Deep-sea sediment compression curves: Some controlling factors, spurious overconsolidation, predictions, and geophysical reproduction

Oedometer test have been carried out on 70 undisturbed surficial clays (at approximately 250 mm below the mudline), mostly collected by free-fall core rs from sites widely scattered throughout the deep-sea North Atlantic. Acou stic measurements were also made, initially on contiguous samples and ultim ately on the same sample using a geophysically instrumented oedometer which also collected electrical resistivity data. Apart from those quiescent are as below the carbonate compensation depth, such as north of the West Indies where very fine clays exist, most of the samples are silty clays whose geo technical-geophysical properties are dependent on the type of clay minerals present (and their ability to take in moisture), the sand-size fraction, a nd the quantity of carbonate present. Thus the pure clays have high compres sibilities which decrease on the addition of coarse particles, while the co nverse is true for the acoustic parameters, these increasing with the sand fraction. Using the notion of the intrinsic compression line for all sample s, and comparison to it of the measured compression curves, it is clear tha t, contrary to some previously held ideas, most deep-sea clays are normally consolidated; the addition of carbonate has the effect of creating an open , stronger sediment skeleton. Interestingly, where information is available , the variation with depth of a sample's acoustic velocity follows the void ratio/pressure relationship of the compression curve. This allows the cons truction of an in-situ sediment compression curve using the in-situ geophys ical observations.