and outwardly therefrom preferably an expansion mechanism 26 is mounted in a gap 28 formed in riser frame 17 for expanding riser frame 17 outwardly into clamping engagement with the manhole frame expansion device 26 can be a turnbuckle as shown in fig3 or other types of expansion mechanisms such as shown in u s pat nos 4 225 226 3 891 337 and 4 097 171 it is this outward expansion which heretofore forced riser frame 17 against conical surface 9 which if angle a fig4 was too great could prevent a secure clamping engagement to be achieved therebetween in accordance with the invention a locking ring indicated generally at 30 is secured to riser frame 17 such as by an annular weld 31 locking ring 30 is preferably formed with a gap 32 fig3 which aligns with gap 28 of riser frame 17 locking ring 30 extends generally vertical downwardly from riser frame 17 and will be expanded radially outwardly with riser frame 17 by expansion device 26 into abutting engagement with the generally vertical surface 11 of manhole frame 5 as shown in fig4 surface 11 may have a slight taper of several degrees but considerably less than that of conical surface 9 and thus is referred to as being generally vertical this provides the desired locking engagement between riser assembly 15 and manhole frame 5 instead of relying upon the heretofore locking expansion engagement of riser frame 17 with conical surface 9 thus the radial outward locking engagement of riser assembly 15 within.