Another tunnel, the Papago Freeway Drainage Tunnel in Phoenix, used both compaction and chemical grouting during tunneling to control surface settlement, ground runs, and the development of large, open chimneys to the ground surface.
	The project was constructed between 1984 and 1987.  It consisted of three soft-ground tunnels.  These tunnels were referred to as the north, east, and west tunnels.  The total length of the tunnels was approximately 10.5 km (6.5 m).  Of the three tunnels, only the west tunnel was grouted.  It was 4.3km (13,970 ft) long with an excavated diameter of 7.6m (25 ft) and a finished diameter of 6.4 m (21 ft).  The crown of the tunnel was between 10.1 and 13.7 m (33 and 45 ft) below the surface.  All three tunnels were excavated through coarse alluvial sand-gravel-cobble deposits in downtown Phoenix.  All but a third of the east and west tunnel lengths were driven above the groundwater table.  In the portion of tunnel excavation below the groundwater table, the groundwater levels were drawn down to below tunnel invert using several large-diameter, high-capacity wells.
	The contractor selecte4d a shield machine with a digger arm.  Almost immediately after starting the east tunnel, which had no ground modification associated with the excavation, a series of large ground runs occured.  The runs resulted in large surface settlements on the order of 2-3 m (6-10 ft) and in large, open chimneys to the ground surface.  Sinc ethe tunnel alignment ran under highway right-of-way with little surface development, no surface structural damage was sustained.  In an effort to control these runs, the contractor modified the tunnel machine by adding poling plates and breasting boards.  The east tunnel was completed with these modifications.
	Based on the ground movements experienced during the east tunnel drive, and the fact that the west tunnel alignment passed under highly developed properties, a ground modification program was considered.  A risk assessment was eprformed based on the types of development along the west alignment.  The development included streets and pavements, high-voltage electric lines, high-pressure gas lines, and fiber-optic telecommunications cable, as well as residential and commercial buildings.  Based on the risk assessment and the experience gained on the east tunnel drive, a program of compaction and chemical grouting was developed for the west tunnel.
	The compaction grouting was performed from the surface as the tunnel advanced.  Grout holes were drilled on 3 m (10 ft) centers along the centerline of the tunnel alignment to within approximately 3 m (10 ft) above the crown of the tunnel.  Each hole was fitted with a 75mm (3 in.) diameter grout pipe.  Compaction grouting was started on a hole just after the tail shield passed beyond a grout pipe.  At first, noncementitious grout was used consisting of well-graded silty sand, fly ash, and water.  The mixture had a slump of approximately 50 mm (2 in.).  The mix was later changed to a readily available native sandy silt without fly ash.  This mix was used for the remainder of normal tunneling operations.  However, when ground losses in excess of 76.5 m^3 (100 cu yd) occured at the face, one bag of portland cememnt was added to every 1.5m^3 (2 cu yd) of grout mixed.  The cement was added to strengthen the ground and reduce the chances of grout flow into the heading around the face of the shield.  Lyman (1988) reported the grouting program was effective in limiting near-surface ground movement.  This effectively minimized the risk of damage to utilities, street pavement, and nearby buildings throughout most of the west tunnel alignment.
	As with jet grouting, compaction grouting is a specialized type of grouting requiring special equipment and procedures.  It is therefore performed by a limited number of contracting firms.  These specialty contractors usually design and construct the compaction grouting system to meet the design engineer's or prime contractor's requirements.  As with jet grouting, to maximize use of this methood specialty contracting fimrs need to be comtacted early in the design process.  A performance specification, as opposed to a method specification, is advixed when compaction grouting is specified.