Plymouth City Council said the pier, which was built in the 1880s, had been subject to many repairs due to storm damage.
The Local Democracy Reporting Service had previously reported the cost of the emergency repair work caused by Storm Bram, which involved pouring about 300 tonnes (300,000kg) of concrete into the pier to close the large hole, would total about £1m.
The council said over the summer it had been pointing the outer walls to help make them stronger, along with extensive investigations of the past remediation which included site sampling.
The authority said a 2019 investigation showed a “degradation of material” and although some “may have stabilised”, it needed to find out its full extent.
The expanding grout material was excavated to the fracture line, which it said was “very hard” as it was about “three times stronger than standard concrete”.
That difficulty “delayed the process which left us exposed to the winter storms”, it added.