CONSOLIDATION CHARACTERISTICS OF A SILTY CLAY: CONCERNING THE EFFECT OF SOIL DISTURBANCE

In this paper, undisturbed specimen of a silty clay constituting of Phu Bai formation (ambQ21-2 pb) was collected from boreholes in Hue city and surrounding areas. The soil, under both undisturbed and disturbed conditions, was then subjected to standard one-dimensional consolidation tests with 7 loading increments. It is shown from the experimental results that the time to the end of primary consolidation (EOP), determined by Log Time method (tLT) and 3-t method (t3T), decreases with the load increment and under the same vertical stress, the primary consolidation of disturbed silty clay finish at a shorter time than those of the undisturbed one. The coefficient of secondary consolidation (Cα) increases with the vertical stress and reaches the maximum values before decreasing. The obtained values of Cα = 0.005 - 0.020 suggest a relatively low secondary compressibility of the silty clay constituting of Phu Bai formation.

PERENCANAAN FONDASI TIANG PANCANG PADA PROYEK TOWER 2 ARANDRA RESIDENCE

ABSTRACTThe pile foundation is an sub-structure to load from the upper structure. Ultimate load carrying-capacity (qu) will be transfered into a hard soil layer by using a deep foundation system. To design the pile foundation, several methods are needed to obtain different bearing capacity values. This study determines the planned pile depth, pile dimensions and pile cap. The purpose of this final project is to plan the pile foundation for the Arandra Residance 2 tower construction project located in Cempaka Putih, Central Jakarta. The method used is the method of Meyerhof, U.S Army Corp, Tomlinson, α and λ. In addition, the calculation of reinforcement, immediate settlement and settlement of primary consolidation was also carried out. The results of the calculation of bearing capacity foundation are different values. The Meyerhof Qu method is 9846,786 kN, the U.S Army Corp method Qu = 11065.11 kN, the Tomlinson Qu method = 10409.68 kN, the method α = 9558.95 kN, and the method λ Qu = 10066.37 kN. Whereas according to Broms, the lateral bearing capacity is 10845 kN. In planning used reinforcement D25-270. Immediate settlement is 50.3 mm, primary consolidation settlement is 9.89 mm, and time rate of consolidation during 1.75 months. Keywords: Foundation, driven pile, bearing capacity, settlement, primary consolidation ABSTRAKFondasi tiang merupakan fondasi yang menyalurkan beban struktur atas dan beban lainnya ke struktur lapisan tanah keras yang mempunyai daya dukung tinggi yang terletak jauh di dalam tanah. Untuk merencanakan fondasi tiang pancang diperlukan beberapa metode untuk mendapatkan nilai daya dukung yang berbeda. Studi ini menentukan kedalaman tiang pancang yang direncanakan, dimensi tiang pancang dan pilecap. Tujuan dari tugas akhir ini adalah merencanakan pondasi tiang pancang untuk proyek pembangunan tower Arandra Residance 2 yang berlokasi di Cempaka Putih, Jakarta Pusat. Metode yang digunakan adalah metode Meyerhof, U.S Army Corp, Tomlinson, α dan λ. Daya dukung lateral menggunakan metode Broms. Selain itu juga dilakukan perhitungan penulangan, penurunan segera, dan penurunan konsolidasi primer. Hasil perhitungan daya dukung fondasi terdapat perbedaan nilai. Metode Meyeherhof Qu = 9846.786 kN, metode U.S Army Corp Qu = 11065.11 kN, metode Tomlinson Qu = 10409.68 kN, metode α = 9558.95 kN, dan metode λ Qu = 10066.37 kN. Sedangkan menurut broms daya dukung lateral sebesar 10845 kN. Pada perencanaan digunakan tulangan D25-270. Penurunan segera terjadi sebesar 50.3 mm, penurunan primer sebesar9.89 mm, dan kecepatan waktu penurunan konsolidasi selama 1.75 bulan. Kata kunci: Fondasi, tiang pancang, daya dukung, penurunan, dan konsolidasi primer

Hydrologic control on natural land subsidence in the shallow coastal aquifer of the Ravenna coast, Italy

Multiple processes contributing to natural land subsidence in a shallow coastal aquifer near Ravenna (Italy) were identified by analysing the relationships among different data set time series (water table level, rainfall, land reclamation drainage, sea level, etc.) and establishing the correlations with vertical ground motion observed at a high-resolution settlement gauge. Our study highlights the presence of three deformation components related to different processes controlling land subsidence: elastic, delayed-elastic, and irreversible (plastic) components. The elastic and delayed-elastic components are closely related to water table fluctuations that change the effective stress in two portions of the coastal aquifer at a daily (in the sandy unconfined portion) and seasonal time scales (in the layered clay-rich semi-confined prodelta portion), respectively. The irreversible component represents the trend in the land subsidence time series and is due to primary consolidation (pore pressure dissipation) of the fine-grained prodelta levels above where the settlement gauge is located. The amplitudes of the elastic component can be up to 0.2–0.3 mm whereas the amplitude of the delayed-elastic component reaches 0.89 mm. The primary consolidation rate of deformation is 0.9 mm yr−1 and constrains the likely age of prodelta sediments deposition to 1300–2800 years before present. The delayed-elastic subsidence rate has similar magnitude to that due to primary consolidation and is connected to poroelastic effects in the prodelta sequence following seasonal variations in water table. Our findings are important for planning land subsidence management and monitoring strategies especially where the surface aquifer structure is heterogeneous due to different depositional settings. The natural land subsidence rate in the Holocene sediments of the shallow coastal aquifer of Ravenna (North eastern Italy) that we measured in this study accounts for 10 %–20 % of the total current land subsidence rate observed in this portion of Ravenna coastal area (10–20 mm yr−1).

The Secondary Consolidation (Creep) due to Geohistorical Overburden Pressure in the Houston-Galveston Region, Texas

The combination of groundwater withdrawal, hydrocarbon extraction, salt-dome movement and faulting have caused widespread subsidence in the Houston-Galveston region (HGR). Subsidence results from primary consolidation consisting of inelastic (nonrecoverable) and elastic (recoverable) compaction caused by subsurface fluid withdrawal and secondary consolidation (creep) over time caused by overburden pressure. Subsidence in the HGR is monitored using borehole extensometers that were installed at 13 locations across Harris and Galveston counties between 1962 and 1980. By 1977, withdrawals from the Chicot and Evangeline aquifers resulted in groundwater-level declines of about 114 and 115 m relative to predevelopment water levels, respectively in parts of Harris County. By 1979, as much as 3 m of land subsidence was estimated to have occurred in localized areas of the HGR. Land subsidence can be hazardous in populated areas because it exacerbates the effects of storm surge and impedes storm-water runoff by decreasing land-surface elevations in areas where water accumulates. To assess aquifer compaction in response to changes in groundwater levels, a bulk land-surface subsidence rate is assumed to be the sum of the primary consolidation rate and the negligibly variable component of overburden pressure referred to as the “pseudo-constant secondary consolidation rate.” From 1931 to 1976, groundwater levels decreased as groundwater withdrawal rates increased from 0.57 to 4.3 million m3 d−1, causing pressure heads in aquitards the Chicot and Evangeline aquifers to continually decline. In response to reductions in groundwater withdrawal rates from 4.3 to 3.0 million m3 d−1 between 1976 and 2001, groundwater levels rebounded, decreasing inelastic compaction rates in some parts of the HGR from as much as about 40 mm yr−1 in the early 1980s to negligible amounts by 2000. Inelastic consolidation from about 1937 to 2000 contributed to land-surface subsidence and its associated effects. Land-surfaces have rebounded in localized areas of the HGR where groundwater levels rebounded significantly. Pseudo-constant secondary consolidation rates were computed at each of the 13 extensometers and ranged from 0.48 to 8.49 mm yr−1 in areas where groundwater levels in the two aquifers were stabilizing. This secondary consolidation subsidence is beyond the control of any groundwater-level management schemes because it is caused by geohistorical overburden pressure on the two aquifers.

Analysis of settlement prediction due to preloading and vertical drain applications on runway construction

Consolidation settlement is a general geotechnical problem particularly found in the area where is composed of soft soil. It is caused by the discharge of pore water pressure induced by the increase of stress in the soil mass. Construction of runway above soft soil requires analysis for stability related to the reduction of consolidation settlement and the recovery. This study aims to analyze the settlement comprehensively using empirical methods of Prefabricated Vertical Drains (PVD) and preloading installation. Preloading is a technique by which consolidation of soil can be achieved to a substantial amount before the imposition of actual construction load. According to soil investigation, the characteristic of the soil layer is clay soil, which has the potential to consolidation settlement. The result of the settlement analysis of the taxiway in the research area is from 33 cm to 214 cm. It takes ten years for primary consolidation to reach a 90% degree of consolidation. However, in the Hansbo method of Prefabricated Vertical Drains (PVD) and preloading are applied, with triangular configurations in depth of 11 meters and duration for variation embankment spacing of 1 m is 79 days, 1.5 m is 202 days and 2 m is 390 days. The conclusion of efficient distance of PVD installation and preloading is spacing of 1 m with 79 days for primary consolidation.

TIME TO THE END OF PRIMARY CONSOLIDATION (EOP) OF SOFT CLAYEY SOILS: CONCERNING THE EFFECT OF ATTERBERG’S LIMIT AND CYCLIC LOADING HISTORY

In order to observe the end of primary consolidation (EOP) of cohesive soils with and without subjecting to cyclic loading, reconstituted specimens of clayey soils at various Atterberg’s limits were used for oedometer test at different loading increments and undrained cyclic shear test followed by drainage with various cyclic shear directions and a wide range of shear strain amplitudes. The pore water pressure and settlement of the soils were measured with time and the time to EOP was then determined by different methods. It is shown from observed results that the time to EOP determined by 3-t method agrees well with the time required for full dissipation of the pore water pressure and being considerably larger than those determined by Log Time method. These observations were then further evaluated in connection with effects of the Atterberg’s limit and the cyclic loading history.