O-Cell Load testing of large-diameter single-drilled shaft foundations for the REM project in Montreal

By Riad Diab, Louis D’Amours, and Malek Bouteldja, SNC-Lavalin, Montreal, Quebec, Canada

Edited by William F. “Bubba” Knight, Fugro Loadtest, Gainesville, FL.

Figure 1. REM project alignment

Introduction

The Réseau Électrique Métropolitain (REM), a $6.5 billion design-build project, is being constructed by a Joint Venture comprised of SNC Lavalin, Dragados Canada, Group Aecon Quebec, Pomerleau, and EBC, partnering with Aecom and SNC Lavalin as the lead design firms. The project, referred to as REM, is a fully automated light rail transit (LRT) proposed by the Caisse de dépôt et placement du Québec (CDPQ) Infra, to serve the major metropolitan areas in Montreal, Canada. The 67 kilometres (km) REM will be one of the world’s largest automated transportation systems in the world. 

As Figure 1 shows, the REM will link downtown Montreal, South Shore, West Island (Sainte-Anne-De-Bellevue), North Shore (Deux-Montagnes), and the Montreal-Pierre Elliott Trudeau International Airport.

The project comprises four segments: South Shore (SS) will cross the St. Lawrence River on the new Champlain Bridge; the Deux Montagnes (DM) segment, which is basically a conversion of the existing Deux-Montagnes line; the Sainte-Anne-De-Bellevue (SADB) segment; and the airport segment. Over 25 km will be constructed on an elevated structure founded on over 650 single-drilled shafts socketed into rock.

One advantage of using a single shaft is the ease of construction, especially close to existing structures. However, the major disadvantage is the lack of redundancy. This highlights the importance of design calibration.

Depending on the structural loads and site conditions, the drilled shafts will range from two to 3.2 metres (m) in diameter with embedment of up to nine metres within the sound rock calibrated with full scale Osterberg Cells Load tests.

Site Geologic Conditions

The general geology of the layout consists of a till deposit overlying the bedrock at varying depths (two to 17 m). The surface of the bedrock is altered and fractured for depths varying from one to three. Three rock types are encountered along the REM alignment. Limestone and dolomite are the main rock formations along DM and SADB. Shale is the only rock type along the SS segment.

Testing objectives

To optimize drilled shaft design using higher resistance values, full-scale Osterberg Cell bidirectional static load test were performed in each of the encountered rock formations; limestone, dolomite and shale. As O-cell tests were performed for, the new Champlain Bridge project in the shale permission was obtained to use those results. Accordingly, one each O-Cell test was performed in the dolomite and limestone. The tests established the drilled shaft side and base resistance ultimate limit state design values for the construction means and methods planned for production foundations. The test also allowed calibrated development of an equivalent top load-settlement curve for production piles for use in for serviceability limit state settlement estimates.

Subsurface Investigation

Load test location soil and rock investigations included SPT borings, pressuremeter, dilatometer and vane shear tests in cohesive soil. The laboratory program consisted of soils sieve and hydrometer analysis and rock unconfined compressive tests and elastic modulus.

At the SADB load test location, the overburden soil consisted of an upper 4.4 m clayey silt deposit which the field vane indicated a 57 kPa undrained shear strength, underlain by a layer of glacial till consisting mainly of very dense silty sand and gravel.

The bedrock surface was a poor quality, weathered metamorphosed conglomerate located at depths of 11.4 to 12.1 m with an RQD of 25 per cent. Below was a dolomite in poor to good quality with a 69 per cent RQD.

At the DM load test location, the upper 3 m of overburden is a silt and sandy silt fill material with an average SPT ‘N’ values of 4 blows/0.3 m, underlain by a four-metre glacial till layer of silty sand, average SPT ‘N’ value 14 blows/0.3 m. The limestone surface encountered at a seven metre depth was of bad quality for 0.75 m, RQD of 22, with quality increasing from good to excellent as exhibited by RQD averaging 83.

Table 1 defines more specific rock parameters for the test sites:

Construction procedure

A hydraulic rig inserted a 1,300 mm diameter permanent steel casing through the overburden and the fractured rock to refusal. After casing seating and cleaning of overburden, an auger and bucket alternated to drill a 1,180 mm shaft socket with a minimum of 300 mm of recess. A cleaning bucket then obtained a relatively flat shaft base. Verticality checks were accomplished by measuring drilling rig alignment with a 1.2 meters long spirit level targeting verticality of zero to two per cent. Figure 2 shows the placement of the casing and the soil excavation.

Casing placement and soil excavation

After base cleaning, the O-cell assembly placement into the excavation and temporarily supported from the outer steel casing. Crosshole Sonic Logging (CSL) tubes attached to the reinforcement cage interior and the five-inch OD tremie pipe pre-inserted through the loading plates to facilitate placement. Figure 3 shows images of the reinforcement cage, instrumentation and O-cell assembly. Figure 3a depicts images of the cage placement and top of the O-cell test shaft.

Concrete placement initiated through the tremie at the shaft base. Continuous monitoring of the volume of concrete in the shaft (top of concrete elevation) to the volume delivered from the trucks occurred at all times. Tremie pipe embedment into the concrete stayed greater than 3M at all times. Confirmation of concrete construction quality control occurred through Ultrasonic Crosshole Testing (CSL) and pile integrity testing.

Load Test Results

Fugro Loadtest performed the O-Cell bi-directional static load tests, on full-scale non-production test shafts, July 9, 2018 in the dolomite and July 12, 2018 for the limestone. Performed in accordance with ASTM D1143 Loading Procedure, A – Quick Test, each test used one (1) 13.8 MN bidirectional embedded jack (O-cell) to load the shaft socket base against the socket side resistance.

Instrumentation & reinforcement cage O-Cell assembly
Reinforcement cage O-Cell assembly placement & top of O-cell test shaft

The concept known as the “Chicago Method”, which consists of utilizing a smaller base area to maximize the unit base pressure, was used. Therefore, the reactions from a 720-millimetre-diameter base area acted against the side shear from a 1,180-millimetre-diameter socket. Table 2 below shows the drilled shaft properties.

The detailed load-displacement curves for the dolomite and limestone test shafts are in Figures 4a and 4b, respectively. Table 2 details the bottom O-cell plate suspended slightly above the shaft base prior to concreting. The measured unit base resistance for the test shafts were modified using a 2(vert):1(horiz) projected area for the concrete-filled clearances below the base plate, dolomite – 100 mm; limestone – 130mm.

Each test shaft had four levels of sister bar vibrating wire strain gages attached diametrically opposite on the reinforcing cage. For the socket below the casing, Zone 1 is between the O-cell and Level 1 strain gages and Zone 2 is between the Level 1 and 2 strain gages. Figures 5a and 5b show the calculated unit skin friction versus pile vertical displacement in these zones. The results indicate that the shaft load response in the cased zone above the socket was negligible. Accordingly, production pile design neglected load capacity in the cased zone for analysis and recommendations.

Table 2: Drilled Shaft Properties