Abstract

Athree-dimensional (3D) ground model was developed to design the stabilisation of a dam founded on both weak and liquefiable units up to about 18 m below ground surface. The ground model covers a linear extent of approximately 800 m and was developed from five separate site investigations completed over a four-year period and digitisation/georeferencing of historic drawings/plans. Combined, the investigation comprised 206 cone penetration tests (CPTs), 37 boreholes and 36 test pits, including several vane shear tests, ball penetrometer tests and sampling. CPT data was processed to identify different material behaviours, generally based on the following features: corrected tip resistance; sleeve friction resistance; pore water pressure ratio; state parameter; and the soil behaviour type index. Each CPT interpretation was compared with information from the nearest borehole using a purpose-built python code. This information was reviewed manually in an iterative process to delineate the various geotechnical unit based on CPT response and the physical logs. This process identified a continuous weak organic layer across the site which had not been previously picked-up by the Engineer of Record (EoR). The works identified nine separate geotechnical units, with one of these subdividable based on its CPT response/grain size. The 3D ground model was built in Seequent Leapfrog Geo using the following information: unit levels specified from each CPT, digitised historic drawings/plans and a topographic survey. This paper describes the process of development and presents the full 3D ground model used as critical input to the stabilisation design of the dam

Abstract

The dilatometer test (DMT) has been shown very useful in providing a number of geotechnical parameters, in different types of soils. The test is regularly carried out every 0.20 m, i.e., it is a discontinuous test. Therefore, its capability of detecting variations in the geotechnical profile is conditioned by the tests interval. The Medusa DMT, however, is able to obtain continuous measurements of the A-pressure reading, thus providing a much better picture of the soil stratigraphy. Medusa DMT tests have been carried out at Sarapuí II very soft clay deposit around Guanabara Bay, in Rio de Janeiro, and a detailed soil profile was obtained. A comparison was made with piezocone tests. The stratigraphy of the deposit obtained by both tests was almost the same.

Abstract

The natural structure of clays has a significant influence on its mechanical behaviour and can be characterized using insitu and laboratory tests. It was reported by Robertson (2016) that soil structure leads to an increased tip resistance (qc) and shear wave velocity (Vs) when performing seismic cone penetration tests. However, only limited studies investigated changes in soil structure by means of in-situ tests. Sensitive, marine clays were investigated within the research project "VIBE – Sustainable Ground Improvement Solution for Oslo" at the Norwegian Geo-Test site Onsøy (Gundersen et al. 2019). Possibilities and limitations of the vibro replacement method were studied for very soft ground conditions based on a full-scale field test. The influence of soil structure on in-situ measurements of piezocone penetration tests (CPTu) and seismic flat dilatometer tests (SDMT) are further studied by intentionally disturbing the soil structure by a vibrator. Results of CPTu and Medusa SDMT, executed before and after treatment, are compared to characterize changes in soil structure. The results indicate that the vibration-induced destructuration led to a significant decrease of CPTu measurements, namely tip resistance (qc), sleeve friction (fs) and measured pore pressure (u2), within medium to high sensitive clays. As the decrease in fs is more significant compared to the decrease in qc, a significant decrease in friction ration (Rf) was observed. In analogy, SDMT resulted in a decrease in shear wave velocity (Vs), horizontal stress index (KD) and dilatometer modulus (ED) after the vibro treatment. It was further shown that the soil behaviour type chart according to Robertson (2016) leads to no sufficient characterization of soil structure in soft, marine clays.