reference planes intersect the first reference surface along a closed continuous piecewise linear first curve and the reference planes intersect the second reference surface along a closed continuous piecewise linear second curve when second curve is large than and exterior to the first curve and those curves are laterally separated by less than approximately 35 feet as a practical matter refrigerant characteristics will generally provide sufficient cooling of the earth to permit the barriers to merge at that separation with a thick walled barrier as may be established by controlling refrigerant flow so that the ice columns from adjacent barriers merge i e overlap the resultant composite barrier may be maintained so that its central region i e between the sets of inner and outer boreholes is at a predetermined temperature such as the optimum temperature 37 u00b0 celsius once this temperature is established in that central region the refrigerant flow may be controlled so that the average barrier width remains substantially constant for example the flow may be intermittent so that during the 34 on 34 time the barrier tends to grow thicker and during the 34 off 34 time the barrier tends to grow thinner due to heat absorption from earth exterior to the composite barrier however during this 34 off 34 time the region between the inner and outer boreholes tends to remain substantially at its base temperature since little heat is transferred to that region by appropriately cycling the on off times the average width is held substantially constant in contrast with intermittent refrigerant flow in a single barrier system during the 34 on 34 time the barrier grows thicker but during the 34 off 34 time.