Addendum Geotechnical Report Dewatering and Settlement Sanitary Sewer Improvements at Bush IAH, Project No. 699 HVJ Project No.

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1 Memorandum Firm Registration No. F-646 Date: To: From: Subject: Broutin Sherrill, P.E. RS&H, Inc. Michael Hasen, P.E. Addendum Geotechnical Report Dewatering and Settlement HVJ Associates, Inc. was requested to review dewatering for the proposed project and comment on dewatering related settlement. Our review was based on our geotechnical report, HVJ Report No. HG dated December 15, 2014 and project plans dated March We were also requested to review Specification for Tunnel and Primary Liner. The project includes replacement sanitary sewers, primarily a 24-inch diameter sewer from the existing Chanute Road lift station west of the airport extending east beneath taxiways and runways to Terminal A. The majority of the sewer is planned for installation in a primary lined tunnel with portions planned for open cut. There is a sand layer encountered in most of the borings, this sand layer is generally below the water table. Following is a summary of the soil, groundwater, and proposed construction based on the plans and our geotechnical report. Approximate Elevation, Feet Boring Tunnel Bore* Open Cut Invert Shaft Bottom Sand Layer Groundwater (Boring) Location** Sta / Sewer Line B-1 69 to 74 NA to B, 0+60 C B-2 69 to NA 67 to B, 1+80 E B-3 68 to to B B-4 NA 69 NA 72 to 78 74*** B B-5 67 to to B, 0+90 D B-6 66 to to A, 1+10 B B-7 65 to 70 NA NA NA A B-8 64 to 69 NA NA 67 to 75 76*** A B-9 63 to 68 NA NA NA A B to to A B-11 NA 63 NA NA A B to to 77 NA 4+20 D * Based on 54-inch diameter tunnel ** In most cases borings are offset from sewer centerline, see plans for details *** Groundwater level in piezometer

2 Page 2 of 5 Dewatering Groundwater conditions are an important factor in tunneling and open cut construction. Dewatering along the tunnel alignment is not usually performed. Tunneling equipment suitable for the conditions indicated in our borings is discussed in the tunneling section below. Dewatering is usually performed at the access shaft locations. Where the shafts penetrate water-bearing sand formations in the Houston area dewatering with relatively closely spaced wellpoints or educators located around the shaft is often used to bring the water level below the base of the shaft. Without such dewatering the shaft sides and base are likely to be unstable with loss of ground around the shaft, unsafe working conditions, and possibly unstable base conditions. Every access shaft extends through a water bearing sand layer except the shaft at B-10, and a water bearing sand layer is located very close to the base of the shaft near B-10. For the open cut operations, trenches that penetrate water-bearing sand formations in the Houston area are usually dewatered with relatively closely spaced wellpoints or educators located along the trench to bring the water level below the base of the trench. Without dewatering the sides and base of the trench are likely to be unstable with loss of ground around the trench, unsafe working conditions, and possibly unstable base conditions. Following is an evaluation of the open cut portions of the project. Sewer line A, Sta 1+00 to 9+46 based on B-10 and B-11 the profile is in clay above the water table. Pump and sump dewatering should be adequate. This alignment is generally away from airfield operations. Sewer line B, Sta 1+00 to 2+10 this is a tie in to an existing sanitary line. Tunneling would require an additional access shaft which will be expensive for such a short drive. Based on B-6 open cut excavation will require dewatering by eductors or similar systems. Sewer line B, Sta 2+10 to 4+10 I recommend that this section be tunneled with the access shaft moved to the location of the manhole at Sta This will move the access shaft at the end of the tunnel along sewer line B away from Taxiway WG which will allow easier access for construction operations and reduce the risk of any settlement around the access shaft impacting the pavement. Sewer line B, Sta 8+27 to Based on B-3, B-4, and B-5 open cut excavation will require dewatering by eductors or similar systems. Since access shafts are already planned at either end of this line it is a good candidate to convert to tunneling. Sewer line E Based on B-2 this line is in clay above the top of the sand layer and the groundwater table, it can be excavated using typical pump and sump dewatering.

3 Page 3 of 5 Tunneling Tunneling equipment needs to be suited to the ground conditions encountered. The best conditions in the Houston area are full face stiff to very stiff clay. By full face we mean that the entire tunnel diameter is within the same ground condition, in this case stiff to very stiff clay. The most difficult conditions normally encountered are full face or mixed face with sand below the water table. By mixed face we mean that a portion of the tunnel diameter is within different ground conditions, usually sand and clay. In these conditions uncontrolled flowing ground can develop which would cause large settlements of the ground surface above the bore. The tunnel bore will encounter sand below the water table at B-1, B-2, B-3, B-5, B-6, B-8, and B-12 in either full face of mixed face conditions. Equipment suitable for tunneling in these conditions is needed for the project. We recommend closed shield tunneling equipment for tunneling on this project. Closed shield refers to tunneling when the tunnel face is not exposed during excavation. The actual digging occurs beyond the closed shield with excavated cuttings and muck transported mechanically behind the shield for removal. Earth pressure balance or slurry shield tunneling equipment are two common types of closed shield tunneling equipment, Figure 1 illustrates slurry shield tunneling. A closed shield allows continuous support of the tunnel face by controlling the volume of muck removed based on the rate of advance of the bore. Figure 1 Slurry Shield Tunneling Settlement There are two types of settlement of concern on the project settlement due to dewatering and settlement due to tunneling. The impact of settlement on facilities depends on their sensitivity to movement. The settlement related concern for this project is the effect on displacement across pavement joints of airfield pavements. We understand from RS&H that if differential settlement at pavement joints exceeds 0.5 inches that pavement maintenance may be required. We recommend adopting a tolerable differential settlement at pavement joints of 0.25 inches for this project. Dewatering Settlement. Water table drawdown can cause settlement in clay. As the water table is lowered the effective stress in clay below the water table increases and this increase in effective stress causes consolidation settlement. The full long-term potential settlement will occur over a period of years. We need mote geotechnical daa to estimate the amount of long-term settlement that may

4 Page 4 of 5 occur and the time rate of such settlement. The amount of settlement will vary gradually over distance from the full amount at the point of dewatering to zero at some distance from the wells. We would need hydrogeologic data to determine the distance that dewatering may influence, but 300 to 600 feet would be reasonable for the full, long-term settlement case. We need to account for the time duration of dewatering. For the full settlement to occur dewatering would probably have to continue for more than five years, possibly much more. Due to the relatively short duration of project dewatering the settlement will be less than the full long-term settlement and the area affected will be smaller. We believe the key factor is that whatever dewatering related settlement does occur will vary gradually over distance, and the greatest amount of settlement will occur close to the wells. Based on experience we don t believe that the settlement will be significant to airport operations since it will vary gradually We recommend keeping dewatering wells as far as possible from airfield pavement but at least 25 feet, and limiting the duration the wells are operated as much as possible consistent with the need for excavation dewatering. Tunneling Settlement. There will always be some settlement over a tunnel due to loss of ground. Good construction practices limit the loss of ground. This is part of the tradeoff for the lack of disruption at the surface. See Figure 2 below. Figure 2 Tunneling Settlement Trough The settlement can be limited by locating the bore in clay. The settlement will be more if it is bored in sand below the water table or in mixed face conditions (i.e. both clay and water bearing sand encountered at the face). The deeper the bore is relative to diameter of the bore the less total settlement will occur over the centerline, but the wider is the area affected by settlement due to tunneling. The amount of settlement is a function of the volume of ground bored per linear foot. A bigger diameter bore will cause more settlement than a smaller diameter bore.

5 Page 5 of 5 In order to reduce tunneling settlement as much a possible we would use the smallest possible diameter tunneling equipment and move the bore so that the crown is at least 1 diameter or 5 feet into a clay layer. Moving the bore deeper helps from a settlement point of view but makes access shafts more expensive. The tunnels for this project are located in sand below the water table for the most part or mixed face conditions. We understand that in order to lower the tunnel into a clay layer there would need to be substantial modifications to the downstream lift station which is probably cost prohibitive. The smallest bore diameter would be possible using microtunneling equipment. However, the 2,100-foot drive length under the runways is a concern. In order for microtunneling to be a practical alternative one or more access shafts would need to be added between active taxiways and runways this location creates substantial limitations for tunneling activities. The additional access shafts add to the cost, and the microtunneling equipment is also likely to be more expensive, so unless tunnel related settlement is of extreme concern we do not think this is a cost-effective option. We understand that in discussions with industry that a 54-inch diameter bore was about the minimum with good availability. If this approach is used then we recommend limiting the bore diameter to no larger than 54 inches. Following is a comparison of estimated settlements above the tunnel centerline beneath the runways (the estimated maximum settlement) for the following scenarios. Tunnel at current profile (primarily in water bearing sand, assume 3.5% ground loss) o 54-inch bore w/steel liner: Est. Settlement 0.25 Inch o 30-inch bore w/o steel liner: Est. Settlement 0.10 Inch Tunnel lowered into clay layer (assume 1.5% ground loss) o 54-inch bore w/steel liner: lower profile about 8 feet, Est. Settlement 0.12 Inch o 30-inch bore w/o steel liner: lower profile about 6 feet, Est. Settlement 0.05 Inch Based on the tolerable differential settlement discussed above a 54-inch diameter bore should be adequate assuming reasonably good construction practices are used. Specification Review For the project specification Tunnel Excavation and Primary Liner as written we have the following comments. Section 1.06C should specifications mention the aircraft wheel loads required for the design? Section 3.03C.2 Be capable of closed face operations Section 3.05A.3 Open-face excavations are not allowed beneath runways and taxiways or within 50 feet of the edge of such facilities. Section 3.07A Perhaps show on plans subsurface settlement monitoring points on the approaches to the runways/taxiways. Install 5 feet above of the top of bore along the tunnel centerline.