississippi. important differences in opinion concerning loess soil is with respect to its origin in Mississippi. Russell ( 10) of Louisiana State University has developed an hypoth­ esis that loess soils of Mississippi are the result of fluvial deposition and soil-forming processes rather than windblown origin. This theory is not concurred in by many other Mississippi and Louisiana geologists ( 11, 12) . Apparently, little disagreement exists con­ cerning the origin of the Nebraska loess soils, i.e., that they are windblown and their origin was mostly in the sandy areas of northwest and north-central SILT I CLAY I Nebraska and northeast Colorado, with minor contrib­ Figure 3. Comparative grain-size uting sources in river valley floodplains (13). curves for Mississippi and Figure 3 shows typical comparative grain-size Nebraska loess soi Is. analyses of the Mississippi and Nebraska loess soils. 12 O.B LOESS SAMPLE 5 (UNDISTURBED ) DEPTH, 12 .0-14.3 FT l--~~__:::.....:::---f-~(AFTER3DAYS~~~~~-l-~~~~~~+-~~~~~--t N 0 . 6 f- DRYING) LL I/) z !AFTER 2 DAYS e FINAL CONDITION ~ 803 SATURATION .!:O I 0. 4 f­ (AFTER 24 HR l'.) z DRYING) w ll'. f­ (AFTER 7 HR l/) ll'. <( w T I/) u. 2: INI T IAL CONDITION 95% SATURATION 0 2 4 26 29 30 32 34 M OIS T U RE I N ~. OF DR Y WE IGH T Figure 4. Change in shear strength with changing moisture content, Mississippi loess. Note that the Mississippi soil tends toward a higher clay content. Figure 4 shows a radical change in shear strength with changing moisture content on undisturbed Missis­ sippi loess soil . A similar graph on Nebraska loess soil could be even more pronounced because many of the Nebraska loess deposits contain more calcium carbonate than the Mississippi deposits and thus have a greater, relative, cohesive strength as a result of cementation produced by the calcium carbonate. In the Nebraska project mentioned earlier, the author had a great amount of experience with Nebraska in situ loess de­ posits and their loss of strength when wetted under loading. Many observations were made on the consolidation of in situ loess soils when wetted. One of the most signifi­ cant experiences of this type was in the filling of a reservoir by storm waters and the loss of the water largely as a result of soaking into the undisturbed thick layer of loess soil beneath. The reservoir bottom developed many sinkholes, and the overall average settlement was estimated to be several feet. Another illustration of consolidation of in situ loess soil concerned a local irrigation project on the high terrace of the Platte River south of Kearney, Neb. As a result of approximately 15 years of irrigation, the overall settlement of the irrigated land ranged between 5 and 7 ft. As would be expected, this settlement produced extremely difficult problems for the farmers in attempting to maintain main and sublateral ditches for distribution of irrigation water. Other features of this project are described later. Figure 5 shows the excavation of a mastodon skull in a vertical bank of a Missis­ sippi highway cut. Factors such as this are excellent checks on the age of a loess deposit. 13 Figure 5. Excavation of mastodon skull in highway cut, Mississippi. Figure 6. Live and unrotted plant roots 15 ft INTERNAL STRUCTURE below the surface, Mississippi loess. Internal structure contributes largely to the behavior of in situ loess, i.e. , steep columnar sloughing, low resistance to erosion and piping, and consolidation and loss of strength when wetted. The latter phenomenon is quite often referred to as collaps­ ible soil. It is generally believed that internal structure of loess soil is developed by one or the other or a combination of (a) mechanical cementation due to calcium car­ bonate, and (b) formation of more or less vertical tubules in the loess as a result of the decay of fibrous plant roots. Quite often these tubules are lined with calcium carbonate and are so small that the naked eye is unable to distinguish them. Figure 6 shows live and unrotted plant roots some 15 ft below the surface. Figure 7 shows the tubule lining of calcium carbonate; the soil has been washed away, and the tubules, as shown, are approximately 1/2 mm in diameter. Figure 8 shows a petrographic thin section across calcium-carbonate-lined tubules. The total width of the view is approximately 4 mm, which means that the longest diameter of the tubules ranges between % and 1 mm. Another phenomenon, which in a sense contributes to the internal structure of loess soil, is the replacement of larger plant roots with calcium-carbonate concretions. The size and shape of these concretions vary considerably. Some may be elongated with a Figure 8. Thin, petrographic section across Figure 7. Tubule lining of calcium carbonate, ca le ium-carbonate- lined tubules, Mississippi loess. Mississippi loess. 14 Figure 9. Elongated and irregularly shaped Figure 10. Rounded to subrounded concretions, concretions, Mississippi loess. Mississippi loess. diameter of% in. and larger; others may be more or less irregular to spherical in shape. Figure 9 shows some of the smaller elongateci anci irregularly shaped loess concretions in Mississippi loess soil. On occasion, some of the nodules have been made up largely of silica. In Nebraska loess, the author has seen in open cuts many yucca root bulb replacements, most of which were silica. Some of these replacements were in the shape of an elongated turnip with the maximum diameter at the top being 8 to 10 in. The replaced yucca roots, unless extremely abundant, would have little to do with the internal structure of loess; however, relatively high percentages (up to 5 per­ cent) of small, elongated particles could have a material effect on the structure ofioess.
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