Site Investigations of the Lacustrine Clay in Taihu Lake, China, Using Self-Boring Pressuremeter Test

Site investigations of the soils are considered very important for evaluation of the site conditions, as well as the design and construction for the project built in it. Taihu tunnel is thus far the longest tunnel constructed in the lake in China, with an entire length of over 10 km. However, due to the very insufficient site data obtained for the lacustrine clay in the Taihu lake area, a series of self-boring pressuremeter (SBPM) field tests was therefore carried out. Undrained shear strengths were deduced from the SBPM test, with the results showing generally higher than those obtained from the laboratory tests, which may be attributed to the disturbance to the soil mass during the sampling process. Degradation characteristics of the soil shear modulus (Gs) were mainly investigated, via a thorough comparison between different soil layers, and generally, the shear modulus would cease its decreasing trends and become stable when the shear strain reaches over 1%. Meanwhile, it was found that a linear relationship between the plasticity index and the shear modulus, and between the decay rate of the shear modulus and the plasticity index as well, could be developed. Further statistical analysis over the undrained shear strength and shear modulus distribution of the soils shows that the undrained shear strength of the soils follows a normal distribution, while the shear modulus follows a log-normal distribution. More importantly, the spatial correlation length of the shear modulus is found much smaller than that of the undrained strength.

Analysis of Strain-gage Records from a Static Loading Test on a CFA Pile

A static test was performed on a 610-mm diameter, 10 m long CFA pile installed through 3 m of clay and sand and into a thick deposit of lacustrine clay. The loading procedure included prolonged load-holding and an unloading-reloading event, which adversely affected the interpretations of the strain records and demonstrated the inadvisability of not performing a test with equal load-increments and equal load-holding durations and avoiding all unloading-reloading sequences. The pile was strain-gage instrumented at three levels and the recorded strains were used to calculate the pile axial stiffness and determine the load distributions for the applied load. Back-calculations using effective stress analysis were fitted to strain-gage determined load distributions and were then used in simulating the measured pile-head load-movement of the test pile.

A sediment-mixing process model of till genesis, using texture and clay mineralogy data from Saginaw lobe (Michigan, USA) tills

We present a sediment-mixing process model of till genesis based on data from surface tills of the Saginaw lobe terrain in lower Michigan. Our research uses a spatial approach to understanding glacial landsystems and till genesis. We sampled calcareous till at 336 upland sites and at 17 sites in lacustrine sediment of the Saginaw Lake plain. The loamy tills have bimodal grain-size curves, with a fine-texture mode near the silt–clay boundary and a sand mode. Spatial grouping analysis suggests that tills can be divided into six groups, each with different textures and clay mineral compositions that vary systematically down-ice. The similarity among groups with respect to the silt–clay mode and clay mineralogy argues for a common origin for the fines—illite-rich lacustrine sediment of the Saginaw Lake plain. Fine-textured sediments were probably entrained, transported, and deposited down-ice as till, which also becomes sandier and enriched in kaolinite, reflecting increasing mixing with shallow sandstone bedrock with distance from the lacustrine clay source. Clayey tills on the flanks of the Saginaw terrain may reflect proglacial ponding against nearby uplands. A process model of progressive down-ice mixing of preexisting fine lake sediments with crushed/abraded sandstone bedrock helps to better explain till textures compared with a purely crushing/abrasion process model.

Koralmbahn Baulos 60.3 St. Kanzian – Herausforderungen für den Spezialtiefbau bei extrem schwierigen Untergrundbedingungen (Seeton)/Koralmbahn lot 60.3 St. Kanzian – Challenges for special foundation works under extremely difficult ground conditions (lacustrine clay)

Zusammenfassung Im Zuge der Errichtung der Koralmbahn wurden im Teilbereich Mittlern – Althofen, beim Baulos 60.3 St.Kanzian die Tunnel Srejach und Untersammelsdorf mit einer Länge von 620 m und 665 m errichtet. Dafür waren umfangreiche Spezialtiefbauarbeiten in extrem herausforderndem Untergrund, bestehend aus schluffigen bis feinsandigen Stillwassersedimenten („Seetone“), notwendig. Basierend auf den Erkenntnissen der umfangreichen Vorversuche des Bauherrn sowie dank innovativer Technik, konnten die Anforderungen an die Düsenstrahlsäulen (DS-Säulen-Bodenvermörtelung mit dem Düsenstrahlverfahren) hinsichtlich Durchmesser, Festigkeit, Rücklaufmenge und -behandlung erfolgreich eingehalten werden. Die im Zuge der Vorversuche getesteten und schlussendlich auf der Baustelle umgesetzten technologischen Weiterentwicklungen können als richtungsweisend für die Weiterentwicklung des Düsenstrahlverfahrens angesehen werden. Bei dem Projekt wurde zusätzlich besonderes Augenmerk auf die Qualitätssicherung gelegt. In einem eigens im Zuge der Baustelle entwickelten Programm wurden alle produktionsrelevanten Daten elektronisch in einer SQL-Datenbank zusammengefasst, täglich ausgewertet und die Ergebnisse dieser Auswertung graphisch dargestellt. Dadurch konnten die Arbeiten mit verhältnismäßig geringem Aufwand mehrfach überprüft werden.