This tutorial provides an introduction to stratigraphy layer sedimentation which is a method of depositing new sediment during a simulation.   Layer sedimentation is defined using two data structures:

 

1Sedimentation_data which defines the control data for deposition of a single layer; e.g. unit name, material, sedimentation type, etc.

2Sedimentation_horizon which defines the target topology for the stratigraphy horizon of the new layer.

 

The sedimentation procedure creates a new sediment layer with geometry defined by the sedimentation horizon and the  existing top surface of the sediment.  In 2-D the algorithm allows  arbitrary intersection of the sedimentation horizon with the geometry of defining the existing sediment, which can lead to new sediment layers that are represented by several unconnected geometry surfaces.  The 3-D sedimentation algorithm is restricted to layer-cake sedimentation; i.e. where each new sediment layer completely covers the model.  

 

Schematic of Layer Sedimentation

 

The new layer is created instantaneously at the sedimentation time and then the gravity load for the layer is ramped up gradually over the specified duration of sedimentation process.  In addition to generating the new layer, the sedimentation algorithm also

1Updates loads and constraints at side boundaries if required; i.e. constraints or load assignments applied to underlying sediment is automatically assigned to the new sediment layer.

2Updates the stratigraphy description;i.e. the  stratigraphy unit and stratigraphy horizon definitions.

 

 

Example Illustrating Progradation of New Sediment on Pre-Kinematic Sediment Overlying a Salt Layer

 

Specific issues considered in the tutorial are:

1Defining the stratigraphy for pre-existing layers.

2Defining a moving sedimentation surface with constant and variable sedimentation rate.

3Deposition of multiple stratigraphy layers.

4Performing simulations with multiple stages (steps).

5Defining material grids and adaptivity for newly sedimented layers.

 

The example documentation assumes that the following examples have been undertaken beforehand:

1Mesh_001 2-D Unstructured Mesh Generation

2Mech_001 Mechanical Analysis Introduction

3Mech_004 Biaxial Test on Sand with Adaptive Remeshing

 

The objective is to deposit ca. 2000m of sediment over a 10 Ma period under uniaxial conditions and evaluate the porosity, horizontal stress and vertical stress throughout the column in the final configuration.   The sediment is considered to be Kimmeridge Westbury Clay (KWC) deposited with a reference porosity of 0.51 (51%) with the associated pre-consolidation pressure (pc) being 0.1 MPa.   The sediment is deposited in 10 stratigraphy layers of approximately 200m thick (the actual layer thickness is dependent on the vertical displacement due to consolidation of the previously deposited layers).    This is achieved by defining a 200m high by 50m initial column, and then depositing nine additional layers using the layer sedimentation procedure.

The model is analysed in uniaxial strain conditions; i.e. the vertical sides of the model are constrained in the horizontal direction, and the base of the model is constrained in the vertical direction.    These conditions are automatically transferred to newly sedimented layers in the sedimentation procedure.    The first stage (step) of the analysis is compaction of the pre-existing sediment due to its self weight over a 1 Ma time frame.  Each new stratigraphy layer is then deposited as a single stage with duration of 1 Ma.

 

 

The material corresponds to "KWC" in the "training.mdb" material database,    The model is elastoplastic with poroelasticity.  The main properties for this material are:

 

 

 

 

The shape of the initial yield surface in the p-q plane, hardening evolution of pc vs epv and the shape of the initial yield surface in the deviatoric plane are shown below.

 

 

Simulation Cases

 

Two Cases are considered:

 

Case 1 Constant Sedimentation Rate

 

Case 2 Variable Sedimentation Rate

 

A full description of data required for defining the sedimentation process is provided for Case 1 Constant Sedimentation rate