Wedge Volume Research Articles

Precise Patellar Tendon Insertion Protection and Osteotomy Surface Advantage of Transtibial Tuberosity-High Tibial Osteotomy.

Medial opening wedge high tibial osteotomy (HTO) is successful in the treatment of knee osteoarthritis with medial compartment stenosis and tibial varus deformity, but patella infera is the main complication. This study aims to design a new medial tibial open osteotomy scheme, transtibial tuberosity-high tibial osteotomy (TT-HTO), which can fully protect the patellar tendon insertion. In addition, the area of the osteotomy surface and wedge volume were evaluated in TT-HTO, biplanar distal tibial tuberosity osteotomy (biplanar-DTO), and uniplanar-DTO to evaluate the potential advantages of this technology in bone healing. The tibial tubercle was divided into four equal sections from proximal to distal, which were defined as zones A, B, C, and D. From September to December 2020, the imaging examinations of 200 patients (95 males and 105 females) with a mean age of 40.6 years (range 19-60 years) were evaluated to observe the zonation of the tibial tubercle where the insertion of the patellar tendon is located. Then, 59 patients (23 males and 36 females) with a mean age 59.6 years (range 43-77 years), for a total of 69 knees (32 right and 37 left), who underwent routine knee surgery were observed and verified. According to the position of the patellar tendon insertion, TT-HTO was designed. Fifteen tibial sawbones were divided equally into three groups: TT-HTO; biplanar-DTO; and uniplanar-DTO. The total area of the osteotomy surface was compared using the graph paper method. The wedge volume at wedge heights of 10 mm was compared among osteotomy types using the plasticine Archimedes principle. One-way repeated-measures analysis of variance was used to compare the total area of the osteotomy surface and the wedge volume. The osteotomy line of TT-HTO passes through the boundary point of zones B and C of the tibial tubercle to fully protect the insertion point of the patellar tendon. The total area of the osteotomy surface in TT-HTO and biplanar-DTO was significantly larger than that in uniplanar-DTO (P < 0.05). The wedge volume in uniplanar-DTO was significantly smaller than that in TT-HTO and biplanar-DTO (P < 0.05). No significant differences in the osteotomy surface and the wedge volume were identified between TT-HTO and biplanar-DTO. TT-HTO can protect the patellar tendon insertion and avoid postoperative patella infera. The osteotomy surface is large and located in an area of cancellous bone, which ensures its good healing characteristics.

The Use of Unmanned Aerial Vehicles (UAVs) for Estimating Soil Volumes Retained by Check Dams after Wildfires in Mediterranean Forests

Check dams act as soil collectors during floods, thus retaining a large amount of sediments. The estimation of the soil volumes stored behind a check dam is a key activity for a proper design of these control works and for evaluation of soil delivery after restoration measures at watershed level. Several topographic techniques have been proposed for this activity, but the sediment wedge mapping tools are complex and time consuming. Conversely, the use of unmanned aerial vehicles (UAVs) has been proposed to support aerophotogrammetric techniques for several survey activities with promising results. However, surveys by UAVs have never applied to calculate the size of the sediment wedge behind check dams that are built in fire-affected watersheds, where soil loss and sediment transport may be high after a wildfire. To fill this gap, this study evaluates the efficiency and efficacy of aerophotogrammetric surveys using UAVs to estimate the volume of the sediments stored behind ten check dams, built as post-fire channel treatment in a forest watershed of Castilla La Mancha (Central Eastern Spain). The results of the aerophotogrammetric technique were compared to traditional topographic surveys using a total station and GNSS/RTK, assumed as reference. The estimation of sediment wedge volume provided by UAVs was more accurate (mean RMSE of 0.432), extensive (density of mapped points of 328 m−2) and quick (two days of fieldwork) compared to surveys using the topographic method (RMSE &lt; 0.04 m, six days of field work and density of mapped points of 0.194 m−2) by the topographic method. The differences in the sediment volume estimated by the two methods were not significant, but the UAV method was more accurate for the larger check dams. Moreover, a significant correlation was observed between the volume estimates provided by the two methods, shown by a coefficient of determination close to 0.98. Overall, these results propose a larger use of the aerial surveys for mapping activities in channels regulated by check dams, such as those built for restoration of fire-affected forest watersheds.

Logjam attenuation of annual sediment waves in eolian-fluvial environments, North Park, Colorado, USA

Sediment waves, a term that describes the fluvial transport of a discrete sediment influx, have been studied in regard to channel response to infrequent catastrophic events, such as mass movements or dam removal. However, few researchers have studied (1) the potential presence of sediment waves of annual or sub-annual scale in mixed eolian-fluvial geomorphic environments or (2) the role of large wood in sediment wave dispersal. This study addresses both topics through observations of North Sand Creek and East Sand Creek, which flow alongside the active sand dunes of North Sand Hills and East Sand Hills, respectively, in North Park, Colorado. The creeks experience similar seasonal, asynchronous cycles of eolian influx and fluvial transport, although North Sand Creek receives a greater volume of eolian sand. We use repeat photography and repeat measurement of ephemeral terraces to determine whether each creek experiences a sediment wave, and we use repeat measurement of logjam sand wedge volume to examine changes in sand storage associated with logjams over time. Results indicate that North Sand Creek experiences a highly translative sediment wave that is attenuated by logjams. East Sand Creek, on the other hand, does not experience a sediment wave.

Increased contact area of flange and decreased wedge volume of osteotomy site by open wedge distal tibial tuberosity arc osteotomy compared to the conventional technique

The purpose of this study was to evaluate the area of the osteotomy surface, including the flange and wedge volume, in open wedge high tibial osteotomy (OWHTO), distal tibial tuberosity osteotomy (DTO), and distal tibial tuberosity arc osteotomy (DTAO) using tibial sawbones. It was hypothesized that the area of the osteotomy surface, including the flange, in DTAO was larger than that in OWHTO and DTO and that the wedge volume in DTAO was smaller than that in OWHTO and DTO. Fifteen tibial sawbones were divided equally into three groups: OWHTO, DTO, and DTAO. The total area of the osteotomy surface in OWHTO, DTO, and DTAO was compared using image analysis software. The contact area of the flange and the wedge volume at wedge heights of 5, 10, and 15mm were compared among osteotomy types. One-way repeated-measures analysis of variance was used to compare the total area of the osteotomy surface, the contact area of the flange, and the wedge volume at 5, 10, and 15mm in OWHTO, DTO, and DTAO. The total area of the osteotomy surface in DTO and DTAO was significantly larger than that in OWHTO (P < 0.05). The contact area of the flange in DTAO was significantly larger than that in OWHTO at each wedge height (P < 0.05). In addition, the contact area of the flange in DTAO was significantly larger than that in DTO at wedge heights of 5 and 15mm (P < 0.05). The wedge volume in DTAO was significantly smaller than that in DTO at each wedge height (P < 0.05). This study demonstrated that an increase in the flange contact area and a decrease in the wedge volume in the opening gap were found in DTAO compared to DTO. This would be an advantage for anteroposterior screw insertion from the flange to the distal tibial fragment and bone union at the osteotomy site.

Imaging perturbed shock propagation in powders.

A novel experimental methodology is presented to study the deviatoric response of powders in shock regimes. The powders are confined to a cylindrical wedge volume, and a projectile-driven shock wave with a sinusoidally varying front propagates through the powder. The perturbed shock wave exhibits a damping behavior due to irreversible processes of viscosity and strength (deviatoric) of the powder with propagation through increasing powder thicknesses. The inclined surface of the wedge is polished and coated to establish a diffuse surface suitable for reflecting incident laser light into a high-speed camera imaging at 5 MHz. Images of the contrast loss upon shock wave arrival at the observation surface are post-processed for qualitative and quantitative information. New data of shock damping behavior with parameters of perturbation wavelength and initial shock strength are presented for powders of copper, tantalum, and tungsten carbide as well as their mixtures. We present the first full-field images showing additional spatial disturbances on the perturbed shock front that appear dependent on particle material and morphology.

An estimate of ice wedge volume for a High Arctic polar desert environment, Fosheim Peninsula, Ellesmere Island

Abstract. Quantifying ground-ice volume on a regional scale is necessary to assess the vulnerability of permafrost landscapes to thaw-induced disturbance like terrain subsidence and to quantify potential carbon release. Ice wedges (IWs) are a ubiquitous ground-ice landform in the Arctic. Their high spatial variability makes generalizing their potential role in landscape change problematic. IWs form polygonal networks that are visible on satellite imagery from surface troughs. This study provides a first approximation of IW ice volume for the Fosheim Peninsula, Ellesmere Island, a continuous permafrost area characterized by polar desert conditions and extensive ground ice. We perform basic GIS analyses on high-resolution satellite imagery to delineate IW troughs and estimate the associated IW ice volume using a 3-D subsurface model. We demonstrate the potential of two semi-automated IW trough delineation methods, one newly developed and one marginally used in previous studies, to increase the time efficiency of this process compared to manual delineation. Our methods yield acceptable IW ice volume estimates, validating the value of GIS to estimate IW volume on much larger scales. We estimate that IWs are potentially present on 50 % of the Fosheim Peninsula (∼3000 km2), where 3.81 % of the top 5.9 m of permafrost could be IW ice.

Air wedge with variable refractive index for precise laser beam steering in a small range

This paper provides the results of work on a new method for metrological laser beam steering for the purpose of angular stabilization. Frequency stabilized metrological lasers are used for interference measurements of length and angle. Angular deviation of the beam may cause cosine errors when measuring displacements. This paper presents a method for metrological laser beam deflection with an air wedge of variable refractive index. The change in the index is achieved by changing the air pressure in the wedge volume. The implemented system produced a range of angular displacements of the 0.1mrad (10-4 rad; 100μrad) beam with a resolution of 10-7 rad.

WEDGE DIFFRACTION AS AN INSTANCE OF RADIATIVE SHIELDING

The celebrated Sommerfeld wedge diffraction solution is reexamined from a null interior field perspective. Exact surface currents provided by that solution, when considered as disembodied half-plane laminae radiating into an ambient, uniform space both inside and outside the wedge proper, do succeed in reconstituting both a specular, mirror field above the exposed face, and a shielding plane-wave field of a sign opposite to that of the incoming excitation which, under superposition, creates both the classical, geometric-optics shadow, and a strictly null interior field at the dominant, plane-wave level. Both mirror and shadow radiated fields are controlled by the residue at just one simple pole encountered during a spectral radiative field assembly, fixed in place by incidence direction $\phi_{0}$ as measured from the exposed face. The radiated fields further provide diffractive contributions drawn from two saddle points that track observation angle $\phi.$ Even these, more or less asymptotic contributions, are found to cancel exactly within the wedge interior, while, on the outside, they recover in its every detail the canonical structure lying at the base of GTD (geometric theory of diffraction). It is earnestly hoped that this revised scattering viewpoint, while leaving intact all details of the existing solution, will impart to it a fresh, physically robust meaning. Moreover, since this viewpoint confirms, admittedly in an extreme limit, the concept of field self-consistency (Ewald-Oseen extinction in more picturesque language), perhaps such explicit vindication may yet encourage efforts to seek exact solutions to scattering/diffraction by electromagnetically permeable (dielectric) wedges, efforts that harness integral equations with polarization/ohmic currents distributed throughout wedge volumes as sources radiating into an ambient, uniform reference medium.

PREDICTION OF ROCK MASS PROPERTIES, TUNNEL STABILITY AND SUPPORT PRESSURE BY GEOLOGICAL STRENGTH INDEX (GSI) IN CROCKER FORMATION: A CASE STUDY

This study was conducted along a tunnel in Crocker Formation. The objectives of this study are to determine the value of Geological Strength Index (GSI) and to predict rock mass properties, very unfavourable discontinuities combination and tunnel support pressure for rock bolts or shotcrete for the tunnel. Engineering geological mapping, rock sampling and estimation of GSI values and the disturbance factor were conducted along the tunnel faces. Laboratory analysis includes Point load and dry density test and data analysis consists of kinematic analysis and limit equilibrium analysis. The rock mass was characterised by 94.88 MPa UCS, 0.024 MN/m3 unit weight, widely space and high persistency of discontinuities. The GSI value is 50 with 0.8 disturbance factor. The cohesion, tensile strength and friction angle are 3.671 MPa, 0.056 MPa and 25.20°, respectively. There are eight possibilities of discontinuities combinations on tunnel roof that have factor of safety (F.O.S) lower than 2 and combination of joints 2, 4 and 6 has the highest maximum wedge volume of 28.37 m3. The maximum support pressure of rock bolts or shotcrete for F.O.S of 2 is 0.04 MN. The individual discontinuity plane has been identified to overestimate friction angle and cohesion values, then the GSI system should be applied in homogeneous or isotropic and not in structurally controlled rock masses.

Predicting the recurrence risk factors and clinical outcomes of peripheral pulmonary adenocarcinoma ≤3 cm with wedge resection

This study was designed to investigate the risk factors of recurrence and survival of clinical stage I lung adenocarcinoma underwent wedge resection by the use of Shanghai Chest Hospital Lung Cancer Database. A total of 746 patients with clinical stage I adenocarcinoma underwent wedge resection from 2010 to 2015 in our database were included in this study. Univariable and multivariable Cox proportional hazards regression were performed successively to select significant risk factors and then nomograms as well as the concordance indexes for RFS, OS and LCSS were developed, respectively. Kaplan-Meier survival curves were performed if necessary, with the identification of log-rank test. The 5-year RFS, OS and LCSS of clinical stage I adenocarcinoma underwent wedge resection were 86.1, 83.6 and 85.2%, respectively. There were three independent risk factors related with RFS (sex, pathology, pleural invasion), two related with OS (sex, volume ratio) and two with LCSS (sex, volume ratio) with the analysis of Cox regression and were selected to develop nomograms. The C-indexes of RFS, OS and LCSS were 0.767 (95% CI 0.667-0.867), 0.782 (95% CI 0.660-0.904) and 0.794 (95% CI 0.669-0.919), respectively. Lymphadenectomy did not show differences statistically but had tendencies of better RFS, OS and LCSS among the subgroup of invasive adenocarcinoma. Sex, pathology and pleural invasion could be recommended as criteria for clinical stage I adenocarcinoma undergoing wedge resection. And the larger the wedge volume and/or the smaller the tumor volume was, the better OS and LCSS were. If the volume ratio reached 10:1 or more, the survival rate was approximately 90% for both OS and LCSS. Whether lymphadenectomy was necessary for WR, especially in invasive adenocarcinoma, needed further research.

Geometric models for measuring sediment wedge volume in retention check dams

AbstractCheck dams play an important role worldwide in sediment retention, although their effectiveness is currently the subject of debate. In this study, 11 geometric models with their field methodology and equations are proposed to estimate the retained sediment quickly, easily and with a sufficient degree of approximation. The models were applied to a sample of check dams in the Saldaña badlands (Spain) in accordance with the shape of the valley to determine the volume of retained sediment and compare them with other widely used methods. Our geometric method gave lower volumes than topographic methods, but higher volumes than simple methods. This method is thus useful as a first approximation to estimate the sediment trapped by check dams and small silted dams, minimizing fieldwork, calculations and cost. Furthermore, it allows the selection of a better fit to the sediment wedge shape and higher accuracy than the simple methods used until now.

Role of ground ice dynamics and ecological feedbacks in recent ice wedge degradation and stabilization

AbstractGround ice is abundant in the upper permafrost throughout the Arctic and fundamentally affects terrain responses to climate warming. Ice wedges, which form near the surface and are the dominant type of massive ice in the Arctic, are particularly vulnerable to warming. Yet processes controlling ice wedge degradation and stabilization are poorly understood. Here we quantified ice wedge volume and degradation rates, compared ground ice characteristics and thermal regimes across a sequence of five degradation and stabilization stages and evaluated biophysical feedbacks controlling permafrost stability near Prudhoe Bay, Alaska. Mean ice wedge volume in the top 3 m of permafrost was 21%. Imagery from 1949 to 2012 showed thermokarst extent (area of water‐filled troughs) was relatively small from 1949 (0.9%) to 1988 (1.5%), abruptly increased by 2004 (6.3%) and increased slightly by 2012 (7.5%). Mean annual surface temperatures varied by 4.9°C among degradation and stabilization stages and by 9.9°C from polygon center to deep lake bottom. Mean thicknesses of the active layer, ice‐poor transient layer, ice‐rich intermediate layer, thermokarst cave ice, and wedge ice varied substantially among stages. In early stages, thaw settlement caused water to impound in thermokarst troughs, creating positive feedbacks that increased net radiation, soil heat flux, and soil temperatures. Plant growth and organic matter accumulation in the degraded troughs provided negative feedbacks that allowed ground ice to aggrade and heave the surface, thus reducing surface water depth and soil temperatures in later stages. The ground ice dynamics and ecological feedbacks greatly complicate efforts to assess permafrost responses to climate change.

2D and 3D GIS-based geological and geomechanical survey during tunnel excavation

The survey of rock mass conditions during the excavation of a tunnel is critical to ensuring the adequacy of the planned rock support measures. Information acquired by mapping the tunnel heading has a limited extension, making it difficult to mentally connect features and visualize the geometry and the systematics of geological structures along the tunnel axis. Therefore, a digital GIS (Geographic Information System)-based method has been developed to perform the geological survey and to automate a part of the geological/geomechanical mapping during tunnel excavation. A rugged tablet PC is used to digitally collect rock mass characteristics into a georeferenced map of the tunnel heading. Data are then added to a central database by a routine that automatically converts the relative coordinates of the face/walls maps to the geographic coordinates of the site. From the converted data, a horizontal cross-section of the encountered geology is drawn, and a 3D model of the surveyed discontinuities is automatically generated by a custom geoprocessing tool. A graph parallel to the tunnel axis, termed a parallel data viewer (PDV), allows one to automatically plot and control data associated with the excavation, such as geomechanical parameters, convergence values, overprofile height, wedge volumes and rock support classes. The horizontal geological cross-section along the tunnel axis and PDV dramatically increase comprehension of interactions between the rock mass and the support measures; they visually summarize a substantial amount of information and describe complex technical concepts in a way comprehensible even to non-specialists. The capacity of anticipating forthcoming geological problems, hence reducing hazards, is improved by representing interpretations of drillings on the site map and in 3D. The 3D model of the geology–tunnel interaction is used to calculate volumes of wedges and to assess the adequacy of rock support using parameters that accurately describe the real geological conditions.

Osteotomy techniques close to the knee. Effect on wedge volume and bony contact surface

Bone geometry following osteotomy around the knee suggests that biplanar rather than uniplanar open wedge techniques simultaneously create smaller wedge volumes and larger bone surface areas. However, precise data on the bone surface area and wedge volume resulting from both open and closed wedge high tibial osteotomy (HTO) and distal femoral osteotomy (DFO) techniques remain unknown. It was hypothesized that biplanar rather than uniplanar osteotomy techniques better reflect the ideal geometrical requirements for bone healing, representing a large cancellous bone surface combined with a small wedge volume. Tibial and femoral artificial bones were assigned to four different groups of valgisation and varisation osteotomy consisting of open wedge and closed wedge techniques in a uniplanar and biplanar fashion. Bone surface areas of all osteotomy planes were quantified. Wedge volumes were determined using a prism-based algorithm and applying standardized wedge heights of 5 mm, 10 mm and 15 mm. Both femoral and tibial biplanar osteotomy techniques created larger contact areas and smaller wedge volumes compared to the uniplanar open wedge techniques. Although this idealized geometrical view of bony geometry excludes all biological factors that might influence bone healing, the current data suggest a general rule for the standard osteotomy techniques applied and all surgical modifications: reducing the amount of slow gap healing and simultaneously increasing the area of faster contact healing may be beneficial for osteotomy healing. Thus, biplanar rather than uniplanar osteotomy should be performed for osteotomy around the knee.

Wedge volume and osteotomy surface depend on surgical technique for distal femoral osteotomy

Biplanar distal femoral osteotomy (DFO) is thought to promote rapid bone healing due to the increased cancellous bone surface compared to other DFO techniques. However, precise data on the bone surface area and wedge volume resulting from both open- and closed-wedge DFO techniques remain unknown. We hypothesized that biplanar rather than uniplanar DFO better reflects the ideal geometrical requirements for bone healing, representing a large cancellous bone surface combined with a small wedge volume. Femoral saw bones were assigned to 4 different groups of varization distal femur osteotomies: group 1, lateral open-wedge uniplanar DFO; group 2, medial closed-wedge uniplanar DFO; group 3, lateral open-wedge biplanar DFO; and group 4, medial closed-wedge biplanar DFO. Bone surface areas of all osteotomy planes were quantified. Wedge volumes were determined using a prism-based algorithm, applying standardized wedge heights of 5, 10 and 15mm. The biplanar osteotomy techniques (groups 3 and 4) created significantly larger femoral surface compared to the uniplanar groups (groups 1 and 2) (p=0.036). Bone surfaces after the lateral biplanar open-wedge technique (group 3) were slightly larger than the medial biplanar closed-wedge technique (group 4) and biplane techniques significantly larger than the uniplanar techniques (groups 1 and 2). Wedge volumes were significantly higher in the lateral uniplanar open-wedge (group 1) and biplanar open-wedge (group 3) techniques compared to the closed-wedge techniques (groups 2 and 4) that have nearly absent wedge volumes. Bone geometry following DFO suggests that the medial biplanar closed-wedge technique simultaneously creates smaller wedge volume and larger bone surface areas compared to the lateral biplanar open-wedge and the uniplanar DFO techniques. The horizontal cuts of the biplane DFO techniques are positioned behind the trochlear area in better healing metaphysial bone, which further enhances bone healing potential. Although this idealized geometric view on bony geometry excludes all biological factors that influence bone healing, the current data confirm the general rule for osteotomy techniques: reducing the amount of slow gap healing and wedge volumes and simultaneously increasing the area of faster contact healing by larger bone surface areas may be beneficial for osteotomy healing.

UNIQUENESS PROPERTIES OF THE INVARIATOR, LEADING TO SIMPLE COMPUTATIONS

It is shown that, for a three dimensional particle (namely an arbitrary compact domain with piecewise smooth boundary in R^3) the mean wedge volume defined on a given pivotal section is equal to the average nucleator estimator of the particle volume defined on that section. Further, if the particle is convex and it contains the pivotal point, then the flower area of a given pivotal section equals the average surfactor estimator defined on that section. These results are intended to throw some light on the standing conjecture that the functional defined on a pivotal section according to the invariator has a unique general expression. As a plus, the former result leads to a computational formula for the mean wedge volume of a convex polygon which is much simpler than the one published recently, and it is valid whether the fixed pivotal point is interior or exterior to the particle.

Wedge volume and osteotomy surface depend on surgical technique for high tibial osteotomy

Biplanar open-wedge high tibial osteotomy (HTO) is thought to promote rapid bone healing due to the increased cancellous bone surface compared to other HTO techniques. However, precise data on the bone surface area and wedge volume resulting from both open- and closed-wedge HTO techniques remain unknown. We hypothesized that biplanar rather than uniplanar HTO better reflects the ideal geometrical requirements for bone healing, representing a large cancellous bone surface combined with a small wedge volume. Tibial saw bones were assigned to 4 different groups of valgisation high tibial osteotomies: group 1: open-wedge uniplanar HTO; group 2: open-wedge biplanar HTO with ascending frontal cut; group 3: open-wedge biplanar HTO with descending frontal cut (retrotubercule osteotomy technique), and group 4: closed-wedge uniplanar HTO. Bone surface areas of all osteotomy planes were quantified. Wedge volumes were determined using a prism-based algorithm, applying standardized wedge heights of 5, 10, and 15 mm. The open-wedge biplanar osteotomy with a descending frontal cut (group 3) created significantly larger bone surfaces compared to the “classic” biplanar technique with an ascending frontal cut (group 2) and compared to all uniplanar techniques. Bone surfaces after the classic open-wedge technique (group 2) were slightly larger compared to all uniplanar techniques (group 1 and 4). No significant differences of wedge volumes were found between the retrotubercle (group 3) and classic open-wedge techniques (group 2). Wedge volumes were significantly higher in the uniplanar open-wedge technique (group 1) compared to the biplanar open-wedge techniques (group 2 and 3). Bone geometry following HTO suggests that the biplanar open-wedge techniques simultaneously create smaller wedge volumes and larger bone surface areas compared to the uniplanar open-wedge techniques. The relatively neglected closed-wedge technique still offers in theory the best healing potential, characterized by an almost absent wedge volume and a large bone-to-bone contact area. Although this idealized geometric view on bony geometry excludes all biologic factors that influence bone healing, the current data suggest a general rule for the applied standard osteotomy techniques and all of their surgical modifications: reducing the amount of slow gap healing and simultaneously increasing the area of faster contact healing may be beneficial for osteotomy healing. Thus, a biplanar rather than a uniplanar osteotomy may be performed for high tibial osteotomy in clinical practice.

Load and unload as interference factors on cyclical behavior and kinematics of Coulomb wedges: Insights from sandbox experiments

Modes of accretion in orogenic wedges are strongly controlled by mass balance and the efficiency of basal detachment. Sandbox models of accretionary wedges have demonstrated that fault systems grow episodically via cycles of alternating wedge thickening and lengthening. Generally, a new thrust plane generates within the footwall of the previous one, following a piggy-back mode of accretion, whereas redistribution of the mass balance in the wedge is associated to underthrusting and the reactivation of previous thrusts. We present the results of five sandbox experiments that model the interaction between tectonic accretion, sedimentation in foredeep area and erosion in the axial zone, in order to analyze the influence of these events on the cyclical behavior of a growing wedge. Modifications of the initial setup were made to analyze the effect on wedge development of syntectonic denudation, syntectonic sedimentation and coeval sedimentation and erosion, which markedly altered mass transfer within the wedge. In particular, lowering the surface slope by syntectonic erosion triggered synchronous accretion and underthrusting modes; by contrast, a sudden syntectonic sediment load in the prowedge region promoted prolonged phases of underthrusting, retarding the accretion of new imbricates at the prowedge toe, whether wedge volume was increased or not. The high length/thickness ratio of the model (120 cm/2.5 cm) allowed us to monitor a complete cycle of recovery of the system after each episode of sedimentation and/or erosion, observing how the wedge reacted to modification both altering its own cyclicity and progressively recovering it.

Analytical Approaches to Stress Intensity Factor Evaluation for Indentation Cracks

In the present paper, an analytical solution for the stress intensity factor in the case of cracks produced by Vickers indenters has been extended to the cases of triangular indenters, i.e., Berkovich and cube‐corner. According to the adopted approach, median/radial cracks produced by indentations are modeled as loaded by either a point‐force or a symmetric disk‐shaped wedge. The wedge diameter is assumed to be equal to the plastic zone size whereas the wedge thickness is evaluated by comparing the wedge volume with the hardness‐impression volume. The point‐force and the disk‐shaped wedge analyses produce an upper and a lower bound, respectively, of the geometry‐dependent parameter appearing in the expression for the fracture toughness. The predictions of the present analysis are in good agreement with similar experimental and numerical results.

Overview of analogue science activities at the McGill Arctic Research Station, Axel Heiberg Island, Canadian High Arctic

The Canadian High Arctic contains several of the highest fidelity Mars analogue sites in the world. Situated at nearly 80° north, Expedition Fjord on Axel Heiberg Island is located within a polar desert climate, with the surrounding landscape and conditions providing an invaluable opportunity to examine terrestrial processes in a cold, dry environment. Through the Canadian Space Agency's Analogue Research Network program, scientific activities based out of the McGill Arctic Research Station (M.A.R.S.) are extremely broad in scope, representing physical, biological, and technological investigations. Some of the most unique hydrogeologic features under investigation near M.A.R.S. are a series of cold saline springs that maintain liquid-state flow year round regardless of air temperature. Previous studies have examined their geomorphic relation to discharge-related formations, water chemistry, temperature monitoring, discharge rates, and combined flow/thermal modeling. Recent investigations have identified microbial communities and characterized biological activity within the springs and within permafrost sections, having direct relevance to astrobiological analogue research goals. Another main thrust of research activities based at M.A.R.S. pertains to the detection, mapping, and quantification of subsurface ice deposits. A long-term study is presently underway examining polygonal terrain, comparing surficial patterns found in the region with those identified on Mars, and using surface morphology to estimate ice wedge volumes through a combination of aerial photography interpretation and ground-based geophysical techniques. Other technological developments include the use of in situ microscopy for the detection of biomarkers and improved permafrost drilling techniques. This paper presents an overview of previous studies undertaken at M.A.R.S. over the past decades and will describe in detail both present and upcoming work.