I THINK MINING

A perfectly bad Vancouver summer day: dry, yet chilly, yet warm as you ride the four hours around the city and climb the bridges and sail down the inclines.  Stop for rice and meat, and later for ice cream.  Who cares how many calories.  This summer riding is shedding the weight and making for loose pants.  I can eat and drink as much as I like and yet the mass goes.   The rewards of summer and preparation for another winter of indulgence.

Back in the home office, I wrote yet another EduMine course.  I have  eight up and they bring me a reasonable income—at least as much and more than social security.  Thus I could not resist the temptation inherent in the challenge: “We have no course on groundwater modeling in mining.  Why don’t you write it—and fast.”

Admittedly it is a long time since I lectured on groundwater flow.  It is even longer since I did a thesis on the topic.  I have worked on projects over the years involving groundwater.  I have even solved groundwater problems using common sense and judgment.

Recently I found myself editing and extensively re-writing poor prose for a guideline for mining-related groundwater modeling.  I get little credit, but boy did I sweat the rewriting and casting of poor prose into plausible English.  Conclusion: groundwater modelers cannot write.  They are nice people.  But write? Better keep them at their computers.

Thus I elected to write in my own voice what I know and think about groundwater modeling. It will be many months before this course appears on EduMine.  In the meantime here is what I wrote today on flow nets:

I like flow nets and believe they are fundamental to understanding groundwater seepage and hence to modeling groundwater at mines and elsewhere.  I urge you to become familiar and adept with flow nets.  Cedergren is the best.  Work through his examples, many of which are applicable to mining groundwater situations.

I have interacted with so-called groundwater modelers who could not draw a flow net.  I dismissed them as being incompetent and incompletely skilled for the task at hand.

I helped a young geologist draw a flow net.  At first he could not get any correspondence between his sketches and the piezometer readings from the field.  We examined the situation.   He had forgotten to include the scree slope at the base of the hills.  These rock debris piles were of high permeability and significantly affected the groundwater flow.  When he included them he was able to sketch reasonable flow nets.

And a bit on boundary conditions:

The constant head boundary.  If you put a piezometer anywhere along such a boundary, the water in the piezometer would rise to the same elevation.  In effect this means that the energy along that boundary is constant.  There may be differences of elevation and differences of water pressure.  But the head, which is defined as the sum of elevation and water pressure, is such that in effect the energy that a particle of water has is constant along such a line.

The No-Flow Boundary.  No seepage occurs across such a boundary.  In real conditions, very low permeability clay layers are no-flow boundaries.  Water simply does not seep across such boundaries.  The water prefers to seep along in the high permeability soils above the low permeability clays.  If you sketch a flow net that incorporates this boundary, the boundary constitutes a flow line, i.e. the path that individual water particles (drops) follow as they seep through the permeable soils and rock.

Water Table (Phreatic Line).  This is a tricky boundary that occurs all too frequently in groundwater flow modeling of mining conditions.  Mathematically, the energy along such a boundary is equal to the elevation of the boundary.  For there is no water pressure along such a boundary; water pressure is zero along this line.  In flow net construction, such a boundary counts as a flow line, that is as long as there is no infiltration along the boundary.  Things get even more tricky if there is infiltration across the water table.  Consult the computer code manual on how to deal with this.

Seepage Face.  Along seepage face boundaries water seeps from the mass of soil or rock into the open air.  In practice this is a nuisance;  wet spots develop; erosion occurs as the seepage flows down the face; and you may have to collect this seepage.  In modeling, the issue is often: will the seepage rate be greater of less than the evaporation rate?  For if less, generally the rocks will remain reasonably dry and you may mine safely.