This page of The Last of Us game guide contains information on where to find the artifacts and pendants in the Tommy's Dam chapter. Those items are collectibles-secrets. Our walkthrough indicates the exact location of each artifact and pendant.
Artifact #56 - Power Plant Map
Once you reach the power plant gate, search through the office space to the left thoroughly.
Inside, there is the map on the wall.
After the turbines go online, follow your brother down and go to the area at the other end of the hall.
On one of the bookshelves, there is one more pendant.
Artifact #57 - Plant Schematics
While fighting the bandits, you will reach back the place where Tommy introduced you to two engineers over the schematics.
You can now collect the documents from the desk.
Once you reach the derelict ranch, enter the nearest building and go to the room at the back of the house (to the right).
You should see a small table there, where there is the next pendant.