WJEC Geology of the Human Environment (GL3) Jan 2005

WJEC – Geology of the Human Environment (GL3) – Jan 2005

2. Figure 2a shows ground subsidence in the Santa Clara valley, California, together with changes in water table levels caused by groundwater extraction.

Figure 2a

(a) What is meant by the term water table? [1]

…………………………………………………………………………………………..

(b) Refer to Figure 2a.

(i) Calculate the average rate of ground subsidence (mm/year) between 1920 - 1930. Show your working. [2]

Average rate of subsidence ……………………………………………mm/year

(ii) Compare the variation in rates of subsidence from 1935 – 1980 with that for 1920 - 1930. [3]

…………………………………………………………………………………………

(c) With reference to Figure 2a, describe the relationship between changes in water table levels and ground subsidence. Explain your answer. [4]

…………………………………………………………………………………………

(d) Mexico City has also subsided as a result of groundwater extraction. Figure 2b shows how buildings of different size, shape and foundation design in Mexico City, have subsided by different amounts compared with street level.

Figure 2b

Refer to Figure 2b.

Give a geological explanation for differences in the amount of subsidence of any one building (A , B or C) compared to the subsidence of the current street level. [3]

Chosen building (A, B or C)

…………………………………………………………………………………………

………………………………………………………………………………………… Total 13 marks

Notes for answers

These represent suggested answers and do not necessarily represent all the possible acceptable alternatives of a mark scheme.

Question 2

(a) Upper surface/level of saturated rock/soil. [1]

(b) (i) 800/10 (range 700 to 900) or 80cm or 0.8m

= 80 mm/yr (70 - 90) [2]

(ii) 1935-1948 – lower/less (little subsidence)

1948-1960 – same rate of subsidence

1960-1968 – greater rate

1968-1980 - lower/less (little subsidence)

(max 3) [3]

When water table drops – ground subsidence occurs

When water table rises – ground subsidence stops

Subsidence only when WT drops below previous low level

Reference to response time of subsidence to WT changes

(3 max)

Explain;

Falling WT = Pore pressure reduced

Particle become better compacted/compressed

Rising WT = Stability

Subsidence not reversed with increased pore pressure

(3 max) [4]

(d) A - Clay dewatered more beneath raft - weight

Clay more compacted beneath building

No deep foundations to compensate/support

Loading subsidence of A > dewatering subsidence

Other sensible observation explained

B - Piles to more component thin sand layer

Clay beneath slightly dewatered/compacted

Basement reduces load

Loading compaction of B = dewatering subsidence

Other sensible observation explained

C - Deep piles - greater support

Thicker/stronger sand - more competent

Clay beneath less affected by weight of building

Subsidence of C < dewatering subsidence of street

Other sensible observation explained

(3 max) [3]

Total 13 marks

ESTA would like to thank the WJEC for allowing this question to be published on GEOTREX. Any comments should be directed to the Subject Officer for Geology at the WJEC 15/04/2007