Contract one – Al Salam Street Tunnel on Al Salam Street in the centre of Abu Dhabi, UAE, is constructed using a top down construction method with a total length of 3.6km. The tunnel is located within a highly developed urban setting with numerous underground utilities.

The formation level of the tunnel is 13m below ground level and the temporary shaft support comprises an 18m-deep diaphragm wall. To allow for excavation within the temporary support, a system of dewatering wells is installed and then the base slab cast in the dry.

Two ground investigations are completed prior to construction. These do not show any abnormalities.

The temporary support and tunnel foundation are designed based on the findings. The wall is a conventionally anchored diaphragm wall and the base slab is a piled raft foundation. However in one area along the line of the tunnel, ground conditions are discovered to be very different during wall construction. Bentonite slurry supporting the sides of the wall excavations disappears completely and the sides collapse.

This results in a complete rethink of the design and construction of the walls and base foundation of the tunnel.

Geological conditions leading to wall failure
Two ground investigations were carried out. One designed and managed by the consultant prior to tender and the other by the main contractor after winning the contract. A geological cross section shows the ground model comprises calcarenite and calcareous sandstone below the dune sand deposit of approximately 8m thick.

The temporary support system comprises mainly diaphragm walls along both sides of the tunnel. The walls are 800mm thick and 18m long. A series of geotechnical instruments are installed in the ground to monitor movement and pore water pressures.

In one area in particular, 30m long diaphragm wall works can not continue because the trench collapses at about 10m depth. To investigate the problem additional ground investigation is undertaken by the main contractor, a Samsung and Saif Bin Darwish JV.

The investigation comprises 17 boreholes each 30m deep. Figure 2 shows the location of the boreholes and the geological cross section indicates the presence of cavities up to 1m thick within the calcarenite and sandstone layers. These new findings are surprising because there is a risk of high settlement to the tunnel and adjacent structures to the tunnel. In order to quantify this risk, a damage classification assessment is carried out.

Buildings Risk Assessment
When assessing the severity of damage for an existing structure, the control of damage should be based on multiple factors such as crack widths, total settlement, differential settlement and angular distortion of the foundation. Relying on only one parameter can be misleading.

The damage classification proposed by Rankin (ref 1) was used to determine the control parameters to the buildings that are applicable to framed buildings with isolated foundations or with pile foundations. This uses two control parameters – distortion and settlement – to classify the damage level. This classification is updated to take into account the vulnerability index of the building likely to be affected by the tunnel construction (ref 2).

This index is evaluated for each building using its structural behavior, foundation type, use of the building, state of conservation and its orientation. For each building a building condition survey is compiled containing all of the information, and a vulnerability index for each building is determined.

The results of Rankin classification and the risk categories through a vulnerability index evaluation are summarised opposite in figures 3 and 4.

Investigation and Analysis of Tunnel Movement
Geophysical investigation
A geophysical survey by multichannel analysis of surface waves (MASW) is carried out to determine the extent of the cavities. The technique provides shear wave velocity distribution as two-dimensional profiles or a series of depth slices. These shear waves were generated by an impact at ground surface. Shear waves are proportional to shear modulus.

The MASW is carried out along 25 lines, which include 13 in east-west direction, and 12 lines in north-south direction. The fieldwork includes the use of 24 low frequency geophones separated 1m from each other. Also an 8kg sledgehammer (shot) fired at 2m centres was used to create the waves and a seismograph recorded the shear wave data.

The data is processed and then plotted in two-dimensions along 25 lines with a depth of penetration limited to 20m. The data is also presented as a series of geological cross sections. Two regions are defined as follows.

The first has shear waves between 200 and 600m/s and is found to be limited at shallow depth less than 9m below ground level, i.e. above base slab level.

The second region has shear waves greater than 600m/s and locally less than 400m/s and is found at depths greater than 10m, i.e. below base slab level. Low velocities indicate weak strata or a cavity.

The low velocity zones shown on the two dimensional sections and depth slices are summarised in figure 5 (right) as anomalies. This shows some localised deep anomalies and large areas of shallow anomalies (ref 3). The shallow weak ground is not a concern as it is located above the base slab and therefore will be excavated, but the deep anomalies are another matter.

Borehole investigation
A program of investigation is carried out to confirm the presence and extent of the deep anomalies recorded in the geophysical investigation comprising six boreholes and six diagraphy holes to 30m depth. To maximise core recovery, a double tube core barrel with a nominal diameter of 90mm with hard liner on short core runs of 0.5–1.5m is used (reference 4). The investigation includes a series of SPT and pressuremeter tests to determine the elasticity modulus of the various strata.

Diagraphy profile (DH) results correlate well with the boreholes, so where diagraphy profiles show high penetration rates, standard penetration test results are almost zero at the adjacent boreholes.

Ground model and analysis
The ground model from the exploratory holes is shown in figure 6. This shows the presence of a soft layer 3m to 4m thick and cavities at 18m, some 5m below the proposed base slab formation.

The presence of the soft layer can induce very high movements in the tunnel. The magnitude of these movements is predicted by a two dimensional finite element analysis using Plaxis (ref 5).

The various stages of construction are analysed, including excavation to install ground anchors, excavation to formation level and applying surcharge behind the wall. Then the weights of the base slab and tunnel structure are applied, and are predicted to experience maximum movements of 31 and 49mm respectively. The effect of constructing the tunnel also induces high levels of movement on the adjacent structures.

These movements are excessive and therefore the category of damage is quantified to be three and four according to Rankin damage classification (figure 4) which implies there is a high risk of damage and hence special monitoring systems together with mitigation measures are required prior to tunnel construction.

Mitigation measures for the cavity
Two solutions are investigated to control movement in the tunnel and adjacent buildings. One is to use a raft piled option with piles of 800mm diameter and 13m in length. The other option is to undertake a ground improvement program. The pile option is disregarded due to costs.

The ground improvement consists of using short jet grouted piles, 4m long, 1m in diameter and spaced on a grid of 3m x 3m. The spacing would be reduced to a tighter grid of 1.5m x 1.5m if a cavity is encountered during improvement works.

This program of improvement is to be followed by confirmatory boreholes to prove the remediation was successful.

A Klemm Bohrtechnik KR-805 is used and this is equipped with sophisticated programmable recording and controlling system installed on the operating panel for quality control of the jetting process. The panel has the capability to produce continuous logging of drilling parameters such as penetration speed, thrust, rotation speed and injection pressure of drilling fluid. This allows for the identification of soft areas. In addition where a sudden drop of the drilling rod or loss of drilling fluid are recorded then this would indicate the presence of cavities.

After the first few holes a criteria for determining thickness of ground to be improved is set as follows:
• Less than 100m/h the rock is hard
• Between 100 and 200m/h the rock is very weak
• Greater 200m/h rock/soil is soft

Drilling parameter profiles for the 96 holes confirm that the soft layer is where it was recorded in the initial boreholes and assumed in the analyses.

Once drilling reaches the stronger strata, high-pressure grout of 400 bar is injected as the rod is withdrawn. This creates a pile of 1m diameter which terminates 1m above the soft layer. This process replaces the soft layer with columns of 1m diameter and with lengths varying from one location to another.

Using this process the Samsung-Saif Bin Darwish JV completes all 96 short piles within 10 days.

Five confirmatory boreholes are drilled and the drilling parameter profiles confirm that the soft layer has been replaced by grout and therefore the remediation has been successful. Subsequent surveys of the tunnel structure show very little movement at this location.

Instrumentation and monitoring
As a consequence of the high category of damage, an instrumentation program is implemented. Its objective is to confirm the remedial works are satisfactory and in addition monitor the tunnel and adjacent structures to ensure movements are below control levels.

As well as the existing instrumentation for the tunnel an additional series of instrumentation is installed in this problematic area to monitor movements in the walls, soil behind the wall and adjacent buildings. This includes five inclinometers in the piles, three prism targets installed on the capping beam of the piles, three extensometer and six piezometers installed in the soil behind the piles.

Ground settlement markers are installed in a grid pattern between the walls and adjacent buildings.

On the buildings themselves a series of prism targets, tilt meters, settlement pins and crack meters are also installed. Two load cells to monitor the loads in each of the ground anchors on the tunnel walls are also installed.

Conclusion
The construction of a tunnel using the cut and cover technique is eventually successfully completed in an area that had complex and extremely weak ground conditions with cavities.

A damage risk assessment together with investigation, assessment and analysis is presented. All deep excavation works are successfully completed without any damage to the tunnel or adjacent buildings and structures.

This confirms that the ground improvement work was the key factor contributing in minimising ground movement, and that remedial measures together with extensive instrumentation and monitoring were essential contributors to a successful completion of the project.


Figure 1, shape of the collapse and slurry flow Figure 2, an investigation of the collapse with borehole and geological profiles Figure 3: Risk damage classification adjusted by the vulnerability index Iv Figure 4: Actions related to damage and risk categories in the building Figure 5, anomaly plan and confirmatory borehole locations Figure 6, predicted ground movement as deformed mesh and geology location