Soft Soil Layer Research Articles

Investigating Surface Deformation Caused by Excavation of Curved Shield in Upper Soft and Lower Hard Soil

Existing studies on surface deformation prediction consider single soil layers and straight-line excavation when investigating deformation caused by metro shield construction. In this study, we provide a new prediction method for studying the soil deformation caused by curved shield tunnel construction in the upper soft and lower hard soil. The deformation equations are derived using the Mindlin solution and random medium theory and are verified using engineering examples and numerical simulation. The influencing factors and laws of the surface deformation caused by the excavation are also identified. The study found that the horizontal settlement trough on the ground surface was distributed asymmetrically during the curve construction, with maximum settlement on the inner side of the curve. The offset and settlement values were affected by the thrust difference coefficients α and β and the turning radius. When constructing in the upper soft and lower hard soil layers, the settlement trough tends to become wider and shallower. The results show that the derived equation is suitable for actual engineering calculations, and the measured data are in good agreement.

Water-Rich Soft Soil Freezing Method Combined With Steel Sleeve Receiving-Shield Technology Field Test Research

The initiation and reception of shields are major risk events for shield construction in water-rich and weak strata. Although the freezing method and the steel sleeve method receiving-shield tunneling technologies both have engineering applications, the environmental safety control effect cannot meet the construction requirements of water-rich soft soil. Considering the shield construction of a typical soft soil layer in Suzhou, China, as a research target, the applicability and safety of the freezing method combined with steel sleeve receiving-shield technology in water-rich soft soil were evaluated based on a field test system. The test results show that, during soil freezing, the temperature change trend of each measuring point in the temperature measuring hole is roughly the same. The freezing process can be divided into five typical stages. The closer the active freezing period of the water-rich soft soil is to the inside of the frozen-soil curtain, the faster the development rate of the frozen wall. The soil cooling gradient increased with an increase in the radial depth. After freezing the curtain circle, the soil frost heave significantly accelerated until the frost heave amount peaked. During the construction process, special attention should be paid to the change in the value of the soil settlement during each stage change to prevent sudden changes in soil displacement. The freezing method, combined with steel sleeve receiving-shield technology, can effectively reduce the environmental disturbance caused by shield construction in water-rich soft soil.

Site Amplifications from Earthquake Data and VS30 in the Fort Worth Basin, Texas

Abstract Following the development of unconventional oil and gas production in the Fort Worth Basin, Texas, a rapid increase in basin-wide seismicity began in 2008 that grew to include earthquakes affecting a substantial portion of the Dallas–Fort Worth metropolitan area. To assess and mitigate the seismic hazard, which in this region is impacted by the thickness of the sedimentary basin and accompanying soft soil layer, we estimate site effects at 22 seismic stations deployed to record these earthquakes. Site responses are derived using two different datasets and approaches: (1) a modified generalized inversion technique (GIT) based on the S-wave Fourier amplitude spectra from earthquakes; and (2) application of the quarter-wavelength approximation (QWA) using estimates of average shear-wave velocities in the upper 30 m, known as VS30. We find that site amplification estimates based on the two techniques are roughly consistent with one another (median of the amplification ratio = 0.92) over frequencies where a quarter-wavelength corresponds to ∼30 m depth. The site amplification factors from the two approaches are found on average to be about 3 at the quarter-wavelength frequency (QWF). These site amplification estimates are not well correlated with geologic characteristics including rock type and geologic age. Finally, QWA values at six sites do not match GIT site amplification at the QWF (outside of ± median absolute deviations boundary), which we attribute to a combination of the underlying assumptions of the QWA, uncertainty in VS30 estimates, and unmodeled site response complexity.

Numerical investigation of railway subgrade stiffening: Critical speed and free-field vibrations

For a train speed close to the speed of elastic waves in the soil, often referred to as “critical speed”, largely elevated vibration responses occur. This can be a practical problem for soft soil sites, where the phenomenon may cause excessive vibrations in the track and also at distances far from the track. To ensure the running safety of the train, the long-term quality of the track and to reduce the vibrations in the surroundings, such effects must be avoided. An effective counter-measure is to increase the stiffness of the soil underneath the track, thereby increasing the critical velocity.In this paper, a 2.5D finite element model is used for studying the critical velocity phenomenon and its mitigation through soil stiffening, for a ballasted track on a layered half-space with very soft soil. Soil improvement under the track, in the shape of a solid block or as various number of panels, with varying depth and stiffness is considered. The effect of the soil improvement is evaluated both in terms of the maximum rail and free-field displacements. It is shown that a shallow soil stiffening increases the critical velocity and reduces the rail and free-field response for load speeds near the shear wave velocity of the soft top soil layer. It is also demonstrated that a deep soil stiffening, by use of panels along the track direction, increases the critical velocity further, and may also be efficient in reducing the response for load speeds near the shear wave speed of the underlying half-space.

Three-dimensional numerical simulation of the geometrical dimension pile group effects on the critical speed of high-speed railway track (Case study: Tehran- Isfahan high speed railway track)

Existence of soft soil layers and high ground water level in the Qom Salt Lake area have caused deflection more than allowable limitation in Tehran- Isfahan high speed track railway. Due to reduce deflection of track railway in this area, improving the stiffness and strength of the soil beneath will be done by means of concrete pile group. But also, due to the high speed of train, resonance phenomenon is expected to happen. Despite conducting some researches on the effect of improving soft ground by pile in recent years, but there are ambiguities about performance of pile on the increasing critical speed of the track-ground system which track has been experienced large dynamic amplification as the train speed approaches that speed. So first, the bearing capacity of pile and soil layers geotechnical parameters have been determined by comparing the numerical modeling results of single pile with data obtained from in situ compressive loading test on that in this paper. Afterwards, the effect of the geometrical dimension pile group on the dynamic behavior and critical speed of track-ground system has been investigated under various speed of train (60–450 km/h) using 3D finite element model. The results indicate that resonance phenomenon has been occurred at speed of 100 km/h, which is equal to Rayleigh speed of first subgrade layer. However, improving of soil layers by pile group increases the value of the critical speed to 250 km/h.

Analysis of Soil Subscription on Soil Lack Combination of PVD and PHD With Analytical and Numerical Methods Using 3D Plaxis in STA 103 + 550 Construction of High Click Toll Road Range, Stage 1

The construction of transportation facilities on soft soil will cause problems where the bearing capacity of soft soil is low and the process of land subsidence will require a long period of time. As is known, the land subsidence factor causes many problems in the process of building transportation facilities, besides that, there will also be problems at the level of service (serviceability). Therefore, this problem must be anticipated before the transportation facility begins to function. There are many methods that can be done. Where in the selection of methods, the efficiency and effectiveness factors are very dominant. One method that can be done is to fill the soil combined with vertical drain and horizontal drain. This method was chosen to be one of the most widely used methods because it is easy to do and quite efficient. The purpose of writing this thesis is to analyze thereduction of settlement each layer of soft soil that is repaired using the soil filling method combined with prefabricated vertical drain and prefabricated horizontal drain using the PLAXIS 3D program. The results of this calculation will then be compared with observations made in the field using a settlement plate for later analysis. Calculation of the size of the consolidation settlement using the Terzaghi 1 dimension consolidation method obtained a decrease of 16.09 cm. This result is relatively close to the results of observations made in the field of 16.3 cm. In the PLAXIS 3D modeling approach using 1 PVD line and 1 PHD line, the result is a decrease of 16.52 cm without taking into account the effect of the smear zone and 16.22 cm taking into account the effect of the smear zone. In the PLAXIS 3D modeling approach using 2 PVD lines and 2 PHD lines, the result is a decrease of 16.54 cm without taking into account the effect of the smear zone and 16.28 cm by taking into account the effect of the smear zone. And when modeling without using PHD the value of the decrease obtained is 9.7 cm. When modeling using sand blanket as a substitute for PHD, the decrease value is 13.3 cm

Development Trend and Stability Analysis of Creep Landslide With Obvious Slip Zone Under Rainfall-Taking Xinchang Xiashan Basalt Slope as an Example

The creep slope is a dynamic development process, from stable deformation to instability failure. For the slope with sliding zone, it generally creeps along the sliding zone. If the sliding zone controlling the slope sliding does not have obvious displacement, and the slope has unexpected instability without warning, the harm and potential safety hazard are often much greater than the visible creep. Studying the development trend of this kind of landslide is of great significance to slope treatment and landslide early warning. Taking Xiashan village landslide in Huishan Town, Xinchang County, Zhejiang Province as an example, the landslide point was determined by numerical simulation in 2006. Generally, the landslide is a typical long-term slow deformation towards the free direction. Based on a new round of investigation and monitoring, this paper shows that there are signs of creeping on the surface of the landslide since 2003, and there is no creep on the deep sliding surface. The joint fissures in the landslide area are relatively developed, and rainfall infiltration will soften the soft rock and soil layer and greatly reduce its stability. This paper collects and arranges the rainfall data of the landslide area in recent 30 years, constructs the slope finite element model considering rainfall conditions through ANSYS finite element software, and carries out numerical simulation stability analysis. The results show that if cracks appear below or above the slope’s sliding surface, or are artificially damaged, the sliding surface may develop into weak cracks. Then, the plastic zone of penetration is offset; In the case of heavy rain, the slope can unload itself under the action of rainfall. At this time, the slope was unstable and the landslide happened suddenly.

Newtonian-noise characterization at Terziet in Limburg—the Euregio Meuse–Rhine candidate site for Einstein Telescope

Limburg, in the border region between Belgium, Germany and the Netherlands, has been identified as the Euregio Meuse–Rhine candidate site for Einstein Telescope. The site hosting this gravitational-wave observatory must minimize the Newtonian coupling of ground vibrations to the core optics of the low-frequency detectors. Newtonian noise depends on the ambient seismic field which is in turn dependent on the site’s geology and the distribution of surface and underground seismic-noise sources. We have characterized the site near Terziet in Limburg in terms of propagation modes, dispersion and angular distribution of seismic noise by employing sensor arrays on the surface. Attenuation of seismic noise with depth was studied with a borehole sensor. Based on the results of these measurements, a realistic seismic-field model has been derived that represents a complete solution of the elastodynamic wave equations for a horizontally-layered soil structure. This seismic-field model allows to estimate the Newtonian-noise contribution to the sensitivity of Einstein Telescope for the characteristic geology and ambient noise conditions in South Limburg. The site’s geology features soft-soil layers on hard-rock and is effective in attenuating Newtonian noise from surface waves below the required sensitivity. A random background of body waves with all possible angles of incidence is expected to constitute the dominant source of Newtonian noise.

Numerical Simulation Study on Construction Effect of Top‐Down Construction Method of Suspended Diaphragm Wall for Deep and Large Foundation Pit in Complex Stratum

In order to study the deformation of a diaphragm wall and the settlement of surrounding soil under the complicated conditions of the deep and soft soil layer, which is underlying inclined rock and top‐down construction method, the construction scheme of suspended diaphragm wall based on the engineering practice of Oujiang North Estuary deep and large foundation pit of Wenzhou S2 railway is proposed, and the research method of numerical simulation is adopted in this article. By using Midas GTS finite element software, a three‐dimensional model of deep and large foundation pit excavation with top‐down construction method is established, and the internal force, deformation, and surface settlement of surrounding soil of suspended diaphragm wall with different depths into rock (1 m, 2 m, and 4 m) are analyzed. Furthermore, the optimization simulation research on the anchor locking scheme is carried out for the determined footed diaphragm wall. Similarly, the anchor locking scheme at a depth of 1 m into the interface between soil and rock is selected as the supporting scheme of the footed diaphragm wall through the simulation results. The research results of this article can guarantee the safety of diaphragm wall construction of deep and large foundation pit in complex stratum.

Numerical Comparative Study on the Effective Replacement Thickness of Traditional Stone Coarse Aggregate or Steel-Slag Aggregate Mixtures in Improved Soft Fine Soils

This study uses Steel Slag Coarse Aggregate (SSCA) as a mixture replacement, preamble material to improve soft soils, which is economic, and has good effect environment. Recently, the development and utilization of by-product, waste and recycle materials must be studied and investigated as a source of improved material for soft soils as, an economic and good effect environmental. The study analyzes effects of both replaced mixtures, (SSCA) or (TSCA) on improved soil bearing capacity and expected settlement after verifying the model. Numerical modeling of the one of real store loaded strip using, PLAXIS, 2D, strain deformation behavior to achieve field visible and measured deformations of untreated soft soil. Numerical studies were devolved to investigate geomechanics parameters improved to compare between using (SSCA) or (TSCA) as, replacement mixture. Results demonstrate that using (SSCA) improved compressibility and strength of shallow soft soil layer significantly than using (TCSA) mixture, while (SSCA) improved strip footing ultimate bearing capacity, (UBC), by 84.4% compared with increase of 20.5% when using (TCSA) mixture at the same thickness. In addition, the study highlights the effective (SSCA) replacement thickness ranges between (0.65 ~ 0.80) footing width.

Research on Numerical Simulation of Top‐Down Construction Effect of Diaphragm Wall of Deep and Large Foundation Pit under Different Working Conditions in Complex Stratum

In order to study the construction effect of deep and large foundation pit excavation and its impact on the surrounding environment under complex stratum conditions, the deep and large foundation pit of the top‐down construction method of shield exit shaft of S2 line of Wenzhou City railway is taken as the research object in this paper. According to the complex geological conditions of deep soft soil layer with underlying inclined rock surface in Wenzhou, the numerical simulation method is used to carry out the related research. The simulation results of the deformation characteristics of the diaphragm wall under two different working conditions which are the same length diaphragm wall and the suspended foot diaphragm wall are compared and analyzed. According to the analysis results, the suspended foot diaphragm wall is determined as the final construction scheme of the diaphragm wall, and it is verified by field measurement. The research results can provide technical support for the construction of similar projects. At the same time, it can also provide basic accumulation for the construction of major projects in Wenzhou.

Geotechnical approaches for preservation of openly exhibited Geo-relics damaged by rainfall infiltration

Excavated geo-relics are vulnerable to damage by natural processes. The aim of this study is to contribute to the establishment of a technical framework for the preservation of openly exhibited geo-relics. This study also examines the preservation of an openly exhibited geo-relic in Japan, which has experienced surface deformation in the soft soil layer due to water infiltration. The surface deformation is numerically investigated by performing seepage-deformation analyses based on unsaturated soil mechanics in order to understand its mechanism and to obtain effective countermeasures. The results show that deformation develops in the surface layer of the slope as the bonding between soil particles, represented by skeleton stress, and decreases when water infiltrates the slope. Although the calculation considers the influence of groundwater, as well as precipitation, the results show that the deformation of the slope is primarily controlled by precipitation, not by groundwater. Furthermore, the elevation of the groundwater does not contribute to the development of surface deformation. Based on the mechanism of the surface deformation, replacing the surface layer with a well-compacted, highly permeable soil is proposed to improve slope stability. It is predicted that this proposed method will be effective because the replaced zone retains sufficient strength and stiffness when it is wet, despite a decrease in the skeleton stress due to rainfall infiltration. This countermeasure has been adopted for the actual restoration of a damaged slope.

Forensic Study on Federal Road Pavement Failures at Rengit and Semerah, Batu Pahat

Forensic investigations in engineering term may be conducted to identify the causes of failure to facilitate the design of proper repairs, or to improve the performance or lifespan of a component, assembly, or structure. This paper presents the combination of geotechnical investigation and geophysical survey method as a forensic tool to predict the causes of pavement failure occurred along the coastal area of federal road FT005. The number and type of field testing are varying on each selected study area at Rengit and Semerah, Batu Pahat as this location constructed on soft soil area. Non-destructive testing (NDT) method using electrical resistivity tomography (ERT) were chosen to be applied at the worst severity of the road failure. Three number of resistivity lines of 100m and 200m were laid out using ABEM Terrameter LS2 with gradient method of electrode arrays covering up to 40-meters depth. Then, further testing of destructive testing (DT) method using borehole drilling conducted near the ERT lines to obtain soil profile with SPT N-value measurement. The disturbed and undisturbed samples were obtained to carried out laboratory test for soil classification. After that, inspection of road pavement layers were implemented at five number of trial pit tests which excavated up to subgrade layer to determine the thickness of the materials used in road construction. Three number of mackintosh probe test were also conducted on top of the subgrade layer after the trenching to obtain the strength of the subgrade layer. The results presented showed that severe longitudinal cracking were the predominant premature failures on the roads studied due to settlement effect of soft soil. The analysis obtained from RES2DINV program stated that the subsurface profiling was dominantly in saturated condition which the resistivity value was less than 100 ohm.m. From the SPT N-value it is observed that, the very soft soil layer is up to 8 m followed by the soft to stiff clays soil. Another cause of failure was resulted from the differential settlement due to the effect of different design of road construction with varies material used. The reliability and efficiency of the instruments used were also discussed in this study.

Novelties in the islands and tunnel project of the Hong Kong–Zhuhai–Macao Bridge

The Islands and Tunnel Project of the Hong Kong–Zhuhai–Macao Bridge is a design–build contract involving the construction of two artificial islands, each with an area of 100000 m2, that are connected by a 6.7-km-long underwater tunnel. The immersed section of the tunnel consists of 33 elements. This paper examines the novel techniques used during the design and construction of this project. To eliminate its environmental impact, deep piles consisting of steel cylinders and auxiliary cells were developed to build the artificial islands. Novel structural solutions the semi-rigid system were also developed to address the challenges of a high surcharge load and unfavorable layers of soft soil. Moreover, a foundation scheme consisting of a composite foundation layer and a combined foundation was developed to control settlement and prevent leakage. The factory method was implemented to prefabricate the 33 tunnel elements, including five plane-curved elements, where the mass of a typical element was 78000 t. Severe sediment-related problems were encountered during the immersion of elements of the tunnel that necessitated special measures as well as the development of a sediment warning and forecasting system. The closure joints were constructed by using a novel method involving deployable elements. The novelties introduced during the construction of this project are geared toward simplicity, which is conducive to green construction and sustainable solutions. The authors also provide suggestions for improvements if this kind of project were to be undertaken again.

ALTERNATIVE IMPROVEMENT OF SUBGRADE AND REINFORCEMENT OF EMBANKMENT ON PELABUHAN KUALA TANJUNG TOLL ROAD, INDERAPURA - KUALA TANJUNG SECTION IN NORTH SUMATRA (STA 0+000 S/D STA 3+500)

Planning the Kuala Tanjung Ruas Port of Inderapura - Kuala Tanjung Toll Road is one of the efforts to support economic growth in North Sumatra. The toll road will be built on a pile with a height of 2 ms / 9 m at STA 0 + 000 to STA 3 + 500. Due to limited land, the toll road section must be built on the poor subgrade. Based on the N-SPT test results, it is known that the subsoil consists of a clay layer with a thickness reaching 36 meters and a layer of sand at the top and bottom with a low N-SPT value. If the soft subgrade is burdened with a high heap, it is estimated that the subgrade will experience landslides, so it is necessary to plan the improvement of the subgrade and reinforcement of the pile.The soil improvement method chosen is the pre-loading method with a combination of the prefabricated vertical drain (PVD). Pre-loading method aims to spend the compression that occurs on the subgrade so that there is no compression during the service life of the road. The combination with PVD is intended to speed up the compression time in thick clay layers. The stability of the embankment will be calculated using a computer-assisted program, and geotextile reinforcement planning will be carried out if the value of the safety factor does not meet the requirements.Based on the analysis of compression computation will be divided into five zones. The amount of compression obtained under various piles of 2 ms / d 9 m was 2.25 m, 1.49 m, 1.75 m, 2.57 m, 3.65 m, 4.3 m, 4.69 m, and 5.06 m with an initial heap height of 5.02 m, 5.26 m, 6.51 m, 8.34 m, 10.41 m, 12.07 m, 13.46 m, and 14.83 m. Compression time required for subgrade is relatively long with a period of more than five years so that the planned acceleration with PVD. To accelerate the compression time to 5-6 weeks, a triangular PVD installation pattern is used with a spacing between PVD of 1.5 m and a depth of PVD installation as thick as a soft soil layer. From the results of the pile stability control, it is predicted that the pile will experience slides so that the reinforcement is installed with geotextile.Rp1.598,213,522,582.08.

Influence of extent of remedial ground densification on seismic site effects via 2-D site response analyses

Remedial ground densification techniques are widely employed in seismically active regions to mitigate geotechnical hazards and reduce the potential for damage to structures. These techniques can substantially modify the local soil conditions and the seismic site response characteristics of the profile. This study provides a quantitative insight into the local site amplification in remedially densified soils. Parametric site response analyses are performed using various subsoil conditions and extents of ground densification. The free-field site responses obtained with and without ground densification are compared using two-dimensional (2-D) finite element models. The results show that the frequency content of ground motions propagating in densified sites is modified, with a shift of soil response harmonics towards higher frequencies. When a densified crust is implemented with unimproved soft soil layers underneath, the PSAs at the ground surface are generally reduced, at frequencies between 5 and 10 Hz. However, the levels of de-amplification decrease as the depth of ground densification increases. In particular, the densification of the full depth of liquefiable soil layers leads to higher PSAs at the ground surface. The results presented provide a better understanding of the geometry and density of the improved zones that can lead to the amplification of ground surface motions in the frequency range of interest for building response, prior to investigating more complex problems that include soil-structure interaction.

Study on the Impact of Hydraulic Fracturing on Surrounding Ancillary Buildings considering SSI

Hydraulic fracturing is a key technology in the development of unconventional oil and gas reservoirs. With the continuous industrialization and large-scale development of shale gas production in China, the workload of hydraulic fracturing is also increasing rapidly, and the induced seismic events are also increasing gradually, resulting in different degrees of damage to the surrounding ancillary buildings. In order to study the impact of hydraulic fracturing on ancillary buildings, the finite element software ABAQUS was used to establish a three-dimensional model of middle and high-rise isolated structures to simulate the earthquake triggered by hydraulic fracturing. Then, considering the SSI (soil-structure interaction) effect of soil-based structure, the nonlinear dynamic response of the structure under the action of ground motion was analyzed. Through the adoption of different types of soil and the foundation depth, the influence of various parameters is discussed. The study found that in the case of not considering SSI, basal shear force, and displacement between floors of the seismic-isolation structure significantly greater than considering SSI, using hard soil layer, base shear displacement is greater than the soft soil layer and interlayer, shows that due to the effect of hydraulic fracturing, making fluid diffusion in soil, the seismic energy dissipation effect. It is also found that the period, base shear, peak displacement, and interlayer displacement of deep foundation pit are increased compared with shallow foundation pit considering SSI effect.

Effect of several patterns of floating stone columns on the bearing capacity and porewater pressure in saturated soft soil

One of the common geotechnical problems is the construction on soft soil and the improvement of its geotechnical properties to meet the design requirements. A stone column is one of the well-known techniques used to improve the geotechnical properties of soft soils. Sometimes thick layers of soft soil imposed the designer to use floating stone columns for improvement of such soil; in this case, the designer will be lost the end bearing of the stone column. In this study, the effects of several patterns of floating stone columns distribution under footing on the bearing capacity of soil and the distribution of excess porewater pressure are investigated. The soft soil used in this study has a very low undrained shear strength (cu) of 5.5 kPa and improved by several patterns of stone columns (single, two linear, triangular, square, and quadrilateral). The stone column has a length of 180 mm and a diameter of 30 mm. The material of the stone column is poorly graded sand has an angle of internal friction (48.5°) at a relative density of 65%. The results indicated a significant increase in the ultimate bearing capacity of soft soil when treated with floating stone columns despite the small ratio of area replacement and reducing the excess porewater pressure and settlement. Also, the ultimate bearing capacity of soil calculated from experimental work is compared with the corresponding values obtained from the proposed equations in the previous studies to evaluate the validity of using such equations.

An Empirical Correlation between the Residual Gravity Anomaly and the H/V Predominant Period in Urban Areas and Its Dependence on Geology in Andean Forearc Basins

The study of site amplification effects is crucial to assess earthquake hazards that can produce great damage in urban structures. In this context, the gravity and the ambient noise horizontal-to-vertical spectral ratio (H/V) are two of the most used geophysical methods to study the properties of the subsoil, which are essential to estimate seismic amplification. Even though these methods have been used complementarily, a correlation between them has not been thoroughly studied. Understanding this correlation and how it depends on geology could be important to use one method as an estimator of the other and to make a distinction between the seismic and gravimetric basement. In this research, a comparison between the residual gravity anomaly and the H/V predominant period is performed using a long dataset from different projects on sedimentary basins in a group of the most important cities in Chile. To simplify the geological information, a seismic classification is used for soils, which considers the Vs30 and the predominant period of vibration (T0). The results of this comparison show a direct correlation between both parameters, the higher the negative residual gravity anomaly the higher the H/V predominant period. This correlation improves when only soft soils are considered, increasing the R2 value in more than a 50% in all the individual cities with respect to the overall correlation. When all the cities are considered, the R2 value for soft soils increases up to 0.87. These results suggest that the ideal geological background for this correlation is when a soft soil layer overlies a homogeneous bedrock. Heterogeneities in the bedrock and in the soil column add dispersion to the correlation. Additionally, the comparison between the depth to basement inferred by both methods show differences of less than 15% in soft sites; in denser sites, the difference increases up to 30% and the definition of a clear H/V peak is more difficult. In general, the gravimetric basement is deeper than the seismic one. However, gravimetric depths to basement can be under/over-estimated in zones with a heterogeneous soil column.

Results of land subsidence measurement using GPS method in the Jakarta groundwater basin in 2015-2019

The development of population and development activities in big cities in Indonesia, especially in the city of Jakarta and surrounding areas is very rapid. From several land subsidence studies, several factors have been identified that cause land subsidence, namely: excessive groundwater extraction, reduction due to building/infrastructure loads, subsidence due to natural consolidation of soft soil layers, and subsidence due to tectonic forces. At present the exploitation of ground water for industrial and residential needs is at a level that needs attention. Excessive pumping of groundwater will cause a decrease in the quantity of ground water, entry of seawater into the land (sea water intrusion) and land subsidence. Symptoms of the negative impact of land subsidence have been felt in several areas, especially in industrial areas located in the northern part of Jakarta. This land subsidence can be measured by GPS or satellite geodetic method, which have begun to develop in Indonesia in the past two decades. Measurements were made using the radial method at 53 GPS points in 2015 up to 100 measurement points in 2019 in Jakarta Groundwater Basin. The result of these campaign GPS surveys that is northern part of Jakarta relatively had higher subsidence rate than the southern. The largest subsidence almost reached 6.2 cm/year in Muara Baru in northern area which is southern area only suffered an average rate of 1.16 cm/year.