Definitely could be. Guess you have to clean the sand out of it and smell. This will tell you if it's connected to the sewer line. If it is, it can probably be used as a clean out. Are you plugged up? After you've used it I would suggest purchasing a clean out with a screw in plug for future use and to keep the kids out of it.

By code definition the buried line from the municipal main or septic tank to the structure is called the "House Sewer" while the main line within the structure is defined as the "House Main Drain". Code states that there must be a "Main Cleanout" at the junction of the House Sewer and the Main Drain. The cleanout is generally required to be within 3Â of the point where the house sewer enters the structure. For houses with basements the main cleanout can generally be found within the first three feet inside the basement wall while for slab construction the cleanout will generally be found 3Â outside the footer wall. For structures with a crawl space often there is a limited access in the crawl space so they elect to put the main cleanout outside in the same manner as a slab construction.
There are two commonly accepted methods of installing the main cleanout on the exterior:
1. A "Rodding Tee" is installed on the line. A rodding Tee is a specialized Tee that has a radius curve in both directions which would permit rodding, or snaking" the line both upstream and downstream from the point of the cleanout. The line is required to be buried a minimum of 6" below the average frost line of in climates where frost is not a issue it must be a minimum of 12" below finished grade. A vertical riser is then attached to the side opening of the rodding tee and it is to be extended up to finished grade and capped. Keep in mind that the cleanout is generally installed long before the final grade-work is completed and it is extended to the theoretical grade level. It is not uncommon for the riser to be slightly buried during the final grade-work or landscaping phase.
2. An alternate method is to install a Wye & 1/8 bend with the side opening of the Wye pointing in the downstream direction and a riser extended from the side opening up to finished grade. A second Wye & 1/8 bend with riser is then installed slightly downstream of the first one but this time the side opening of the Wye is pointed in the Upstream direction. In this manner the line can be snaked in both directions dependant upon which riser you select. Here again, the risers are initially installed to the theoretical finished grade elevation so they may have been slightly covered during final grading or landscaping.

Ok a plumber came to my house and told me to look for the clean out under the ground cover. It's going to take for ever. otherwise no sign of the clean out. The white pvc tubing was a "dummy" according to the plumber. I m not plugged up, but really really want my clean out.
Thank you guys lots of details from you

Philippe