Amenemhet I was the first pharaoh of the 12th dynasty. He reigned for 30 yrs (including a 10 yr coregency with his son Sensuret I) and then he was assassinated.
Unlike his predecessors, Amenemhet I had a different attitude towards the Israelites who, by the beginning of the 12th dynasty, had grown to number well over a million. He feared that the Israelites would side with Egypt's enemies and decided to oppress them lest they rebel.
The pharaohs of the 12th dynasty required a large slave labour force for the construction of their pyramids and other public works.
The Pyramids of the 12th dynasty, unlike those of earlier dynasties, were made almost entirely of mud bricks which were finished with a limestone veneer. Over the centuries, the limestone veneer has fallen away due to erosion, earthquakes and pilfering. What remains is the inner cores of the pyramids which contain millions of mud bricks.
The Jews were given the task of making these mud bricks and working in the fields. The 12th dynasty lasted almost 200 years and at least seven pyramids were constructed during this time.
The Hebrew Oppression probably began with Amenemhet I, the first pharaoh of the 12th dynasty. By the time of Sesostris II, a workers villages had been built to house the slaves working on the pyramids at Hawara and Dashur. These villages were occupied up until the time of the Neferhotep I when the Exodus occurred.
The Pharaohs of the Oppression therefore included: Amenemhet I, Sesostris I, Sesostris II, Amenemhet II, Sesostris III, Amenemhet III, Sobeknefru and the 13th dynasty pharaohs from Sobekhotep I to Neferhotep I.
Middle Kingdom pyramids were built mainly out of mud bricks (the inner core) which were then faced with a limestone veneer. This may have been due to a shortage of limestone and possibly the loss of ability and know how required to construct pyramids with solid limestone blocks as with the Old Kingdom pyramids.
The Amenemhet I pyramid at El-Lisht is severely eroded. It the oldest of the 12th dynasty pyramids which had mud brick cores and limestone veneers. Over the years, the limestone veneer has fallen away due to earthquakes, pilfering and erosion. All that remains is the mud brick core which has been severely eroded.