how to add a tax_query to pre_get_posts in wordpress

functions.php

<?php
add_action( 'pre_get_posts', array( $this,'function_pre_get_posts' ) );

function function_pre_get_posts(){
// you can't use the query->set here for tax_query
					// as tax query has already been made
					// so you need to need add youself to any 
					// existing tax query
					$tax_query = array(
									'taxonomy' => 'tax_name', 
									'field' => 'slug', 
									'terms' => 'term_name',
            			'operator'=> 'IN' );
					$query->tax_query->queries[] = $tax_query; 
   					$query->query_vars['tax_query'] = $query->tax_query->queries;
}