Korelasi Antara Kecepatan Gelombang Geser dan NSPT di Area Pulau Nuklir, Tapak RDE Serpong

Karakteristik gerakan tanah akibat gempabumi dipengaruhi oleh variasi nilai kecepatan gelombang geser (Vs). Ketersediaan nilai Vs suatu daerah belum tentu lengkap dan siap, hal ini disebabkan oleh penyelidikan langsung guna memperoleh nilai Vs memerlukan waktu relatif lama dan biaya yang tidak sedikit. Selain itu juga dipengaruhi oleh tingkat keterjangkauan suatu daerah pengujian. Berbeda halnya dengan data jumlah pukulan (N) dari uji Standard Penetration Test (NSPT) yang lebih banyak tersedia. Dewasa ini telah banyak dilakukan penelitian persamaan korelasi nilai Vs dari data NSPT, meskipun demikian tingkat keandalan antara satu dengan lainnya tidaklah sama. Studi ini bertujuan mengembangkan korelasi yang reliabel antara Vs dan NSPT untuk kategori semua jenis tanah. Metode yang digunakan adalah dengan mengumpulkan dan menganalisis data hasil pengujian geofisika dan geoteknik kemudian melakukan analisis regresi non-linear. Dari hasil perhitungan diperoleh korelasi Vs = 98,759NSPT0,3101 untuk kategori semua jenis tanah di area pulau nuklir Tapak RDE, Serpong. Korelasi ini mampu memprediksi Vs dengan cukup baik dan cepat, tanpa harus melakukan pengukuran yang memerlukan biaya lebih banyak dan waktu lebih lama.

Interpretation of Cone Penetration Test Data of an Embankment for Coupled Numerical Modeling

The Nerang Broadbeach Roadway (NBR) embankment in Australia is founded on soft clay deposits. The embankment sections were preloaded and surcharged-preloaded to limit the post-construction deformation and to avoid stability failure. In this paper, we discuss the NBR embankment’s geology, geotechnical properties of the subsurface, and long-term field monitoring data from settlement plates and piezometers. We demonstrate a comparison of cone penetration test (CPT) and piezo cone dissipation test (CPT-u) interpreted geotechnical properties and the NBR embankment’s foundation stratification with laboratory and field measured data. We also developed two elasto-viscoplastic (EVP) models for long-term performance prediction of the NBR embankment. In this regard, we considered both the associated and the non-associated flow rule in the EVP model formulation to assess the flow rule effect of soft clay. We also compared EVP model predictions with the Modified Cam Clay (MCC) model to evaluate the effect of viscous behavior of natural Estuarine clay. Both EVP models require six parameters, and five of them are similar to the MCC model. We used the secondary compression index of clay in the EVP model formulations to include the viscous response of clay. We obtained numerical models’ parameters from laboratory tests and interpretation of CPT and CPTu data. We observed that the EVP models predicted well compared with the MCC model because of the inclusion of soft clay’s viscosity in the EVP models. Moreover, the flow rule effect in the embankment’s performance predictions was noticeable. The non-associated flow rule EVP model predicted the field monitoring settlement and pore pressure better compared to the MCC model and the associated flow EVP model.

Characterization by Cone Penetration Tests of the decalcified Zandvliet Sand (Lillo Formation, North Belgium)

The sandy Zandvliet Member represents a particular, decalcified facies in the top of the Pliocene Lillo Formation in northern Belgium. Based on the correlation with nearby boreholes at the type locality of the Zandvliet Member, we were able to characterize this unit on Cone Penetration Tests. Compared to the underlying Merksem Member, the Zandvliet Member generally shows markedly lower cone resistance values. Since besides the decalcification, the Zandvliet Member is lithologically nearly identical to the underlying Merksem Member, the lower cone resistance values in the Zandvliet Member compared to the Merksem Member can only be the result of the decalcification of the Zandvliet Member. Indeed, the partly decalcified top of the Merksem Member also gives similar cone resistance values as the Zandvliet Member. Decalcification of the Eocene Brussel Sand in central Belgium is also known to have resulted in lower cone resistance values. Our Cone Penetration Test interpretations show that the thickness of the Zandvliet Member strongly varies across short distances (>10 m across 1 km). As the Zandvliet Member thickens, the underlying Merksem Member thins and vice versa. This trend is not in line with that of the under- and overlying strata, i.e. intraformational, nor with the depositional environment of these units. The thickness changes of the Zandvliet Member therefore purely reflect changes in depth of the post-depositional decalcification into the original shell-bearing sand (i.e. original Merksem Member). This confirms the existing hypothesis that the Zandvliet Member actually represents the decalcified part of the Merksem Member. The anomalous heavy mineralogy of the Zandvliet Member compared to the other members of the Lillo Formation cannot be readily explained by the acid chemical weathering which caused the decalcification. This may rather be related to a change in the primary heavy mineral signal of the upper part of the Merksem Member and equivalent Zandvliet Member compared to the underlying sequences of the Lillo Formation. The reason for the post-depositional decalcification could be similar to the Pleistocene changes in soil acidity invoked for decalcification of time-equivalent Red Crag sand in England.