Overburden Depth Research Articles

TBM tunneling in marble rock masses with high in situ stress and large groundwater inflow: a case study in China

Two of four headrace tunnels in the Jinping II hydropower project were constructed using tunnel boring machines (TMBs). The geology along the tunnel alignment is dominantly massive to highly fractured marble and the maximum overburden depth is 2,525 m. The paper discusses the problems encountered during the TBM tunneling, including instability of the tunnel wall and face induced by high in situ stresses, high-pressure groundwater inflows and excessive cutter and cutterhead damage. Measures taken to overcome these problems involved modifications to both the machines and the mode of operation as well as changes to the support parameters.

Discussion of “Enhancement of the Shear Strength of Wastewater Residuals Using Industrial Waste By-Products” by C. Kayser, T. Larkin, and N. Singhal

The authors have reported a laboratory study with two main themes: (1) an assessment of various waste products as substitutes for lime in amended biosolids (sewage sludge); and (2) the correspondence between undrained shear strength, su, deduced by shear-vane and triaxial compression (TC) tests. This discussion concerns itself with the second theme in which the authors, having studied strain rate (_ e) effects for a very soft site-specific material (su 1⁄4 3–5 kPa) in TC tests, concluded that suvane > sutriaxial by a factor (F) of 1.8 and 1.6 for biosolids and 20% lime-amended biosolids, respectively. Specifically, these F-values were deduced by the authors from a time to failure (tf) of ∼5 s observed for suvane 1⁄4 6–10 kPa using a Geonor handheld shear-vane apparatus, compared with tf 1⁄4 60 min for 75-mm diameter by 150-mm long specimens sheared at _ e 1⁄4 ∼0.33%=min in TC under a confining pressure (σ3) of 100 kPa. However, the factor F is not exclusively governed by relative differences in _ e between shear-vane and TC tests. The different apparatuses approach the estimation of strength in different ways, with torsional-shear failure occurring around a cylindrical surface in vane shear compared with general ductile bulging in TC. The deduced strength value is material specific and depends, among other factors, on its consistency (water content), degree of saturation (gas voids content), specimen boundary conditions, and applied confinement pressure in addition to scale effects concerning the specimen size in TC to the size and aspect ratio of the cruciform vane. For example, the vane specimens prepared and tested in PVC tubes by the authors would have only experienced lateral confinement (i.e., applied vertical stress was 0 kPa). Hence, the mean confining pressure applied in the authors’ vane tests was ∼0 kPa compared with ∼100 kPa for the sutriaxial data used in deducing the values of F reported in Eqs. (5) and (6) of the original paper. These confinement pressures would correspond to overburden depths in a biosolids monofill of 0 and ∼9.1 m, based on the reported in situ bulk unit weight of 1.1 t=m. Furthermore, aging effects on test specimens that are allowed to stand and cure (undisturbed and at constant composition) over an extended period before strength testing cause a strength gain (thixotropy). In the case of unamended biosolids (i.e., for pH < 11), internal reactions also occur, including chemical changes of the solids and pore fluid; biodegradation of organics, and accumulation of biogas produced by ongoing microbial activity. The slow but steady rate of biogas accumulation over the course of long-duration tests progressively reduces the degree of specimen saturation and also the effective confining pressure, with the latter calculated as the applied confining pressure minus the pore fluid pressure (O’Kelly 2006). This is accentuated under the undrained condition in the reported TC tests, for which the test specimen was fully enclosed by a rubber membrane. Consequently, the pore fluid pressure increases with elapsed time between setting up the specimen in the test apparatus and the moment at which shear failure occurs under TC, thereby reducing the mobilized strength (O’Kelly 2005a). Another consideration is that the shear-vane test might not be performed under a truly undrained condition for test material of slurry or very soft consistency on account of some flow of material occurring outward from between the blades and around the cruciform vane (Landva 1980) rather than the development of a purely cylindrical failure surface enclosing the vane, with suvane calculations based on the latter scenario. Next, some data is presented of remolded undrained shear strength (sur) for freshly prepared unamended biosolids with a degree of saturation of 94.1–97.4%, which was tested in both vane shear and TC. This biosolids material was comprised of ∼70% volatile solids by dry mass, a specific gravity of 1.55 (similar to values reported by the authors), and Atterberg liquid limit (LL) and plastic limit (PL) values of 314% and 53%, respectively. The physiomechanical and chemical properties of this biosolids material have been reported in full elsewhere (O’Kelly 2005a, b, 2006, 2008). The drive motor of the laboratory vane apparatus rotated the vane-head at an angular rotation of 9°=min, thereby transmitting a known torque through a calibrated spring to the 12.7 × 12.7 mm cruciform vane, which was embedded in the test specimen. 38-mm diameter by 76-mm long specimens were sheared undrained in singleand multistage TC tests (British Standards Institution 1990) using _ e 1⁄4 1.6%=min, with standard corrections applied to the mobilized deviator stress for the reinforcing effect of the specimen membrane. Water content (w) has been expressed in terms of liquidity index (IL) [Eq. (1)], with IL values of unity and zero corresponding to the LL and PL conditions, respectively:

From two-way time to depth and pressure for interpretation of seismic geometries and velocities offshore

SUMMARY The effect of water and rock loading on seismic velocities and consequently on interpreted geometries is often underestimated in offshore studies. Direct comparative analysis of interval velocity patterns between areas of significantly different water depth and thickness of rock overburden requires various pressure related changes in velocity to be accounted for. Presentation of velocity models as a function of pressure rather than two-way time, or depth, emerges as a possible solution. An accurate velocity model is essential for meaningful timeto-depth conversion of interpreted seismic horizons. Ideally, it should be based on integration of seismic velocities from well log measurements, refraction seismic surveys and from stacking of multi-channel marine reflection data. In some cases velocities derived from stacking of high quality long streamer marine reflection seismic data correlate reasonably with well log measurements and velocities derived from refraction seismic studies, and provide clues to reasonable depth conversion and lithology interpretation.

Numerical Investigation of Shield Cross Tunnel Minimum Cover Depth

With the development construction of subway in the city, the cross tunnel is becoming common. However, due to the geological conditions of the subway which limited the minimum depth of the shallow overburden of the tunnel is not the same. In this paper, according to the mechanical characteristics of the cross subway tunnel in several typical soil, a three-dimensional finite element mode is established. And to find out the discipline of minimum cover depth in vertical cross the tunnel by researching tunnel shield excavation the ground surface deformation characteristics.

Behavior of pressure-grouted anchors in gravel

This study consisted of field tests conducted on nine vertical and three inclined low-pressure-grouted anchors to investigate their behavior in gravel. An anchor can be categorized as a deep anchor when the overburden depth (free length) Z exceeds 8D (D is the diameter of the anchor). The shape of the heave on the ground surface of a shallow anchor is similar to a normal distribution curve. The extended diameter of the heave was between 170 and 300 cm, which could be divided into two zones, primary and secondary, based on the failure mode of the ground. As the fixed length of a shallow anchor increased, the extended diameter also increased. The ultimate load on an anchor increased with the free length and, to a greater degree, with the fixed length of an anchor: a fixed length of only 3 m generated an ultimate load of over 1100 kN. However, the permissible load, determined from the creep coefficient, is inapplicable for short anchors in gravel. The earth pressure coefficient K of vertical anchors was approximately 29 and for an anchor shaft inclined at 25º it was approximately 17.7.

Numerical Simulation for the Safety of Release Hole at Lower Position of Cavity in Tunnel

The energy release and depressurization method is often used for the hidden cavity with extra big mass and high pressure in a tunnel. It is critical to determine the minimal depth of safe overburden for the release hole at lower position and the pressure at high position to be released. According to actual data collected from a cavity of Maluqing tunnel of Hubei, this paper performs the computer simulation and analysis for the safety of the release hole at lower position in the cavity and determines the critical safe depth to disclose the cavity is determined tentatively by using the finite differential software of FLAC3D. The simulated results in accord well with the actual situation show the rationality and validity of this numerical simulation.

On mapping seafloor mineral deposits with central loop transient electromagnetics

Electromagnetic methods are commonly employed in exploration for land-based mineral deposits. A suite of airborne, land, and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose. In contrast, although the commercial value of marine mineral deposits has been recognized for decades, the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy. One particularly interesting electromagnetic method, which could be used to image a mineral deposit on the ocean floor, is the central loop configuration. Central loop systems consist of concentric transmitting and receiving loops of wire. While these types of systems are frequently used in land-based or airborne surveys, to our knowledge neither system has been used for marine mineral exploration. The advantages of using central loop systems at sea are twofold: (1) simplified navigation, because the transmitter and receiver are concentric, and (2) simplified operation because only one compact unit must be deployed. We produced layered seafloor type curves for two particular types of central loop methods: the in-loop and coincident loop configurations. In particular, we consider models inspired by real marine mineral exploration scenarios consisting of overburdens 0 to 5 m thick overlying a conductive ore body 5 to 30 m thick. Modeling and resolution analyses showed that, using a [Formula: see text] transmitting loop with 20 A of current, these two configurations are useful tools to determine the overburden depth to a conductive ore deposit and its thickness. In the most extreme case, absolute voltage errors on the order of 10 nV are required to resolve the base of a 30 m thick ore deposit. Whether such noise floors can be achieved in real marine environments remains to be seen.

Damage-free coring technique for rock mass under high in-situ stresses

Rock sampling with traditional coring method would cause initial damage to rock samples induced by in-situ stress relief during coring. To solve this problem, a damage-free coring method is proposed in this paper. The proposed coring scheme is numerically modeled first, and then it is verified by comparative laboratory tests using rock samples both obtained by conventional coring method and the proposed damage-free coring method. The result indicates that the in-situ stresses in sampling area could be reduced by 30%–50% through drilling a certain number of destressing holes around the whole sampling area. The spacing between adjacent destressing holes is about 10cm. The average uniaxial compressive strength (UCS) of rock samples obtained by the damage-free coring method in Jinping II hydropower station with overburden depth of 1 900m is higher than that of samples obtained by the conventional coring method with the same depth by 5%–15% and an average of 8%. In addition, the effectiveness of damage-free coring method can also be verified by acoustic emission (AE) monitoring. The AE events monitored during uniaxial compression test of damage-free coring samples is fewer than that of conventional coring samples at the primarily loading phase.

On the possibility of using suspension-log data to estimate historic overburden in shallow unconsolidated sediments

Suspension-log data are typically used for engineering and seismic studies in shallow unconsolidated sediments. At the USDOE Savannah River Site, suspension data were acquired in a series of deep boreholes (up to ∼400 m) to support high-resolution geological and geophysical investigations in the Upper Atlantic Coastal Plain. Although regional coastal plain stratigraphy is historically inferred from core, seismic, and log data, it became apparent that both major and minor unconformities and possibly formation contacts in the unconsolidated sediments of the Upper Atlantic Coastal Plain were strongly correlated to distinct variations in suspension data that translated from the deep to very shallow horizons. Overall, the suspension data contained more information (character) than typical wireline sonic logs, most likely because wireline logs are optimized for consolidated sediments (shales, sandstones, carbonates) while the suspension log is more optimized for unconsolidated sediments. Most coastal plain wireline-log stratigraphic correlations are based on gamma and resistivity data that often do not show much character in the freshwater-saturated sediments and correlations from paleontological and palynological analyses are often difficult because of the scarcity and oxidation of fossils. However, because of the correlation of suspension data with high-resolution near-surface engineering data from direct push, core samples, and shallow geophysics, it was noticed that it might be possible to correlate a deep regional engineering stratigraphy using the assumptions that coastal deposition proceeded to a zero stress level (i.e., subaqueous or subaerial exposure), that remnant burial stresses remain in deeper sediments, and that the Poisson's ratio determined from the P and SH data captured the remnant stress history. To explore this possibility, suspension-log data were correlated to stratigraphic horizons from geophysical, wireline, and direct push data. Correlated intervals were then analyzed using simple best-fit regression curves extrapolated to a value where the Poisson's ratio intersected a value for surface sediments (zero overburden) based on literature data. Initial results indicated that for some intervals an interpreted depth of historic overburden was reasonable. Overall results were mixed, but intriguing, suggesting that this technique might have value in estimating historic overburden and in providing better stratigraphic correlations. However, a better understanding of the suspension stratigraphic correlation and possibly a better model are needed.

Principle of in-situ 3D rock stress measurement with borehole wall stress relief method and its preliminary applications to determination of in-situ rock stress orientation and magnitude in Jinping hydropower station

As a main constituent of geological body, the rock masses have distinct differences from other materials, one of which is that rock masses are initially stressed in their natural states. Hence, it is an extremely challenging and significant research project to know the present residual stress of the rock masses in the earth’s crust. Although some regularities of distribution of in-situ rock stresses can be deduced, the basic means to study the state of rock stress is in-situ stress measurement. After a brief review of several measuring methods of in-situ 3D rock stress, a new one, borehole wall stress relief method (BWSRM) to determine the in-situ 3D rock stress tensor in a single drilled borehole was proposed. Based on the principle of in-situ rock stress measurement with BWSRM, an original geostress measuring instrument was designed and manufactured. Preliminary experiments for determination of in-situ stress orientation and magnitude were carried out at an experimental tunnel in Jinping II hydropower station in China, where the buried depth of overburden was about 2430 m. The results showed that it was feasible to measure the in-situ 3D rock stresses with BWSRM presented in this paper. The BWSRM has a broad prospect for in-situ 3D rock stress measurements in practical rock engineering.

A Top Pilot Tunnel Preconditioning Method for the Prevention of Extremely Intense Rockbursts in Deep Tunnels Excavated by TBMs

The headrace tunnels at the Jinping II Hydropower Station cross the Jinping Mountain with a maximum overburden depth of 2,525 m, where 80% of the strata along the tunnels consist of marble. A number of extremely intense rockbursts occurred during the excavation of the auxiliary tunnels and the drainage tunnel. In particular, a tunnel boring machine (TBM) was destroyed by an extremely intense rockburst in a 7.2-m-diameter drainage tunnel. Two of the four subsequent 12.4-m-diameter headrace tunnels will be excavated with larger size TBMs, where a high risk of extremely intense rockbursts exists. Herein, a top pilot tunnel preconditioning method is proposed to minimize this risk, in which a drilling and blasting method is first recommended for the top pilot tunnel excavation and support, and then the TBM excavation of the main tunnel is conducted. In order to evaluate the mechanical effectiveness of this method, numerical simulation analyses using the failure approaching index, energy release rate, and excess shear stress indices are carried out. Its construction feasibility is discussed as well. Moreover, a microseismic monitoring technique is used in the experimental tunnel section for the real-time monitoring of the microseismic activities of the rock mass in TBM excavation and for assessing the effect of the top pilot tunnel excavation in reducing the risk of rockbursts. This method is applied to two tunnel sections prone to extremely intense rockbursts and leads to a reduction in the risk of rockbursts in TBM excavation.

Finite element analysis of a vertical reinforced earth wall

Reinforced earth wall system has become very popular because of its aesthetic value and ease of construction. In order to optimize the design, it is important to understand its behavior during and after construction. This can be achieved either through field instrumentation or numerical simulation. In this paper, a numerical simulation of a reinforced earth wall construction is described and a parametric study using a finite element method is conducted to investigate the performance of the wall during and after construction. Different types of boundary conditions are imposed and the behavior of the wall is evaluated based on certain stress and deformation criteria. The results show that if the wall is allowed to move laterally, horizontal pressures at the connection increases with the depth of overburden until a depth of 0.6 H (height of wall) is reached. Beyond 0.6 H, the horizontal pressure starts to reduce with further depth. If no lateral movement is allowed, the horizontal pressure line follow more or less the KO line until a depth of 0.6 H is reached. Below 0.6 H, the horizontal pressure starts to drop rapidly. The increasing compressibility of the foundation soil induces redistribution of stresses that results in larger tensile forces in the reinforcements at lower levels and smaller tensile forces at around 0.6 H depth of overburden. As the foundation becomes more compressible, the horizontal displacement of wall facing increases. Key words: Anchor blocks, computer simulation, instrumentation, reinforced earth.

우산망 보강(Umbrella Arch) 공법 보강재의 보강효과

이 논문은 연약한 지반조건이나 주변 구조물이 있는 경우에 터널 굴착에 따른 주변지반의 변형을 억제하여 터널의 안정성 및 주변 구조물의 안정성을 확보하기 위하여 적용되는 보조공법 중 국내에서 주로 적용되고 있는 우산망 보강(Umbrella arch) 공법의 보강재에 의한 보강효과를 파악하기 위한 연구이다. 이를 위해 이 연구에서는 토피고, 보강재의 횡방향 및 종방향 설치 조건을 변화시키면서 토사 및 풍화암 지반조건에 대해 3차원 수치해석을 수행하였다. 수치해석에서는 그라우팅에 의한 지반개량 효과는 고려하지 않았다. 수치해석을 통해 터널 천단부 전방 지반의 침하량을 산정하였으며, 이를 이용하여 터널 막장면의 거동 각도를 계산하였다. 수치해석결과에 의하면 터널 굴착에 의하여 발생하는 지반의 침하량은 토피고에 따라 선형적으로 증가하며, 보강재의 중첩길이가 증가할수록 침하량이 감소하는 것으로 나타났다. This paper deals with the reinforcing effects of the reinforced material in the Umbrella Arch Method(UAM). Recently, UAM has been commonly used in Korea to decrease ground movement around a tunnel and stabilize the surface structure adjacent to a tunnel in tunnel construction. In this study, 3-dimensional finite element analyses were carried out for two ground conditions(soil and weathered rock) and for several overburden depths and installation angles of a reinforced material. But ground improvement by grouting was ignored. Displacements at the front of the tunnel crown were obtained from numerical analysis, and the behavior angles in the tunnel face were calculated by using the displacements. The results showed that settlement by tunnel excavation was increased with the increase of overburden depth but was decreased with the increase of overlap length of the reinforced material.

Study on the antipollution of phreatic system in Northern Shaanxi, China

The concept and evaluation method of the anti-polluting capacity of groundwater was assessed in this study to describe the vulnerability of groundwater system in northern Shaanxi Province of China. With a total area of 22,604 km2, the study area was divided into nodes of 80,719 and triangular elements of 125,377. The values of the seven evaluation parameters at each site were identified based on data from the overburden depth, boreholes, local precipitation, groundwater quality, and hydrogeological information. An antipollution map for the area of study was prepared, which suggested that the phreatic system in Northern Shaanxi can be classified into 3 grades of antipollution zones by a comprehensive index of vulnerability. Based on such index, the loess hilly region has the highest capacity of antipollution with an area of 11,300 km2, while the valley area and the area of wind- blown sand with a buried depth of less than 1.5 m have the lowest antipollution with an area of 3,391 km2.

Tempo-spatial characteristics and influential factors of rockburst: a case study of transportation and drainage tunnels in Jinping II hydropower station

Jinping II hydropower station is located in a high in-situ stress region in Southwest China. During the excavations of the transportation and drainage tunnels, more than 460 rockburst events were recorded in the transportation tunnel and 110 in the drainage tunnel, which has a serious and negative influence on the tunnels’ construction and the safety of staff and equipment. In the paper, the characters of rockburst patterns are analyzed for the transportation and drainage tunnels. The results are illustrated as follow: (1) Most of intensive rockbursts occur in the layer T2b, and continuous occurrences of rockbursts are more frequently observed than those in other layers. (2) The critical overburden depth of rockburst in the transportation tunnel is 600 m, and the length of the continuous occurrence section of rockburst is smaller than 25 m. The damaged depth of the rockburst has the tendency to increase with the increasing overburden depth, and the maximum damaged depth is over 3.5 m. (3) From east to west (west to east) in Jinping II hydropower station, the rockburst usually takes place in the right (left) side of tunnel working face, and then the left (right) or roof of the tunnel. The total length of the continuous occurrence section of rockburst is 57.4%–62.2% of the overall rockburst length, followed by the rockbursts of flake-splitting type and other types. (4) Compared with the transportation tunnel, the intensity of rockburst in the drainage tunnel is higher while the length of the continuous occurrence section of rockburst is smaller. The rockburst section with length less than 10 m and depth of 1 m mainly occurs in the layer at a depth of 1 800–2 000 m. The influences of opening geometry and excavation method on the characteristics of the adjacent zone are great, but the influence of the stress among the tunnel group induced by excavation is relatively low.

Study on Calculation of Rock Pressure and Determination of Depth for Shallow Asymmetric Tunnel

The method to calculate rock pressure for shallow asymmetric tunnel is analyzed by means of taking a mountainous tunnel with semicircular crown and straight sidewall as the object in this paper. The calculation method of tunnel rock pressure has been presented with consideration of both tunnel structure size and its overburden depth. Finally the way to determine the shallow or profound depth of asymmetric tunnel is also obtained.

A Numerical Study on the Uplift Behavior of Under-Reamed Anchors in Silty Sand

Ground anchors are common to be employed in geotechnical engineering. Under a limited space of an urban district or a restricted thick of an anchored stratum, an under-reamed anchor could meet both the requirements of a short length and a high anchorage capacity. To investigate the anchorage mechanism of the anchor, a series of triaxial tests was performed to obtain the parameters demanded for a constitutive model, SHASOVOD (A continuous strain hardening/ softening and volume dilatancy model for cohesionless soil during stressing). A numerical program was then developed to study the uplift behavior of the under-reamed anchor in silty sand. Analyzing results show that, for two under-reamed anchors located in different test sites, the complete load-displacement curves estimated numerically were consisted with those measured from the field test. The total load, friction load and end bearing do not reach their peaks simultaneously. According to the behavior of the end bearing, a critical overburden depth H of 6D was found to classify an under-reamed anchor as a shallow anchor or a deep anchor.

Large-scale geomechanical model testing of an underground cavern group in a true three-dimensional (3-D) stress state

The stability of a large cavern group at great depth is discussed on the basis of large-scale three-dimensional (3-D) geomechanical model tests and numerical simulations. The model tests are described in detail. Improvements in the tests were made in terms of experimental techniques and advanced measurement methods. The model tests utilized active loading on six sides of a rock mass in a true 3-D stress state. During the model construction, precast blocks were fabricated and monitoring holes were defined prior to test initiation. Newly developed combination ball-sliding walls were installed on each of the major loading surfaces to reduce the friction induced by model deformation. A unique grouting and installation technique employing prestressed cables was adopted in the tests. A digital photogrammetric technique, displacement sensing bars using fiber Bragg grating (FBG) technology, and mini multipoint extensometers were developed for measuring deformation. Overloading tests were then conducted for different overburden depths, and 3-D numerical analyses were performed to simulate the testing procedures. Conclusions regarding the stability of the cavern group were developed based on a comparison between the experimental and numerical results.

Design Aspects of an LNG Loadout Jetty in Yemen

SummaryThis paper discusses the design problems associated with the site and how they were overcome in consultation with the construction contractor to best suit their expertise and equipment. The facility consists of an 800 m long approach trestle giving vehicular access to the loading platform and carrying the product pipelines, six mooring dolphins, four berthing dolphins, and a multi-level loading platform with four LNG loading arms. Conventional piled headstocks were chosen for the approach trestle, but deeper water along the berthing face led to jacket structures for each of the berthing and mooring dolphins, and the loading platform. The site geology generally consists of sandy sediments overlying Basaltic rock, with the depth of overburden varying from around 15 m at the berth to exposed rock onshore. Low tension capacity of driven piles required a significant number of tension anchors to be drilled into rock. The 7.8 m design wave at the site was a breaking wave over about one-third of the length of the trestle, and the headstock level was chosen to clear the wave crest by 1.5 m. The design of the topworks was being carried out by the main contractor at the same time, which resulted in uncertain pipe loads during the preliminary design phase. Prefabrication was carried out in Malaysia and transported to Yemen by barge, along with all equipment and construction materials. Provisions for sea transport of the fabricated jacket structures and the mechanics of their handling and installation were important parts of the design process.

Effects of overburden, rock strength and pillar width on the safety of a three-parallel-hole tunnel

During the excavation of three-parallel-hole tunnel, the tunnel might collapse due to over-stress as a result of inadequate rock pillar width. Treating the rock overburden depth, rock strength, and rock pillar width as variables, a series of 3D numerical analysis was carried out to examine the effect of each variable on the safety of the tunnel, in particular the rock pillar. A stress strength ratio (SSR) was used to define whether the safety of the rock pillar was exceeded. A simple design chart for the case of three-parallel-hole tunnel, which took into account the influence of overburden depth, rock pillar width, and rock strength, was also proposed for used in the preliminary design stage.