Non-linear Finite Element Dynamic Analysis Of The Effect Of Compaction On Underground Conduits

Construction loads are an important but often overlooked factor in underground pipe design and installation. Common compaction equipments used in pipeline construction can be a major contributor to pipe damage. To reduce load concentrations in excess of the design loads, construction equipment should be kept at a safe distance above the pipe location.This study investigates the behavior of a reinforced concrete pipeline system under a compaction process. The full-scale experimental test was conducted using a Type-2 standard trench installations soil and minimum compaction requirement. Using a compaction machine "hoe pack" and including the weight of a backhoe, a maximum dynamic compaction force was applied to the entire area of the pipe-soil installation system. Induced pressure and deformation to the pipe wall were measured via load cells and strain gauges respectively. Damage to the pipeline was also monitored by an inspection camera installed inside the pipeline. The test results showed the pipeline system was damaged due to the compaction force when only the first 6-in (15-cm) layer of backfill soil above the pipe's crown was completed. The most critical compaction location is at the joint of the pipeline system. Compaction here led to crack and fracture in the pipe wall. Thus, the test results ensured the effect of a heavy compaction force on the failure of the pipeline system during the construction process.The finite element (FEM) model was developed based on the three-edge bearing experimental test known as "D-Load test." To verify the finite element (FEM) algorithm, the D-Load tests were conducted on eight full-scale reinforced concrete pipes with 18-in. (46-cm), 36-in. (91-cm), and 54-in. (1.37-m) diameters manufactured per the ASTM C76-08. The experimental tests closely exhibit the FEM results both when comparing load-deformation and crack initiation and propagation. Further, the significance of crack width in pipe stiffness characteristics is discussed. Complete three-dimensional (3-D) models of the "D-Load" test conducted on the reinforced concrete pipes is simulated by using FEM method. The simulation will predict the test up to failure by scaled dynamic analysis and discrete crack model. The crack model uses the constitutive material law for concrete coupled with tension stiffening algorithm. Also, the failure modes observed for different pipe diameters are documented and reported.Consequently, a parametric study of a pipeline under the compaction process in trench installation was conducted to specify the minimum backfill depth above pipes before compacted. The 3-D FEM modeled two spans of a concrete pipe and surrounding soils. The concrete brittle cracking criterion was applied for the post-failure behavior of a pipe model. In the surrounding soil zone, the Mohr-Coulomb criterion was used for the material property. A surface-to-surface contact property was employed in the interface between each two regions of a pipe and surrounding soils which employs a nonlinear incremental solution algorithm. The compaction force composed of a static weight of backhoe and a dynamic force from hoe pack applied simultaneously. All standard pipe sizes used in this parametric study are 24-in. (61-cm), 36-in. (91-cm), 48-in. (1.22-m), and 54-in. (1.37-m) diameters. The interesting parameters include the geometry related, material properties, and loading locations. This study shows the effect of backfill height on the stress reduction for each pipe size. At the most critical region on a pipeline, a minimum backfill soil cover above a pipe is defined.