All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.
The scholars held two different views regarding mudding the graves (plastering with wet mud); the majority of the scholars held that it is permissible, and the other scholarly view is that it is disliked.
Ibn Qudaamah wrote, "Imaam Ahmad was asked about mudding the grave, and he replied, 'I hope there is no harm in that.' Al-Hasan and Ash-Shaafiʻi deemed it permissible..." [Al-Mughni]
An-Nawawi wrote, "As for mudding the grave, Imaam Al-Haramayn (Al-Juwayni) and Al-Ghazaali held that it is disliked. Abu ʻEesa At-Tirmithi stated in his book Jaamiʻ At-Tirmithi that Ash-Shaafiʻi said, 'There is no harm in mudding the grave.' The scholars of the Shaafiʻi School of jurisprudence did not address this matter. Hence, the correct scholarly view is that it is not disliked, as stated by Ash-Shaafiʻi, and no prohibition was reported in this regard..." [Al-Majmooʻ]
The Hanafi book Radd Al-Muhtaar reads, "It is disliked to mud the graves. The chosen view is that it is not disliked..."
The Maaliki book Minah AL-Jaleel reads, "It is disliked to mud a grave or cover it with lime. Most of the scholars' statements are about mudding the exterior of the grave and some scholars were reported to have meant mudding the interior of the grave..."
Based on the above, you bear no sin for giving the keeper soil to rectify the condition of the graves, since the most extreme ruling regarding mudding graves is dislike (not prohibition).
If there is a real need for mudding the graves so as to preserve them and protect the dead from being carried away by floods or the like, then the dislike is waived. For more benefit, please to fatwa 123484.
Allaah knows best.