Reading lines of integers into a list of lists
A simple possibility could be to enclose each row between parentheses and use read-from-string (assuming that the file does not contain incorrect data):

(defun read-data (&amp;optional (file "data.txt"))
  "Returns the numerical data in FILE as a list of lists."
  (with-open-file (data file)
    (let (*read-eval*)   ; set *read-eval* to NIL to avoid malicious code in the input file
      (loop :for line = (read-line data nil nil)
            :while line
            :collect (read-from-string (concatenate 'string "(" line ")"))))))