SAUGUS – Pre-cast rocks deemed not solid enough for Boston’s Big Dig were reportedly used to build the retaining wall on Hitching Hill Road that partially collapsed in July.An engineering investigation report ordered by Town Manager Andrew Bisignani has confirmed what Hesper Street homeowners already knew: The 400-plus foot retaining wall is not stable.A 30-foot section of the wall crumbled in July, sending nearly a ton of debris tumbling down a steep 70-foot slope toward the homes below on Hesper Street. While none of the homes were damaged, the rockslide was enough to send residents scurrying into the street and keep them out of their homes for the night.Town officials have been reticent to reach out too far to homeowners lest the town become liable for an incident that happened on private property. However, in a quest to offer some guidance, Bisignani met with Hesper Street residents and had a geo-technical survey on the remaining wall completed by UTS of Massachusetts Inc.According to the survey there were at least three different blocks used in constructing the wall that was designed to provide support for the unfinished houses at 2, 4, 6 and 8 Hitching Hill Road. The primary blocks, called “CastleBlock” were reportedly rejects from the Big Dig project and are no longer being manufactured.The survey also states that, “in many areas blocks were over-hanging lower blocks, which results in reduced stability and poor wall construction.”Other concerns show that many of the blocks do not appear to have an interlocking notch at the base and that some of the geogrid material used was facing in the wrong direction, making it weaker. Also the base preparation for the entire wall “is considered suspect and a probable cause for future instability.””Based on the study, it our technical opinion that the wall is not safe,” reads the report. “There are a number of design and construction issues that impact the wall stability. Both the internal and external stability of the wall were reviewed for this study.”Bisignani says he believes the study speaks for itself. He said copies of the report would be forwarded to the Hesper Street homeowners put in jeopardy by the wall, the Town Council and the mortgage company that is in the process of taking the homes in a foreclosure.Bisignani said he has also spoken to the construction company is working to have a chain link fence put up to block the area off from sightseers.”The area has been re-taped and the houses were posted again. But the town can’t secure it because then we accept liability.”Bisignani said he would also seek compensation for the survey from the vendor or whoever ends up with the property.”Essentially they won’t get a building permit until they pay,” he said. “The cost for the survey was about $20,000. It’s an extensive survey.”Bisignani wonders however if the houses would ever be occupied since according to the report, at least two have cracks in the foundations.”It will be a headache for whoever gets the property,” he said. “That wall will have to be taken apart and put back correctly. It’s the only way I could see doing it.”