code: public static String calcGravePlusDensity(String slsnd, String slcly, String omPct, String slcf) {
  
          String slbdm = calcNormalDensity(slsnd, slcly, omPct);
          String alpha = divide(slbdm, "2.65");
          String slcfv = divide(product(alpha, slcf), substract("1", slcf, product("-1", slcf, alpha)));
          String ret = sum(product(slbdm, substract("1", slcfv)), product(slcfv, "2.65"));
          LOG.debug("Calculate result for Bulk soil density (matric plus gravel), g/cm-3 is {}", ret);
  
          return ret;
      }
docs: Equation 20 for calculating Bulk soil density (matric plus gravel),
  g/cm-3
  
  @param slsnd Sand weight percentage by layer ([0,100]%)
  @param slcly Clay weight percentage by layer ([0,100]%)
  @param omPct Organic matter weight percentage by layer ([0,100]%), (=
  SLOC * 1.72)
  @param slcf Grave weight fraction by layer (g/g)