Test 31 - Cofferdams and Caissons

Question 1 ప్రశ్న 1

A bridge pier is to be constructed in a river using a temporary sheet-pile enclosure, while the final pier is supported on a sunk reinforced-concrete well. Which distinction between the two works is correct?

A The sheet-pile enclosure is necessarily the permanent deep foundation, and the well is only a drainage device. B Both the enclosure and the well must be removed before the pier is loaded. C The cofferdam temporarily excludes water and soil from the work area, whereas the well or open caisson forms part of the permanent foundation. D A caisson is always a temporary pump enclosure, whereas a cofferdam transfers pier loads to the bearing stratum.

Question 2 ప్రశ్న 2

A short-duration foundation excavation is proposed in shallow, slow-moving water. Ample space is available for broad side slopes, and suitable locally available cohesive fill can be protected against erosion. Which cofferdam type is a reasonable preliminary choice?

A A sloped earth-fill cofferdam with suitable seepage and erosion protection. B An unsupported single sheet-pile wall designed without considering lateral pressures. C A pneumatic caisson used solely as a temporary perimeter bund. D A floating box caisson assumed to block seepage through its open sides.

Question 3 ప్రశ్న 3

In a proposed rock-fill cofferdam, the available rock is free-draining and the structure must retain a substantial difference in water level. Which detail most directly addresses the permeability of the rock-fill mass?

A Use rock fill alone because large stones are inherently watertight. B Make the external slopes vertical to stop underseepage. C Avoid filters because migration of fines cannot occur in coarse fill. D Include a suitably designed impervious element or cutoff, with compatible filter and seepage-control details.

Question 4 ప్రశ్న 4

A rectangular excavation is enclosed by a single row of interlocking steel sheet piles. The planned excavation depth is such that the wall cannot safely act as an unbraced cantilever. What is the function of walers and struts in this arrangement?

A They replace sheet-pile embedment and eliminate all earth pressure. B Walers and struts transfer and resist lateral wall loads, limiting deformation as excavation and dewatering proceed. C They make the soil at the excavation base completely impermeable. D They provide the finished foundation toe-bearing capacity after the sheet piles are removed.

Question 5 ప్రశ్న 5

A river construction project requires a cofferdam wider and more stable than a single sheet-pile line. Two parallel sheet-pile walls are connected by ties, and the space between them is filled with suitable material. How should this system be classified?

A A pneumatic caisson with an air-pressurized working chamber. B An open well foundation with permanent bottom plug. C A double-wall sheet-pile cofferdam. D A single unbraced cantilever sheet-pile wall.

Question 6 ప్రశ్న 6

A large temporary river enclosure is proposed using interconnected circular sheet-pile cells filled with granular material. Which stability interpretation is most appropriate?

A The cellular arrangement relies on its mass and cell geometry, while sheet-pile interlock tension and sliding, overturning and foundation stability require checks. B The cells act as airlocks for compressed-air excavation below the riverbed. C The enclosed granular fill must be removed before hydrostatic pressure develops. D The cells are supported entirely by roof beams, so soil and water pressures may be ignored.

Question 7 ప్రశ్న 7

A vertical cofferdam wall has still water to a depth of 5 m on one side and is dry on the other. Neglect soil pressure, waves and seepage, and take the water unit weight as 10 kN/m3. What is the resultant horizontal hydrostatic force per metre length of wall?

A 50 kN/m. B 100 kN/m. C 250 kN/m. D 125 kN/m.

Question 8 ప్రశ్న 8

A vertical cofferdam wall retains still water 4 m deep against a dry excavation. Assume triangular hydrostatic pressure only, with gamma_w = 10 kN/m3. What are the horizontal force per metre length and its overturning moment about the bottom of the wall?

A 80 kN/m and 160 kN m/m. B 80 kN/m and 106.7 kN m/m. C 40 kN/m and 53.3 kN m/m. D 160 kN/m and 80 kN m/m.

Question 9 ప్రశ్న 9

A dewatered cofferdam excavation in saturated sand has calculated upward exit hydraulic gradient iexit = 0.60. The specified critical gradient for the soil is icr = 0.90. Using the simplified factor of safety against boiling FS = icr/iexit, which result and interpretation are correct?

A FS = 0.67; the exit gradient is greater than critical. B FS = 0.30; the soil must undergo general shear failure. C FS = 1.50; the exit gradient is below the critical value, but adequacy depends on the required design safety margin. D FS = 1.50; therefore all seepage, piping and uplift checks can be omitted.

Question 10 ప్రశ్న 10

A rectangular cofferdam excavation has a horizontal base area of 8 m by 6 m. For a simplified uplift check, assume a uniform upward hydrostatic pressure corresponding to a water head of 3 m, gamma_w = 10 kN/m3, and a stabilizing downward force of 1150 kN. What are the upward force and unbalanced vertical force?

A 1440 kN upward and 290 kN net upward. B 240 kN upward and 910 kN net downward. C 1440 kN upward and 290 kN net downward. D 1150 kN upward and zero net force.

Question 11 ప్రశ్న 11

A temporary dewatering system for a cofferdam must handle a design inflow of 34 litres per second. Identical pumps each deliver 20 litres per second at the required head. If the specified operating criterion is to meet the design inflow even with any ONE pump unavailable, what is the minimum number of installed pumps?

A Two pumps. B Four pumps. C One pump. D Three pumps.

Question 12 ప్రశ్న 12

A deep braced sheet-pile cofferdam is to be excavated below the river water level. Which construction control is most appropriate during successive excavation and dewatering stages?

A Remove the lowest strut before excavating deeper to reduce compression forces. B Install and maintain supports in the designed sequence, control drawdown and monitor wall movement, support loads and groundwater conditions. C Pump out all water before checking base heave and piping resistance. D Assume temporary works require no further review once the final permanent foundation is designed.

Question 13 ప్రశ్న 13

A reinforced-concrete foundation unit is sunk through riverbed soil by excavating or dredging material from within its hollow interior. During sinking it is open at both top and bottom and has a cutting edge. Which foundation type is described?

A Closed-bottom box caisson positioned on a prepared bed. B A temporary earth-fill cofferdam. C An open caisson or well foundation. D A pneumatic caisson necessarily containing a permanently pressurized working chamber.

Question 14 ప్రశ్న 14

In a conventional open well foundation, which pairing of lower components and functions is correct?

A The steel cutting edge aids penetration at the base, while the reinforced-concrete well curb supports the steining and transfers forces around the lower edge. B The well cap performs all dredging, while the bottom plug acts as the cutting edge. C The top plug is the primary cutting tool, while the steining is a temporary pumping line. D The internal sand filling is used to pressurize the excavation with compressed air.

Question 15 ప్రశ్న 15

During sinking of an open caisson, what is the principal role of the steining, and which movement-related checks remain necessary?

A Steining is the temporary water pump, and only its motor capacity requires checking. B Steining is a permanently closed bottom slab, so dredging cannot occur inside. C Steining is an airlock for workers entering a high-pressure chamber in every open caisson. D Steining is the main wall or shaft of the well, whose integrity and alignment must be maintained while sinking against surrounding soil and water forces.

Question 16 ప్రశ్న 16

An open caisson begins to tilt while sinking because the soil resistance and dredging depth are not uniform around its perimeter. Which response is most appropriate?

A Continue unrestricted dredging at the deepest point because tilt always self-corrects. B Measure tilt and shift and apply a controlled, engineered correction involving dredging, loading or other measures appropriate to the ground conditions. C Install the permanent well cap and ignore the deviation from its specified position. D Immediately dewater the entire riverbed without checking stability.

Question 17 ప్రశ్న 17

An open well has reached its specified founding level and water remains in its interior. Which procedure is most appropriate for forming a bottom seal before subsequent dewatering, where dewatering is part of the construction plan?

A Pump the interior dry first regardless of hydraulic gradient and omit a plug. B Place loose sand as the only bottom seal and immediately apply full uplift pressure. C Clean the founding surface and place a suitably designed underwater concrete bottom plug by an appropriate method such as tremie placement, then allow the required strength and verify stability before dewatering. D Use compressed air without a sealed working chamber to replace all bottom-plug requirements.

Question 18 ప్రశ్న 18

Which sequence is most consistent with construction and completion of a conventional open well foundation, assuming the design requires internal filling and both bottom and top plugs?

A Construct the curb and steining, sink by controlled dredging, complete and cure the bottom plug, place specified internal filling and top plug, and construct the well cap. B Cast the well cap first, fill the well with sand, and then dredge through the sealed bottom plug. C Fill the hollow well with concrete before sinking, then remove the cutting edge. D Install a compressed-air bulkhead and airlocks for every open well, then omit the cutting edge.

Question 19 ప్రశ్న 19

A marine structure is to be supported by a prefabricated hollow reinforced-concrete unit with a permanently closed bottom. The unit can be floated or transported to a prepared seabed location and ballasted into position rather than excavated through its base. Which type is appropriate?

A A conventional open caisson with an open bottom during sinking. B A pneumatic caisson that must be sunk by workers excavating inside compressed air. C A double-wall sheet-pile cofferdam that is always removed after construction. D A box or floating caisson.

Question 20 ప్రశ్న 20

A bridge foundation is to be advanced through a water-bearing stratum while workers need relatively dry access to remove obstructions at the base. The selected caisson has an airtight bulkhead, a pressurized working chamber and personnel access through locks. Which method is described?

A Ordinary open caisson sinking with an unpressurized base. B Pneumatic or compressed-air caisson construction. C Floating box caisson placement on a prepared bed. D An earth-fill cofferdam without access below its base.

Question 21 ప్రశ్న 21

A pneumatic caisson working chamber is located at a level corresponding to a hydrostatic water head of 10 m above it. Ignoring additional pressure allowances and variations, take gamma_w = 10 kN/m3. What approximate compressed-air GAUGE pressure is required merely to balance the external hydrostatic pressure at chamber level?

A 10 kPa. B 50 kPa. C 100 kPa. D 200 kPa.

Question 22 ప్రశ్న 22

Workers must enter and leave the pressurized working chamber of a pneumatic caisson repeatedly. Which safety measure directly addresses a characteristic health risk of this method?

A Controlled compression and decompression procedures, appropriate airlocks and medical safeguards against decompression sickness. B Immediate transfer from working pressure to atmospheric pressure to prevent gas bubbles. C Elimination of all ventilation because compressed air contains sufficient oxygen indefinitely. D Substitution of a sheet-pile cofferdam for personal protective and medical procedures inside the working chamber.

Question 23 ప్రశ్న 23

A floating box caisson has a rectangular plan measuring 5 m by 4 m and displaces water over a submerged depth of 3 m. For this idealized check take gamma_w = 10 kN/m3, self-weight = 420 kN and neglect other forces. What are the buoyant force and minimum additional downward ballast needed for neutral vertical force balance at this immersion?

A 420 kN and 180 kN. B 600 kN and 420 kN. C 180 kN and 600 kN. D 600 kN and 180 kN.

Question 24 ప్రశ్న 24

A sinking open well approaches its founding level, but monitoring indicates a developing lateral shift and possible obstruction beneath part of its cutting edge. Which site action is most appropriate?

A Increase dredging indiscriminately until the well suddenly drops into place. B Review the sinking log and ground information, inspect or investigate the obstruction as feasible, and plan controlled corrective measures with continued tilt and shift monitoring. C Declare the well satisfactory because final depth alone governs acceptance. D Seal the well cap before identifying the cause of shift and assume the obstruction cannot affect bearing performance.

Question 25 ప్రశ్న 25

A water-intake bridge pier must be founded below the anticipated scour zone in a flowing river. Temporary work also requires control of water around a construction area. Which combined foundation-and-construction assessment is correct?

A A temporary sheet-pile cofferdam alone necessarily provides permanent deep bearing resistance below scour. B A box caisson can always be sunk through boulder-bearing riverbed strata simply by dredging through its closed bottom. C A suitable open or other designed caisson may provide the permanent pier foundation, while a separately designed cofferdam may be used for temporary water exclusion; scour, current, stability and construction constraints govern selection. D A pneumatic caisson automatically eliminates all scour and seepage concerns after construction.

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