Martin Feinendegen

Abstrakt: Verschleiß und Verklebung können bei vielen Bautätigkeiten kosten‐ und leistungsbestimmend sein. Der DGGT‐AK 1.11 erarbeitet praxisnahe Empfehlungen zur Beschreibung und Bewertung von Verschleiß‐ und Verklebungspotenzialen für Lockergesteine und veränderlich feste Gesteine sowie zu deren Wechselwirkungen mit den Prozessen der Bauausführung. Zentraler Leitfaden für Aufbau und Inhalt sind zwei neue „Einflussgrößendiagramme“, die die Faktoren, die zu Verschleiß‐ oder Verklebungserscheinungen führen können, in einer von der Planung bis zur Ausführung gedachten Struktur ordnen. Im oberen Teil der Diagramme steht der Baugrund, dessen Eigenschaften – sowohl im natürlich anstehenden als auch im durch äußere Einwirkungen ggf. veränderten Zustand – das grundlegende Potenzial bestimmen, Verschleiß oder Verklebung hervorzurufen. Die zutreffende Beschreibung dieser Eigenschaften liegt üblicherweise im Verantwortungsbereich des Bauherrn. Im unteren Teil der Diagramme werden die Abbau‐ und Umwandlungsprozesse während der Ausführung des Projekts beschrieben. Sie liegen üblicherweise in der Verantwortung des Bauausführenden. Die tatsächlich auftretenden Verschleiß‐ und/oder Verklebungserscheinungen resultieren immer aus der Kombination von Baugrundeigenschaften und Bauausführung.

Considering the recent article on Flow Index and the basic correlation with some geotechnical parameters such as liquid limit, plastic limit, plasticity index, Cation Exchange Capacity, Specific Surface Area, Activity, and water uptake by Spagnoli et al. (2019), the authors reply to the discussers regarding their interesting comments.

Clogging can be a massive problem during the mechanized excavation of tunnels. Soil conditioning with foams and/or polymers is normally used in these cases to decrease the adverse effect of clogging. In a research project the cone pull‐out test was used to assess the variation of adherence for four sticky clays with four commercial additives. Results show that the additives have also an impact on the basic mechanical properties of the clays such as liquid limit, plasticity index and shrinkage limit. The tests were performed on the one hand by spraying the additives on the metal surface of the cone and on the other hand by directly mixing them to the clays to reach the critical consistency index (i.e. where the maximum adherence with water was observed). Sprayed cone tests show for two clays a decrease in adherence. However, by directly mixing the additives to the clay, adherence considerably decreases, except for one kaolinitic clay. The results can be useful to select suitable additives or to develop new ones considering the geotechnical and mineralogical properties of the excavated clays.

The flow index (FI), which is the slope of the water content versus log10 of the blows number plot for the Casagrande cup method, is the measure of the rate at which soil mass loses its shear strength with increased water content. FI was plotted against the plasticity index (PI) of 59 clays; each clay had a different mineralogy and origin, and together they accounted for a wide range of plasticity values. A linear relationship with R2 = 0.91 was observed. However, if FI is plotted against the liquid limit (LL) and the plastic limit (PL) separately, although a good correlation with LL is observed, FI poorly correlates with PL. FI was also correlated with the activity of soil, cation exchange capacity, specific surface area, and water uptake. Predicted equations from PI and activity were established on 13 independent samples confirming that a good correlation against measured values and estimated values (from FI) exists. From the literature review, FI was also used to indirectly estimate PL. Results show a trend, albeit the results are scattered (R2 = 0.66 for the Casagrande cup method and R2 = 0.76 for the cone penetrometer method), thus indicating that PL cannot be directly obtained from FI.

With the revision of the German contract conditions VOB in 2015, the previously usual soil and rock classes were replaced by homogeneous areas, into which the ground is to be divided according to the condition of soil and/or rock before excavation. Numerous properties and characteristic values also have to be specified with their determined bandwidths. In the tendering process for many works in specialised civil engineering, earthworks and tunnelling, the abrasiveness now always has to be assessed and, if necessary, tested for soil using the LCPC test according to the French standard NF P18‐579. This test was originally designed to determine the abrasion and crushability of artificial aggregates and is normally carried out under conditions that are in most cases unrepresentative of the behaviour of natural (mixed grained) soils in‐situ. Transfer of the results to the contractually binding ground report required by the VOB is therefore extremely critical, as this article points out based on new investigations with the LCPC test.

The detection of the plastic limit of clays is subject to human error. Several attempts have been made to correlate across studies the geotechnical properties of fine-grained soils (water content, liquidity index, shear strength, etc.). Based on the premise that the liquidity index and water content ratio can be correlated directly, an alternative method to obtain indirectly the plastic limit is suggested here. The present study investigated 40 natural clayey samples of various mineralogies and origins and other publicly available data, where Atterberg limits and undrained shear strength values obtained with the vane shear tests were given. The liquidity index and water-content ratio correlate very well for defined undrained shear strength values of the clays. Solving the liquidity index equation for the plastic limit, estimated plastic limit values obtained by the liquidity index/water-content ratio relationship were compared with laboratory plastic-limit values. Preliminary results based on 62 values show an exponential trend with a multiple regression coefficient of 0.79. The data need to be confirmed on a larger database, however.

Beim maschinellen Tunnelvortrieb in bindigen Böden oder veränderlich festen Gesteinen treten häufig Verklebungen an den Abbau- und Fördereinrichtungen auf, die zu erheblichen Mehraufwendungen und zeitlichen Verzögerungen beim Bauablauf führen. Da bislang noch kein allgemein anerkanntes Verfahren existiert, um das Verklebungspotenzial sicher und vor allem auch so quantifizierbar vorherzusagen, dass damit eine verlässliche Kalkulation durchgeführt werden kann, kommt es immer wieder zu Streitigkeiten zwischen Bauherrn und Unternehmen, wer für die Folgen der Verklebung aufzukommen hat. In diesem Beitrag werden zunächst die derzeit gültigen Regelungen in deutschen und internationalen Normen und Empfehlungen vorgestellt. Weiterhin werden die bislang existierenden Bewertungsverfahren für das Verklebungspotenzial erläutert. Anhand eines Praxisbeispiels wird gezeigt, dass mit dem noch jungen Konuszugversuch erstmals ein quantifizierender Vergleich der Verklebungsneigung verschiedener Böden möglich ist. Die Ergebnisse werden im Hinblick auf ihre bautechnische Aussagekraft und ihren Mehrwert gegenüber den bisherigen Klassifikationsverfahren bewertet. Schließlich werden Empfehlungen für im Rahmen der Baugrunderkundung durchzuführende Untersuchungen gegeben.