Pick up Lambe and Whitman's Soil Mechanics (SI Version) - Chapter 26 explains Pore Pressure Developed during Undrained Loading. Heavily overconsolidated soils tend to expand upon loading and develop negative pore pressures (see section 26.6).
Not sure if I can improve on BigH, but the Holtz Kovacs Sheahan textbook (previously just Holtz Kovacs) also has a decent explanation of pore pressure, dilative soils, and Dr. Skempton's theoretical basis and coefficients. Chapters 12 and 13. If you have the older text, similar material. In very simple terms, you might think of dilation as a negative poisson ratio, as compared to typical structural materials.
I found the original paper from Skempton. See attached. The paper shows undrained triaxial tests with pore pressure measurements which was used to derive the A coefficients...
