Repeated Load Triaxial Testing of Recycled Excavation Materials Blended with Recycled Phyllite Materials

Recycled Excavation Materials (REM) are becoming viable alternative construction resources due to their economic benefits. However, REM may be composed of weak rocks, e.g., phyllites, limiting the use in a base layer. The present paper attempts to further the knowledge of the mechanical performance of REM by performing Repeated Load Triaxial Tests (RLTT). REM are mixed with Recycled Phyllite Materials (RPM) in systematic blends of 0%, 25%, 50%, and 100%. The batches’ resilient modulus (MR) and permanent deformation (PD) characteristics were assessed to establish the maximum RPM allowed into REM while maintaining the required performance. Hicks and Monismith’s and Uzan’s models were used to characterize the stiffness behavior. A wide variation in the stiffness between the two materials was observed. Batches comprised of 0% RPM–100% REM and 25% RPM–75% REM showed high stiffness performance. The Coulomb model assessed the PD behavior, and the results showed a similar response for all batches. Unlike the stiffness, blended mixtures did not show sensitivity to increased RPM content in the PD. This study may help end-users to understand the performance of REM given the documented threshold on the allowable quantity of RPM in REM.

Stabilization of Deep Roadways in Weak Rocks Using the System of Two-level Rock Bolts

Large rock mass deformation around deep roadways in the weak rocks was a significantproblem in mining activities in Vietnam and other countries. The excavation of roadways leads to highreleasing stress, which exceeds the peak strength of spalling surrounding rock and causes it to enter thepost-failure stage. Tensile failures then initiate and develop around the roadways, which causes thefragmentation, dilation, and separation of surrounding rock. The capacity of the primary support systemis low, which results in a severe contraction in the whole section of roadways, which requires findingsolutions to prevent the deformation of rock mass around roadways and technical solutions fromstabilizing for deep roadways. To stability analysis of roadways can be applied analytical, experimental,semi-experimental, and numerical methods. This paper introduces the prevention mechanism of largedeformation of rock mass around roadways using 2-level rock bolts. The research results show that usingthe system of two-level rock bolts can reduce the values of tensile stress on the boundary of roadwaysrange from 10 to 15% compared with only one. The importance of the total displacement of rock mass onthe boundary of roadways will be reduced from 3.47 to 13.85% using six long cable bolts.

Role of Geomechanics in Identification of Possible Mechanisms for Non-productive Times NPT and Improving Drilling Operations in a Mature Field in Offshore Sarawak, Malaysia

This paper presents a case study on the role of geomechanics to identify possible failure mechanisms for non-productive time (NPT), avoid drilling risks and minimize costs in a field development drilling campaign, Offshore Sarawak Malaysia. Drilling optimization and reducing NPT for the drilling campaign was one of the key focus for maintaining the drilling time and costs. Drilling of moderately to highly deviated wells in this field has proven to be extremely challenging. Numerous lost-time incidents due to tight hole, stuck pipe, pack-off, casing held up were experienced, particularly when drilling through the shallow overburden shales and deeper reservoirs interbedded with shales and coals. Faced with continually increased NPTs, a geomechanical model was developed using regional offset wells to understand the mechanism of failures. A geomechanical model was developed to quantify the minimum recommended mud weights and optimize the wellbore trajectories. The outcome of this study was used as key input for casing and mud design. The in-situ stress state derived from field wide geomechanical model indicates the field is associated with a normal faulting stress regime, i.e., Shmin < SHmax < SV. The presence of relatively weak rocks means the field is potentially subject to stress-induced wellbore instability problems. However, observations of numerous time-dependent failures imply secondary influences must also be considered to arrive at possible remediation strategies. It was observed that the combination of weak rocks and numerous time-dependent failures using different types of mud system have contributed to wellbore stability problems. The wellbore stability is due to reactive shale, which is time sensitive as majority of the drilling problems are observed after drilling. The major contributor to the time-dependent deterioration process is mechanical and chemical imbalances between shale and drilling fluids compounded by large open-hole exposure area and contact time resulting in rising pore pressure caused by the invasion of drilling fluid into the formations, and then exacerbated by less-than-optimal drilling practices. This finding, together with improved geomechanical understanding of the field helped to evaluate the safe mud weight windows, formulate the mud designs and optimize drilling practices. All the planned wells were drilled successfully without any loss time incidents and non-productive time. This paper presents an integrated approach and workflow that combines the drilling data and formation response to identify the most likely causative mechanisms of the time-delayed wellbore instabilities in a mature field. This knowledge was then used to develop strategies for optimizing future drilling operations in the field.

Modified Standard Penetration Test for Drilled Shaft Design in Weak Fine-Grained Rocks

Standard penetration tests (SPTs) have been used to estimate strength parameters of soils and weak rocks when it is difficult to obtain high-quality samples for laboratory shear testing. SPTs require 45 cm (18 in.) of split-spoon sampler penetration to determine the blowcounts per 0.3 m (1 ft), which is difficult to impossible to obtain in weak rock, that is, intermediate geomaterials. As a result, a modified SPT is presented here for sampler penetrations less than 45 cm (18 in.) in weak rocks. This new procedure is termed the modified standard penetration test (MSPT) and uses the penetration rate, not the sum of penetration blowcounts per 0.3 m, to estimate the unconfined compressive strength for the design of drilled shafts in weak fine-grained rocks. The penetration rate is the inverse of the linear slope of the penetration depth versus blowcount relationship. With this new test and interpretation procedure, 45 cm (18 in.) of sampler penetration is no longer required to estimate the unconfined compressive strength of weak rocks. An empirical correlation between MSPT penetration rate and laboratory-measured unconfined compressive strength is presented here for weak Illinois shale. This correlation could be used to estimate the unconfined compressive strength for the design of drilled shafts in weak rocks.

Compositional Features and Swelling Potential of Two Weak Rock Types Affecting Their Slake Durability

Weak and weathered rocks are well known for their sensitivity to changes in moisture content. Degrading behavior is common in weak rocks with moisture-sensitive mineral components and present numerous stability problems. The slake durability is a measure of the resistance to weakening and disintegration of rock materials which quantitatively distinguishes durable from non-durable rock materials. Several rock material parameters interact on the process of disintegration when exposed to cyclic moisture changes, whereby the content of clay is believed to play a major role. This manuscript evaluates the overall material composition of flysch and serpentinite rocks cored from the wall of the shotcrete-lined headrace tunnel of a hydropower project, including minerals, structure, porosity, the presence of micro-discontinuities, and swelling potential, and links these properties to the slake durability. Further, the different methods used to assess compositional features affecting the durability of weak rocks are evaluated and discussed. The manuscript argues that the mineralogical composition and microstructures present in the intact rock and the content of moisture-sensitive constituents, as swelling clays, control the long-term durability of weak rock material. It is demonstrated that XRD assessments are not sufficient to detect the content of brucite and swelling components, and that methods as thin section and SEM analyses should be carried out in the assessment of weak and weathered rock mass.