Mishnayos Eruvin Perek 7 Mishnah 3

With regard to a ditch between two courtyards that is ten handbreadths deep and four handbreadths wide, it is considered a full-fledged partition, and the residents of the courtyard establish two eiruvin, one for each courtyard, but they may not establish one eiruv. Even if the ditch is filled with straw or hay, it is not regarded as sealed and is therefore not nullified. However, if the ditch is filled with dirt or pebbles, the residents establish one eiruv, but they may not establish two eiruvin, as the ditch is nullified and considered nonexistent.

חָרִיץ שֶׁבֵּין שְׁתֵּי חֲצֵרוֹת עָמוֹק עֲשָׂרָה וְרָחָב אַרְבַּע, מְעָרְבִין שְׁנַיִם וְאֵין מְעָרְבִין אֶחָד, אֲפִלּוּ מָלֵא קַשׁ אוֹ תֶבֶן. מָלֵא עָפָר, אוֹ צְרוֹרוֹת, מְעָרְבִין אֶחָד, וְאֵין מְעָרְבִין שְׁנָיִם: